An organic reaction occurs between a substratum (an organic compound) and an attacking agent termed a reagent.
1. Bond Cleavage (Homolytic or Heterolytic Cleavage)
The bonds in the organic compound break and reaction intermediates are formed. These reactions intermediates are:
1. Free radicals
2. Carbocations (An ion that has carbon atom with positive charge)
3. Carbanion (An ion with carbon atom with negative charge)
4. Carbene (singlet and triplet)
5. Radical ion
Homolytic Fission
Free radicals form as intermediates
Heterolytic Fission
Carbocations, carbanions and carbenes form during this type of fission
2. Reagents
Electrophiles, Nucleophiles and Ambiphiles are reagents.
Electrophiles:
This reagent requires electrons.
Examples: ions like NO2+, Cl+
and atoms in molecules like S03 and BF3.
Nucleophiles
They are negatively charged and attack positively charged atoms.
Examples ions like OH-, CN-, H- or
atoms in molecules like N in NH3 and O in H2O.
Ambiphiles
H2O It has an electron attacking and electron repelling site also.
3. Inductive Effect:
The study of this effect is important because of its implication for the stability of reaction intermediates.
When an electron-withdrawing or electron-releasing group (substituent) is attached to carbon chain, polarity is induced on the carbon atom and on the substituent attached to it. This is called inductive effect or simply as I-effect.
Negative Inductive Effect (-I Effect)
Postive Inductive Effect (+I Effect)
4.Electromeric effect
In presence of an electrophile, there is complete transfer of π electrons from one atom to other to produce temporary polarity on atoms joined by mutliple bonds. It is called electromeric effect.
Positive Electromeric effect
Negative Electromeric effect
5. Mesomeric effect
In conjugate systems (compounds having alternate π and sigma bonds) π-electron shifting takes place consecutively giving permanent polarity on the chain. This π-electron shift in conjugate systems is called mesomeric effect or conjugate effect.
Positive
Negative
6. Hyperconjugation
When an alkyl group is attached to a double bond (unsaturated system in general), the hydrogen in the alkyl group releases an electron and becomes positive. And in the process there is no bond formation between C and H+. This type of electron release is called hyperconjugation.
7. Types of Reactions
Addition
Elimination
Substitution
Rearrangement
7a. Addition Reactions
This reaction is characteristic of the compounds having double or triple bonds.
7b. Elimination Reactions
In most elimination reactions, two groups on adjacent atoms are lost and a double bond is formed. Thus, the product of an eliminatin reaction is an alkene.
7c. Substitution Reactions
In this reaction, one atom or group of atoms in a molecule is replaced by another.
Join IIT JEE Academy Orkut Community for more interaction
http://www.orkut.co.in/Main#Community.aspx?cmm=39291603
Tampilkan postingan dengan label July-Dec revision. Tampilkan semua postingan
Tampilkan postingan dengan label July-Dec revision. Tampilkan semua postingan
Minggu, 21 Desember 2008
Minggu, 28 September 2008
Tautomerism and Enols - July Dec Revision
Tautomerism
Tautomerism may be defined as the phenomenon in which a single compound exists in two readily interconvertible structures that differ markedly in the relative position of at least one atomic nucleus, generally hydrogen. The two different structures are known as tautomers of each other.
In the presence of acid or base, ketone or aldehyde with α-hydrogen is converted into hydroxyl ene (called enol), hence this type of isomerism in which keto and enol forms exist in dynamic equilibrium is also called ket-enol isomerism.
Enols
Preparation of aldehydes and ketones from alkynes
Hydration of alkynes in the presence of dilute sulphuric acid and HgSO4 as catalyst gives aldehydes and ketones.
Water adds to alkynes to form unstable enol intermediates which rearrange to form aldehydes or ketones.
Hydration of acetylene gives acetaldehyde.
Hydration of alkynes other than acetylene gives ketones.
Addition of water to alkynes (hydration of alkynes)
In the presence of acid (H2SO4) and HgSO-4, a molecule of water adds to the triple bond at 348K. The catalyst in this reaction is HgSO4 (Mercuric sulphate). The final products of this reaction are carbonyl compounds aldehydes and ketones.
Initially enol is formed which is raidly converted into an equilibrium mixture containing keto form in excess. Enol is so called because it contains 'ene' (double bond) and an alcoholic group (ol).
IIT JEE Revision - Acid Catalysed Hydration of Alkynes
In the presence of acid (H2SO4) and HgSO-4, a molecule of water adds to the triple bond at 348K.
The catalyst in this reaction is HgSO4 (Mercuric sulphate).
The final products of this reaction are carbonyl compounds aldehydes and ketones.
Initially enol is formed which is raidly converted into an equilibrium mixture containing keto form in excess.
Enol is so called because it contains 'ene' (double bond) and an alcoholic group (ol).
Example:
Addition of water to Ethyne or acytelene: Acetylene is passed into water (at about 330K) containing 60% H2SO4 and about 1% mercuric sulphate (HgSO4) as a catalyst, acetaldehyde is formed.
In the first step 'ethenol' is formed and in the second step the rearrangement of it takes place and its isomer 'acetaldehyde' is formed.
The conversion of enol form into keto form is termed tautomerism
Tautomerism may be defined as the phenomenon in which a single compound exists in two readily interconvertible structures that differ markedly in the relative position of at least one atomic nucleus, generally hydrogen. The two different structures are known as tautomers of each other.
In the presence of acid or base, ketone or aldehyde with α-hydrogen is converted into hydroxyl ene (called enol), hence this type of isomerism in which keto and enol forms exist in dynamic equilibrium is also called ket-enol isomerism.
Enols
Preparation of aldehydes and ketones from alkynes
Hydration of alkynes in the presence of dilute sulphuric acid and HgSO4 as catalyst gives aldehydes and ketones.
Water adds to alkynes to form unstable enol intermediates which rearrange to form aldehydes or ketones.
Hydration of acetylene gives acetaldehyde.
Hydration of alkynes other than acetylene gives ketones.
Addition of water to alkynes (hydration of alkynes)
In the presence of acid (H2SO4) and HgSO-4, a molecule of water adds to the triple bond at 348K. The catalyst in this reaction is HgSO4 (Mercuric sulphate). The final products of this reaction are carbonyl compounds aldehydes and ketones.
Initially enol is formed which is raidly converted into an equilibrium mixture containing keto form in excess. Enol is so called because it contains 'ene' (double bond) and an alcoholic group (ol).
IIT JEE Revision - Acid Catalysed Hydration of Alkynes
In the presence of acid (H2SO4) and HgSO-4, a molecule of water adds to the triple bond at 348K.
The catalyst in this reaction is HgSO4 (Mercuric sulphate).
The final products of this reaction are carbonyl compounds aldehydes and ketones.
Initially enol is formed which is raidly converted into an equilibrium mixture containing keto form in excess.
Enol is so called because it contains 'ene' (double bond) and an alcoholic group (ol).
Example:
Addition of water to Ethyne or acytelene: Acetylene is passed into water (at about 330K) containing 60% H2SO4 and about 1% mercuric sulphate (HgSO4) as a catalyst, acetaldehyde is formed.
In the first step 'ethenol' is formed and in the second step the rearrangement of it takes place and its isomer 'acetaldehyde' is formed.
The conversion of enol form into keto form is termed tautomerism
Jumat, 12 September 2008
Chiral Molecules - July Dec Revision
Chiral molecules
A molecule is said to be chiral if it lacks symmetry and its mirror images are not superimposable. To be chiral a molecule must lack symmetry, that is, a chiral molecule can not have any type or symmetry.
Carbon atoms with four sp3 hybridized orbitals can enter into up to four different bonds about the central carbon atom.
When the central carbon bonds with differing atoms or groups of atoms the carbon is termed an asymmetric carbon atom.
Bromochlorofluoromethane is an example of such a molecule. The central carbon, with four sp3 bonds oriented (pointing) to the corners of a tetrahedron, is bonded to a bromine, chlorine, fluorine and methane atoms. There is no symmetry to this molecule.
To explain things in a more simple way, Chiral molecules are like right hand left hand. Both hands have identical fingers but in different orientations. The little finger is in clockwise direction from the middle finger in right hand, where as the little is in the counterclockwise direction in the left hand. So you can fix a right hand to another person's left wrist. Right hand and left hand are different even though all the five diffetent fingers are more or less similar in both the hands.
Enantiomers are optical isomers. A chiral molecule will have enantiomers.
A solution in which both enantiomers of a compound are present in equal amounts is called a racemic mixture, or racemate.
A molecule is said to be chiral if it lacks symmetry and its mirror images are not superimposable. To be chiral a molecule must lack symmetry, that is, a chiral molecule can not have any type or symmetry.
Carbon atoms with four sp3 hybridized orbitals can enter into up to four different bonds about the central carbon atom.
When the central carbon bonds with differing atoms or groups of atoms the carbon is termed an asymmetric carbon atom.
Bromochlorofluoromethane is an example of such a molecule. The central carbon, with four sp3 bonds oriented (pointing) to the corners of a tetrahedron, is bonded to a bromine, chlorine, fluorine and methane atoms. There is no symmetry to this molecule.
To explain things in a more simple way, Chiral molecules are like right hand left hand. Both hands have identical fingers but in different orientations. The little finger is in clockwise direction from the middle finger in right hand, where as the little is in the counterclockwise direction in the left hand. So you can fix a right hand to another person's left wrist. Right hand and left hand are different even though all the five diffetent fingers are more or less similar in both the hands.
Enantiomers are optical isomers. A chiral molecule will have enantiomers.
A solution in which both enantiomers of a compound are present in equal amounts is called a racemic mixture, or racemate.
Jumat, 05 September 2008
Fluid - July Dec revision
There is continuity between the gaseous and liquid state. The term fluid is used for either liquid or a gas to recognize this continuity. A liquid can be viewed as a very dense gas. Liquid and gas can be distinguished only when the fluid is below its critical temperature and its pressure and volume lie under the dome (in isotherm) since in that situation liquid and gas are in equilibrium and a surface separating the two phases is visible.
Chapter: States of Matter
Join Orkut community
Join Orkut community IIT-JEE-Academy for interaction regarding various issues and doubts
http://www.orkut.co.in/Community.aspx?cmm=39291603
Chapter: States of Matter
Join Orkut community
Join Orkut community IIT-JEE-Academy for interaction regarding various issues and doubts
http://www.orkut.co.in/Community.aspx?cmm=39291603
Sabtu, 30 Agustus 2008
Relationship between Solubility and Solubility product
Relationship between solubility (S) and solubility product (Ksp)
Consider MqAr a sparingly soluble salt.
Where
q = Number of cations (Mr+) and
r = Number of anions (Aq-)
That is we have in dissolved state
MqAr ↔ qMr+ + r Aq-
Then
Ksp = [Mr+]q [Aq-]r
If solubility is S, according to the definition of solubility product
We have
[Mr+]q = q.S mol/dm³
[Aq-]r = r.S mol/dm³
Hence Ksp = [q.S] q [r.S] r
= Sq+r. qq.rr
For example for the salt, calcium Phophate, Ca3(PO4)2
Ca3(PO4)2 ↔ 3Caaq2+ + 2PO4(aq)3-
Ksp = [Ca2+] 3 [PO43-]2
= S3+2.33.22
= 108S5
Past JEE Question
For a sparingly soluble salt ApBq, the relationship of its solubility product (Ksp) with its solubility (s) is
a. Ksp = sp+q.pp.qq
b. Ksp = sp+q.pq.qp
c. Ksp = spq.pp.qq
d. Ksp = spq.(pq)p+q)
(2001)
Answer: a
Consider MqAr a sparingly soluble salt.
Where
q = Number of cations (Mr+) and
r = Number of anions (Aq-)
That is we have in dissolved state
MqAr ↔ qMr+ + r Aq-
Then
Ksp = [Mr+]q [Aq-]r
If solubility is S, according to the definition of solubility product
We have
[Mr+]q = q.S mol/dm³
[Aq-]r = r.S mol/dm³
Hence Ksp = [q.S] q [r.S] r
= Sq+r. qq.rr
For example for the salt, calcium Phophate, Ca3(PO4)2
Ca3(PO4)2 ↔ 3Caaq2+ + 2PO4(aq)3-
Ksp = [Ca2+] 3 [PO43-]2
= S3+2.33.22
= 108S5
Past JEE Question
For a sparingly soluble salt ApBq, the relationship of its solubility product (Ksp) with its solubility (s) is
a. Ksp = sp+q.pp.qq
b. Ksp = sp+q.pq.qp
c. Ksp = spq.pp.qq
d. Ksp = spq.(pq)p+q)
(2001)
Answer: a
Jumat, 29 Agustus 2008
Solubility Product - July Dec Revision
Solubility product of a salt at a given temperature is equal to the product of the concentrations of its ions in the saturated solution, with each concentration term raised to the power equal to the number of moles of ions produced on dissociation of one mole of the substance.
Law of Mass Action - July-Dec Revision
Law of Mass Action
For the reaction
2 NO2 = N2O4
in a sealed tube the ratio
[N2O4]
-------
[NO2]²
is a constant. This phenomenon is known as chemical equilibrium. The ratio is called equilibrium constant (K).
[N2O4] and [NO2] are molar concentrations of N2O4 and NO2.
Such a law of nature is called the law of mass action or mass action law.
Of course, when conditions, such as pressure and temperature, change, a period of time is required for the system to establish an equilibrium.
For systems that are not at equilibrium yet, the ratio calculated from the mass action law is called a reaction quotient Q. The Q values of a closed system have a tendency to reach a limiting value called equilibrium constant K over time. A system has a tendency to reach an equilibrium state.
The law of mass action may be written as:
The rate of a chemical reaction at any particular temperature is proportional to the product of the molar concentrations of reactants with each concentration term raised to the power equal to the number of molecules of the respective reactants taking part in the reaction.
In the chemical kinetics chapter we come to know that chemical reactions can be elementary reactions or complex reactions having number of elementary reactions.
Law of mass action is valid for elementary reactions.
For the reaction
2 NO2 = N2O4
in a sealed tube the ratio
[N2O4]
-------
[NO2]²
is a constant. This phenomenon is known as chemical equilibrium. The ratio is called equilibrium constant (K).
[N2O4] and [NO2] are molar concentrations of N2O4 and NO2.
Such a law of nature is called the law of mass action or mass action law.
Of course, when conditions, such as pressure and temperature, change, a period of time is required for the system to establish an equilibrium.
For systems that are not at equilibrium yet, the ratio calculated from the mass action law is called a reaction quotient Q. The Q values of a closed system have a tendency to reach a limiting value called equilibrium constant K over time. A system has a tendency to reach an equilibrium state.
The law of mass action may be written as:
The rate of a chemical reaction at any particular temperature is proportional to the product of the molar concentrations of reactants with each concentration term raised to the power equal to the number of molecules of the respective reactants taking part in the reaction.
In the chemical kinetics chapter we come to know that chemical reactions can be elementary reactions or complex reactions having number of elementary reactions.
Law of mass action is valid for elementary reactions.
Chemical Equilibrium - July Dec Revision
In most of the reaction carried out in closed vessels, reaction does not go to completion under given set of conditions of temperature and pressure. Initially, in the vessel, only reactants are present, and as the reaction proceeds, the concetration of reactants will decrease and that of products will increase.
After some time a stage is reached when no further change in concetrations of reactants and products is observed. This state is called equilibrium state and some of the important questions regarding this phenomenon are:
1. why do reactions seem to stop before they reach completion?
2. What is the extent to which a reaction proceed?
3. Can we modify the conditions to improve the yield of products?
Equilibrium - The phenomenon
Equilibrium is the state at which the concentrations of reactants and products do not change with time.
It is important to remember that equilibrium is achieved in closed vessel reactions only.
The important aspect of reaction equilibrium is the reversibility. The products combine and form reactants. At equilibrium, both the forward and backward reactions are taking place. The rates of forward and backward reactions are same or equal at the equilibrium. As a result, the concentration of each species becomes constant.
The equilibrium is termed as dynamic reaction equilibrium. Dynamic means at a microscopic level, the system is in motion. But at macroscopic level, concentrations are not changing.
Chemical reactions may be classified as reversible reactions and irreversible reactions.
Example of irrereversible reaction
Decomposition of potassium chlorate into potassium chloride and oxygen. Even in a closed vessel this reaction is not reversible.
Example of reversible reaction
1. Decompositon of calcium carbonate. When solid calcium carbonate is heated in a closed vessel at 1073 K, it decomposes into solid calcium oxide and gaseous carbon dioxide. Due to gaseous CO2 there is pressure of gas in the vessel which can be measured. At a constant temperature it can be observed that pressure becomes constant after some time, which means no further CO2 is being produced even though calcium carbonate is still there in the vessel. The constant pressure indicates to us that reaction equilibrium is reached.
Characteristics of chemical equilibrium
1. Chemical equilibrium is dynamic in nature (already explained).
2. the properites of the system become constant at equilibrium and remain unchanged thereafter unless external or internal conditions are changed.
3. The equilibrium is attained only if the system is closed one.
4. As the reactions are reversible and happen under the same conditions, equilibrium can be attained from either direction.
5. A catalyst does not alter the equilibrium point. The catalyst increases the rate of reaction, and at equilibrium it increases both forward and backward reaction rates. But it does not alter equilibrium point, the concentrations of products and reactants at a given set of conditions. But the equilibrium is reached earlier in the presence of a catalyst.
After some time a stage is reached when no further change in concetrations of reactants and products is observed. This state is called equilibrium state and some of the important questions regarding this phenomenon are:
1. why do reactions seem to stop before they reach completion?
2. What is the extent to which a reaction proceed?
3. Can we modify the conditions to improve the yield of products?
Equilibrium - The phenomenon
Equilibrium is the state at which the concentrations of reactants and products do not change with time.
It is important to remember that equilibrium is achieved in closed vessel reactions only.
The important aspect of reaction equilibrium is the reversibility. The products combine and form reactants. At equilibrium, both the forward and backward reactions are taking place. The rates of forward and backward reactions are same or equal at the equilibrium. As a result, the concentration of each species becomes constant.
The equilibrium is termed as dynamic reaction equilibrium. Dynamic means at a microscopic level, the system is in motion. But at macroscopic level, concentrations are not changing.
Chemical reactions may be classified as reversible reactions and irreversible reactions.
Example of irrereversible reaction
Decomposition of potassium chlorate into potassium chloride and oxygen. Even in a closed vessel this reaction is not reversible.
Example of reversible reaction
1. Decompositon of calcium carbonate. When solid calcium carbonate is heated in a closed vessel at 1073 K, it decomposes into solid calcium oxide and gaseous carbon dioxide. Due to gaseous CO2 there is pressure of gas in the vessel which can be measured. At a constant temperature it can be observed that pressure becomes constant after some time, which means no further CO2 is being produced even though calcium carbonate is still there in the vessel. The constant pressure indicates to us that reaction equilibrium is reached.
Characteristics of chemical equilibrium
1. Chemical equilibrium is dynamic in nature (already explained).
2. the properites of the system become constant at equilibrium and remain unchanged thereafter unless external or internal conditions are changed.
3. The equilibrium is attained only if the system is closed one.
4. As the reactions are reversible and happen under the same conditions, equilibrium can be attained from either direction.
5. A catalyst does not alter the equilibrium point. The catalyst increases the rate of reaction, and at equilibrium it increases both forward and backward reaction rates. But it does not alter equilibrium point, the concentrations of products and reactants at a given set of conditions. But the equilibrium is reached earlier in the presence of a catalyst.
Rabu, 27 Agustus 2008
Chemical Kinetics - July December Revision
Jee Syllabus
Chemical kinetics:
Rates of chemical reactions;
Order of reactions;
Rate constant;
First order reactions;
Temperature dependence of rate constant (Arrhenius equation).
--------------------------
The topic "Chemical kinetics" consists of reaction rate and reaction mechanism.
Reaction rate is the speed with which a reaction takes place. This shows the rate or speed at which the reactants are consumed and products are formed.
Reaction mechanism is the path by which a reaction takes place.
Rate of reaction
The rate of reaction is a quantity that tells how the concentration of reactants or product changes with time.
So this can be expressed as Δ concentration/Δ time. That is change in concentration divided by time taken for the change.
Molar concentration i.e., moles per liter (M), is used in these equations.
The brackets, [ ] are always used to to indicate molar concentrations.
Rate law
The rate for a reaction is a mathematical expression that relates the rate of reaction to the concentrations of the reactants.
For the reaction aA + bB → products
The rate law is expressed as, rate of reaction is proportional to [A]^x[B]^y.
x and y are determined experimentally. These values can be whole or fractional numbers or zero.
Law of Mass Action
In 1867, Cato Guldberg, and Peter Waage, proposed this law. According to this law, for the rate determining step in a reaction, the rate of reaction is proportional to the product of the concentrations of the reactants, each raised to the power of its coefficient in the balanced equation.
For the reaction aA + bB → cC (when it is a rate determining step)
Rate of reaction is proportional to [A]^a[B]^b
The above proportionality can be written as an equation, by putting in a proportionality constant k.
Rate = k *[A]^a[B]^b
K is called the specific rate constant
Order of Reaction
From the rate law for a reaction order of reaction can be determined.
For a particular species or reactant, the order is equal to the exponent for that species in the rate law.
For example for Rate = k *[A][B]^2
for B the order of reaction is 2. For A it is 1.
The overall order of reaction is equal to the sum of all the individual orders of reactants.
Temperature
As temperature increases, the average kinetic energy increases. So there are more molecules with activation energy and hence reaction rate increases.
As a general approximation, the rate roughly doubles for each 10°C rise in temperature.
-----------
Molecularity
I recently read the lesson in NCERT Book Part I for Class XII.
There is a section on Molecularity of a Reaction
The number of reacting species which much collide simultaneously in order to bring about a chemical reaction is called molecularity of a reaction.
In the case of reaction
NH4NO2 --> N2 + 2H20
Only one molecule of the reactant decomposes. It is a unimolecular reaction. Its molecularity is one.
In case of the reaction
2HI --> H2 + I2
Two molecules of HI are involved in the reaction. It is a bimolecular reaction and its molecularity is two.
In case of the reaction
2NO + O2 --> 2NO2
2 molecules of NO and one molecule of O2 are involved. Hence it is a trimolecular reaction. The probability that more than three molecules can collide and react simultaneously is very small.
Molecularity above three is not observed. Hence reactions involving many molecules take place in steps.
Hence rate determining step will be there. Law of mass action is applicable to that step.
Chemical kinetics:
Rates of chemical reactions;
Order of reactions;
Rate constant;
First order reactions;
Temperature dependence of rate constant (Arrhenius equation).
--------------------------
The topic "Chemical kinetics" consists of reaction rate and reaction mechanism.
Reaction rate is the speed with which a reaction takes place. This shows the rate or speed at which the reactants are consumed and products are formed.
Reaction mechanism is the path by which a reaction takes place.
Rate of reaction
The rate of reaction is a quantity that tells how the concentration of reactants or product changes with time.
So this can be expressed as Δ concentration/Δ time. That is change in concentration divided by time taken for the change.
Molar concentration i.e., moles per liter (M), is used in these equations.
The brackets, [ ] are always used to to indicate molar concentrations.
Rate law
The rate for a reaction is a mathematical expression that relates the rate of reaction to the concentrations of the reactants.
For the reaction aA + bB → products
The rate law is expressed as, rate of reaction is proportional to [A]^x[B]^y.
x and y are determined experimentally. These values can be whole or fractional numbers or zero.
Law of Mass Action
In 1867, Cato Guldberg, and Peter Waage, proposed this law. According to this law, for the rate determining step in a reaction, the rate of reaction is proportional to the product of the concentrations of the reactants, each raised to the power of its coefficient in the balanced equation.
For the reaction aA + bB → cC (when it is a rate determining step)
Rate of reaction is proportional to [A]^a[B]^b
The above proportionality can be written as an equation, by putting in a proportionality constant k.
Rate = k *[A]^a[B]^b
K is called the specific rate constant
Order of Reaction
From the rate law for a reaction order of reaction can be determined.
For a particular species or reactant, the order is equal to the exponent for that species in the rate law.
For example for Rate = k *[A][B]^2
for B the order of reaction is 2. For A it is 1.
The overall order of reaction is equal to the sum of all the individual orders of reactants.
Temperature
As temperature increases, the average kinetic energy increases. So there are more molecules with activation energy and hence reaction rate increases.
As a general approximation, the rate roughly doubles for each 10°C rise in temperature.
-----------
Molecularity
I recently read the lesson in NCERT Book Part I for Class XII.
There is a section on Molecularity of a Reaction
The number of reacting species which much collide simultaneously in order to bring about a chemical reaction is called molecularity of a reaction.
In the case of reaction
NH4NO2 --> N2 + 2H20
Only one molecule of the reactant decomposes. It is a unimolecular reaction. Its molecularity is one.
In case of the reaction
2HI --> H2 + I2
Two molecules of HI are involved in the reaction. It is a bimolecular reaction and its molecularity is two.
In case of the reaction
2NO + O2 --> 2NO2
2 molecules of NO and one molecule of O2 are involved. Hence it is a trimolecular reaction. The probability that more than three molecules can collide and react simultaneously is very small.
Molecularity above three is not observed. Hence reactions involving many molecules take place in steps.
Hence rate determining step will be there. Law of mass action is applicable to that step.
Senin, 25 Agustus 2008
Chemical Energetics - Basic Terms - July Dec Revision
System
a specified part of universe which is under observation is called the system.
A system is homogeneous system if physical properties nad chemical composition are identical throughout the system. It is heterogeneous if it consists of parts each of which has different physical and chemical properties.
surroundings
The remaining portion of the universe which is not part of the system is termed the surroundings.
Open system: A system which can exchange matter as well as energy with the surroundings is called an open system.
Closed system: A system which can exchange energy but not matter with the surroundings is called a closed system.
Isolated system: A system which can neither exchange matter nor energy with the surroundings is called an isolated system.
Macroscopic properties
Pressure, Volume etc. are related to the behavior of the bulk of the material. These properties are called macroscopic properties.
the macroscopic properties are divided into types.
1. Intensive properties 2. Extensive properties
1. Intensive properties: These properties have no relation to the amount of substance present in a system. Examples: temperature, pressure, viscosity, surface tension, refractive index etc.
2. Extensive properties: The value of these properties depends upon the amount of substance present in the system.
Examples: Mass, volume, surface area, energy, enthalpy, entropy, free energy, heat capacity
State Variables and State Functions
The state of a system is described by macroscopic properties when they are stable and have definite values. If any of the macroscopic properties of the system changes, the state of the system changes.
We describe a system by its state variables. A system having ideal gas can be described by three state variables. These three variables are : temperature (T), pressure (p) and volume (V). Once these three variables are specified all the other variables will be definite and can be easily calculated.
State function is a property of the system whose value depends only upon the state of the system and is independent of the path or manner by which the state is reached.
A system is said to be in thermodynamic equilibrium when the macroscopic properties do not change with time.
Processes
Isothermal: Temperature of the system is constant.
Adiabatic: No heat flows into or out of the system.
Isochoric: volume of the system remains the same.
Isobaric: Pressure of the sytem remains the same.
Reversible: The system changes in infinitesimal steps and they can be reversed.
Irrevesible: Real life systems do not satisfy the reverbility criterion and hence irreversible.
Cyclic: a process in which the system undergoes a series of changes and ultimately returns to its original state is called a cyclic process.
Modes of transfer of energy between system and surroundings
1. Heat (Q): Energy is exchanged between the system and the surroundings as heat if they are at different temperatures.
2. Another modes of transfer of energy is work. Work is said to be performed if th point of application of a force is displaced in the direction of the force.
Pressure volume work
Pressure volume work is mechanical work. It is the work done when the gas expands or contracts against external pressure.
It is equal to force multiplied by distance moved or pressured mulitiplied by change in volume.
Units of Heat and Work
S.I. unit of heat is joule or kilojoule
S.I. unit of work is also joule or kilojoule
Joule and calories are related by the relation
1 cal = 4.184 J
1 kcal = 4.184 kJ
a specified part of universe which is under observation is called the system.
A system is homogeneous system if physical properties nad chemical composition are identical throughout the system. It is heterogeneous if it consists of parts each of which has different physical and chemical properties.
surroundings
The remaining portion of the universe which is not part of the system is termed the surroundings.
Open system: A system which can exchange matter as well as energy with the surroundings is called an open system.
Closed system: A system which can exchange energy but not matter with the surroundings is called a closed system.
Isolated system: A system which can neither exchange matter nor energy with the surroundings is called an isolated system.
Macroscopic properties
Pressure, Volume etc. are related to the behavior of the bulk of the material. These properties are called macroscopic properties.
the macroscopic properties are divided into types.
1. Intensive properties 2. Extensive properties
1. Intensive properties: These properties have no relation to the amount of substance present in a system. Examples: temperature, pressure, viscosity, surface tension, refractive index etc.
2. Extensive properties: The value of these properties depends upon the amount of substance present in the system.
Examples: Mass, volume, surface area, energy, enthalpy, entropy, free energy, heat capacity
State Variables and State Functions
The state of a system is described by macroscopic properties when they are stable and have definite values. If any of the macroscopic properties of the system changes, the state of the system changes.
We describe a system by its state variables. A system having ideal gas can be described by three state variables. These three variables are : temperature (T), pressure (p) and volume (V). Once these three variables are specified all the other variables will be definite and can be easily calculated.
State function is a property of the system whose value depends only upon the state of the system and is independent of the path or manner by which the state is reached.
A system is said to be in thermodynamic equilibrium when the macroscopic properties do not change with time.
Processes
Isothermal: Temperature of the system is constant.
Adiabatic: No heat flows into or out of the system.
Isochoric: volume of the system remains the same.
Isobaric: Pressure of the sytem remains the same.
Reversible: The system changes in infinitesimal steps and they can be reversed.
Irrevesible: Real life systems do not satisfy the reverbility criterion and hence irreversible.
Cyclic: a process in which the system undergoes a series of changes and ultimately returns to its original state is called a cyclic process.
Modes of transfer of energy between system and surroundings
1. Heat (Q): Energy is exchanged between the system and the surroundings as heat if they are at different temperatures.
2. Another modes of transfer of energy is work. Work is said to be performed if th point of application of a force is displaced in the direction of the force.
Pressure volume work
Pressure volume work is mechanical work. It is the work done when the gas expands or contracts against external pressure.
It is equal to force multiplied by distance moved or pressured mulitiplied by change in volume.
Units of Heat and Work
S.I. unit of heat is joule or kilojoule
S.I. unit of work is also joule or kilojoule
Joule and calories are related by the relation
1 cal = 4.184 J
1 kcal = 4.184 kJ
Rabu, 20 Agustus 2008
Kinetic theory of gases - July-Dec 2008 revision
Kinetc theory of gases is also called kinetic molecular theory of gases.
The model takes into account molecular concept and the kinetic concept of gas molecules.
The theory was put forward by Bernoulli and was further developed and extended by Clausius, Maxwell, Boltzmann and others.
Postulates of the theory
1. All gases are made up of large number of minute particles called molecules.
2. The molecules are separated from one another by large distances.
3. The molecules are in a state of ceaseless and random motion in all directions. They keep colliding with other molecules and walls of the container and change their directions.
4. Molecular collissions are perfectly elastic (See physics for concept of elastic collisions)
5. There are no forces of interaction (attrative or repulsive) between molecules.
6. The pressure exerted by the gas is due to the collisions of the its molecules on the walls of the container per unit area.
7. Teh average kinetc engery of the gas molecules is directly proportional to the absolute temperature.
Kinetic gas equation
From the postulates of kinetic molecular theory, an equation was derived for the pressure of the gas. This equation is known as kinetic gas equation and is
pV = 1/3 mNu²
where
m = mass of a molecule
N = the number of molecules in the volume V
u = root mean square velocity of the molecules. u² is the mean square velocity of molecules. The velociytof each molecule is first squared and then its average is taken.
Averge Kinetic Energy of Molecules of a gas
The average translational kinetic energy of a molecule is
1/2 mu²
where
m = mass of a molecule
u = root mean square velocity of the molecules. u² is the mean square velocity of molecules. The velociytof each molecule is first squared and then its average is taken.
The total kinetic energy of the whole gas is
Ek = 1/2 mNu²
From the formulas for pV and Ek we can get
pV = 2/3 Ek
If we take one mole of gas and define Ek as total kinetic energy of one mole of gas
pV = RT = 2/3 Ek
=> Ek = 3/2RT for one mole of gas
For n moles of gas
Ek = 3/2 nRT.
If we want averge kinetic energy of one molecule, we divide Ek of one mole by Avogadros' number NA(6.022*1023).
averge kinetic energy of one molecule = (3/2) RT/NA = 3/2kbT
Where kb = R/NA is called Boltzmann constant.
Thus Ek α T
As Kinetic energy is proportional to u²
u² α T
u α √T
Molecular velocity of any gas is directly proportional to the square root of the absolute temperature.
This molecular motion is also referred to as thermal motion of the molecules. It will be zero when T = 0.
Past IIT JEE Questions
1. Helium atom is two times heavier than a hydrogen molecule. At 298 K, the average kinetic energy of a helium atom is
a. same as that of a hydrogen molecule
b. two times that of a hydrogen molecule
c. four times that of a hydrogen molecule
d. half that of a hydrogen molecule
(JEE 1982)
Answer: (a)
2. State whether the statement is True or False.
Kinetic energy of molecules is zero at 0°C. (JEE 1985)
Ans: False
The equation relating energy of molecules to temperature is in absolute temperature and not centigrade temperature.
3. the average velocity of an ideal gas molecule at 27°C is 0.3 m/s. the average speed at 927°C will be:
a. 0.6 m/s
b. 0.3 m/s
c. 0.9 m/s
d. 3.0 m/s
(JEE 1986)
Answer: (a)
4. Eight grams of oxygen and hydrogen at 27°C will have the total kinetic energy in the ratio of __________________.
(JEE 1989)
Answer: 1:16
Reason: For a mixture of gases, in thermal equilibrium, average kinetic energy of all molecules is same.
(½)*m1* v1² = (1/2)*(m2*v2²
Oxygen’s molecular weight is 32 and hence 8 grams will have 8/32 = ¼ moles.
Hydrogen’s molecular weight is 2 and hence 8 grams will have 8/2 = 4 mols
Total kinetic energy of oxygen moleculues = (¼ )* (½)*m1* v1²
Total kinetic energy of hydrogen molecules = 4*(1/2)*(m2*v2²
The ratio will be (1/4)/4 = 1/16 as remaining terms or equal in both cases.
The model takes into account molecular concept and the kinetic concept of gas molecules.
The theory was put forward by Bernoulli and was further developed and extended by Clausius, Maxwell, Boltzmann and others.
Postulates of the theory
1. All gases are made up of large number of minute particles called molecules.
2. The molecules are separated from one another by large distances.
3. The molecules are in a state of ceaseless and random motion in all directions. They keep colliding with other molecules and walls of the container and change their directions.
4. Molecular collissions are perfectly elastic (See physics for concept of elastic collisions)
5. There are no forces of interaction (attrative or repulsive) between molecules.
6. The pressure exerted by the gas is due to the collisions of the its molecules on the walls of the container per unit area.
7. Teh average kinetc engery of the gas molecules is directly proportional to the absolute temperature.
Kinetic gas equation
From the postulates of kinetic molecular theory, an equation was derived for the pressure of the gas. This equation is known as kinetic gas equation and is
pV = 1/3 mNu²
where
m = mass of a molecule
N = the number of molecules in the volume V
u = root mean square velocity of the molecules. u² is the mean square velocity of molecules. The velociytof each molecule is first squared and then its average is taken.
Averge Kinetic Energy of Molecules of a gas
The average translational kinetic energy of a molecule is
1/2 mu²
where
m = mass of a molecule
u = root mean square velocity of the molecules. u² is the mean square velocity of molecules. The velociytof each molecule is first squared and then its average is taken.
The total kinetic energy of the whole gas is
Ek = 1/2 mNu²
From the formulas for pV and Ek we can get
pV = 2/3 Ek
If we take one mole of gas and define Ek as total kinetic energy of one mole of gas
pV = RT = 2/3 Ek
=> Ek = 3/2RT for one mole of gas
For n moles of gas
Ek = 3/2 nRT.
If we want averge kinetic energy of one molecule, we divide Ek of one mole by Avogadros' number NA(6.022*1023).
averge kinetic energy of one molecule = (3/2) RT/NA = 3/2kbT
Where kb = R/NA is called Boltzmann constant.
Thus Ek α T
As Kinetic energy is proportional to u²
u² α T
u α √T
Molecular velocity of any gas is directly proportional to the square root of the absolute temperature.
This molecular motion is also referred to as thermal motion of the molecules. It will be zero when T = 0.
Past IIT JEE Questions
1. Helium atom is two times heavier than a hydrogen molecule. At 298 K, the average kinetic energy of a helium atom is
a. same as that of a hydrogen molecule
b. two times that of a hydrogen molecule
c. four times that of a hydrogen molecule
d. half that of a hydrogen molecule
(JEE 1982)
Answer: (a)
2. State whether the statement is True or False.
Kinetic energy of molecules is zero at 0°C. (JEE 1985)
Ans: False
The equation relating energy of molecules to temperature is in absolute temperature and not centigrade temperature.
3. the average velocity of an ideal gas molecule at 27°C is 0.3 m/s. the average speed at 927°C will be:
a. 0.6 m/s
b. 0.3 m/s
c. 0.9 m/s
d. 3.0 m/s
(JEE 1986)
Answer: (a)
4. Eight grams of oxygen and hydrogen at 27°C will have the total kinetic energy in the ratio of __________________.
(JEE 1989)
Answer: 1:16
Reason: For a mixture of gases, in thermal equilibrium, average kinetic energy of all molecules is same.
(½)*m1* v1² = (1/2)*(m2*v2²
Oxygen’s molecular weight is 32 and hence 8 grams will have 8/32 = ¼ moles.
Hydrogen’s molecular weight is 2 and hence 8 grams will have 8/2 = 4 mols
Total kinetic energy of oxygen moleculues = (¼ )* (½)*m1* v1²
Total kinetic energy of hydrogen molecules = 4*(1/2)*(m2*v2²
The ratio will be (1/4)/4 = 1/16 as remaining terms or equal in both cases.
Selasa, 12 Agustus 2008
Criterion of Spontaneity and Free Energy
July-December Revision
The flow of heat takes from a body at high temperature to a body at low temperature through conduction, convection or radiation. Similarly a liquid at higher level flows to a lower level. In both these cases the action occurs without any additional support. But if a liquid at lower level has to go higher level additional supporting activity is required.
Similarly in chemical reactions some reactions take place if the reactants are in contact. Some reactions will not take place through contact but require additional inputs like heat, catalysts etc. Reactions that take place due to contact alone are called spontaneous reactions. The rate of reaction is not the issue here. Even if the rate of reaction is very slow, if the reaction is taking place, it is a spontaneous reaction.
What determines the spontaneity of a chemical reaction?
Is decrease in enthalpy in the reaction a criterion for spontaneity?
In exothermic reactions, enthalpy of products is less than that of reactants. Thus some persons postulated that a spontaneous chemical reaction may be due to decrease in energy of the products. It sounds reasonable. But some scientists found that some endothermic reactions are also spontaneous. Therefore it is concluded that enthalpy may be a contributory factor for spontaneity, but it is not the complete explanation.
Is entropy a criterion for spontaneity?
Entropy is a thermodynamic function. It can be interpreted as measuring disorder in the system. A gas is more disordered than a liquid and a liquid is more disordered than a solid. In a chemical reaction, if the disorder in products is more than that of reactants, entropy increases. It is found in examples like diffusion of gases etc. that in spontaneous activities disorder increases.
As heat is added to the system, solids become liquids and liquids become gases. Hence heat increases entropy. Entropy is defined as
ΔS = qrev/T for a reversible reaction.
The criterion of spontaneity is defined by the total entropy change of system and surrounding. The total entropy change (ΔStotal) for the system and surroundings of a spontaneous process is given by
ΔStotal = ΔSsystem + ΔSsurrounding > 0
Gibbs energy of Free energy
Gibbs energy or Gibbs function is a thermodynamic functions defined by
G = H-TS
For a constant temperature reaction
ΔGsys = ΔHsys -T ΔSsys
Criterion for spontaneity in terms of Gibbs energy or function is that
If ΔG is negative or< 0, the reaction will be spontaneous.
This condition comes from the condition that was given above only. That is
ΔStotal = ΔSsystem + ΔSsurrounding > 0
ΔSsurr = ΔHsurr/T = -ΔHsys/T (because what system loses surrounding gains and vice versa)
Hence
ΔStotal = ΔSsystem - ΔHsys/T
=> TΔStotal = TΔSsystem - ΔHsys
As spontaneity criterion is ΔStotal > 0
RHS must be greater than 0.
=> TΔSsystem - ΔHsys > 0
=> -( ΔHsys - TΔSsystem) > 0
=> ( ΔHsys - TΔSsystem) < 0
The flow of heat takes from a body at high temperature to a body at low temperature through conduction, convection or radiation. Similarly a liquid at higher level flows to a lower level. In both these cases the action occurs without any additional support. But if a liquid at lower level has to go higher level additional supporting activity is required.
Similarly in chemical reactions some reactions take place if the reactants are in contact. Some reactions will not take place through contact but require additional inputs like heat, catalysts etc. Reactions that take place due to contact alone are called spontaneous reactions. The rate of reaction is not the issue here. Even if the rate of reaction is very slow, if the reaction is taking place, it is a spontaneous reaction.
What determines the spontaneity of a chemical reaction?
Is decrease in enthalpy in the reaction a criterion for spontaneity?
In exothermic reactions, enthalpy of products is less than that of reactants. Thus some persons postulated that a spontaneous chemical reaction may be due to decrease in energy of the products. It sounds reasonable. But some scientists found that some endothermic reactions are also spontaneous. Therefore it is concluded that enthalpy may be a contributory factor for spontaneity, but it is not the complete explanation.
Is entropy a criterion for spontaneity?
Entropy is a thermodynamic function. It can be interpreted as measuring disorder in the system. A gas is more disordered than a liquid and a liquid is more disordered than a solid. In a chemical reaction, if the disorder in products is more than that of reactants, entropy increases. It is found in examples like diffusion of gases etc. that in spontaneous activities disorder increases.
As heat is added to the system, solids become liquids and liquids become gases. Hence heat increases entropy. Entropy is defined as
ΔS = qrev/T for a reversible reaction.
The criterion of spontaneity is defined by the total entropy change of system and surrounding. The total entropy change (ΔStotal) for the system and surroundings of a spontaneous process is given by
ΔStotal = ΔSsystem + ΔSsurrounding > 0
Gibbs energy of Free energy
Gibbs energy or Gibbs function is a thermodynamic functions defined by
G = H-TS
For a constant temperature reaction
ΔGsys = ΔHsys -T ΔSsys
Criterion for spontaneity in terms of Gibbs energy or function is that
If ΔG is negative or< 0, the reaction will be spontaneous.
This condition comes from the condition that was given above only. That is
ΔStotal = ΔSsystem + ΔSsurrounding > 0
ΔSsurr = ΔHsurr/T = -ΔHsys/T (because what system loses surrounding gains and vice versa)
Hence
ΔStotal = ΔSsystem - ΔHsys/T
=> TΔStotal = TΔSsystem - ΔHsys
As spontaneity criterion is ΔStotal > 0
RHS must be greater than 0.
=> TΔSsystem - ΔHsys > 0
=> -( ΔHsys - TΔSsystem) > 0
=> ( ΔHsys - TΔSsystem) < 0
Rabu, 30 Juli 2008
Ch.1 Basic concepts of Chemistry - July-Dec 08 Revision
JEE Syllabus
General topics:
The concept of atoms and molecules;
Dalton's atomic theory;
Mole concept;
Chemical formulae;
Balanced chemical equations;
Calculations (based on mole concept) involving common oxidation-reduction, neutralisation, and displacement reactions;
Concentration in terms of mole fraction, molarity, molality and normality.
---------
Accuracy
Accuracy is a measure of the difference between the true value (the value to be measured) and the value measured by an instrument.
Precision is depends totally on the instrument and it is the difference between measurements of the same dimension made number of times. It is expressed as the difference between one measurement and the arithmetic mean of the number of measurements.
Significant figures: Significant figures in a number are include all the certain digits plus one doubtful digit.
If a number has 4 significant figures or digits, it means 3 of them are certain and fourth one is doubtful.
Rules for determining the number of significant figures
1. All non-zero digits are significant
Decimal place does not determine the number of significant figures.
2. A zero becomes significant in case it comes between two non-zero numbers.
3. The zeros at the beginning of a number are not significant.
For example 0.0004 has only one significant figure.
4. All zeros placed to right of a number are significant. They represent the precision of the measuring scale.
For example 267.000 has six significant figures.
(The precision does not come by writing the number. It comes because the instrument has the ability to read a number certainly up to that level of measurement.)
Rules of calculations involving significant figures
Rule 1. The final result of addition or subtraction should be reported up to the same number of decimal places as are present in the term having the least number of decimal places.
Example- addition of three numbers
6.414
2.3
0.501
------
9.215 is the answer but the answer should be reported up to one decimal place only as 2.3 is the term having least number of decimal places.
Hence correct answer is 9.2
Rule 2. In multiplication or division, the final result should be reported up to the same number of significant figures as are present in the term with the least number of significant figures.
Example: 4.2345*1.25 = 5.293125
The final result should be reported up to three significant figures only as 1.25 has three significant figures. Hence the correct answer to be reported is 5.29.
Rounding off figures of retention of significant figures.
i) If the digit coming after the desired number of significant figures happens to be more than 5, the preceding digit or figure is increased by 1.
ii) If that digit is less than 5, it is neglected and hence the preceding significant figure remains unchanged.
iii) If that digit happens to be 5, the preceding digit is increased by one in case it is odd number. If preceding digit is an even number, it remains the same.
If the problem has number of steps, the rounding off is to be done at the final answer level only.
Chemical classification of matter
1. Element
Further classification: Metals, non-metals, metalloids
2. Compound
Further classification: Inorganic and organic
3. Mixture
Further classification: Homogeneous mixtures - they are called solutions.
Heterogeneous mixtures: They have visible boundaries of separation between the different constituents and they can be easily seen with naked eye.
Laws of chemical combination
a. Law of conservation of mass
During any physical o chemical change, the total mass of the products is equal to the total mass of reactants.
b. Law of constant proportions
A pure chemical compound always contains same elements combined together in the same definite proportion by weight.
c. Law of multiple proportions
When two elements combine to form two or more than two compounds, the weights of one of the elements which combine with a fixed weight of the other, bear a simple whole number ratio.
d. Law of reciprocal proportions
When two different elements combine separately with the same weight of a third element, the ratio in which they do so will be the same or some simple multiple of the ratio in which they combine with each other.
e. Gay Lussac’s law of combing volumes
Under similar conditions of temperature and pressure, whenever gases react together, the volumes of the reacting gases as well as products (if gases) bear a simple whole number ratio.
---------------
Dalton’s atomic theory
To provide theoretical justification to the laws of chemical combination which are experimentally verified, John Dalton postulated a simple theory of matter. The basic postulates of Dalton’s atomic theory are:
a. Matter is made up of extremely small indivisible and indestructible ultimate particles called atoms.
b. Atoms the same element are identical in all respects ie., in shape, size, mass and chemical properties.
c. Atoms of different elements are different in all respects and have different masses and chemical properties.
d. Atom is the smallest unit that takes part in chemical combinations.
d. Atoms of two or more elements combine in a simpler whole number ratio to form compound atoms (molecules).
e. Atoms can neither be created nor destroyed during any physical or chemical change.
f. Chemical reactions involve only combinations, separation or rearrangement of atoms.
Modern atomic theory
As a result of new discoveries made after Dalton developed his postulates, some modifications were done to atomic theory. They are:
1. Atom is no longer considered to be indivisible: It is found that atom is made up of subatomic particles such as electrons, protons and neutrons. We now state how many electrons are there, protons are there in an atom.
2. Atoms of same element may not be similar in all respects. Atoms of same elements have different atomic masses. These different atoms are called isotopes.
3. Atoms of different elements may have similar one or more properties. Atomic mass of calcium and argon (40 a.m.u.) are same. So the property of atomic mass is same for atoms of different elements. Isobars or elements or atoms having the same atomic mass.
4. Atom is the smallest unit which takes part in chemical reactions. Though electrons and protons are there, it is atom which takes part in chemical reactions and electrons exchange takes place between atoms.
5. The ratio in which the different atoms combine may be fixed and integral but may not always be simple. For example in sugar molecule the ratio of C,H and O atoms is 12:22:11, which is not simple.
6. Atom of one elements may be changed into atoms of other element. Transmutation is the process by which atoms one element can be changed inot elements of other elements by subjecting it to alpha rays.
7. The mass of atom can changed into energy. Mass and energy are inconvertible. The equation give for such conversion is E mc². Hence we cannot say that mass is not destructible. But in chemical reactions, atom remains unchanged and its mass is not destroyed to liberate energy.
----------
Mole concept
A mole is measuring unit like for example dozen.
A mole is a collection of 6.022*1023 particles
Stochiometric coefficients or numbers: The numbers which appear before the chemical symbols in a chemical equation.
Chemical equation gives information about moles of various reactants and products. Hence molar masses involved in the reaction and molar masses of products.
Mass percentage of substance in a system
Mole fraction of a substance in a system
--------------
Concentration in terms of mole fraction, molarity, molality and normality.
Mole fraction of a substance in a system: It is the ratio of number of moles one component to the total number of moles (solute and solvent) present in the solution. It is denoted by x.
So mole fraction of solute = Moles of solute/(Moles of solute + Moles of solvent)
Mole fraction of solvent = Moles of solvent/(Moles of solute + Moles of solvent)
MOLARITY = Amount of a substance (in mol)/Volume of solution expressed in dm^3
It is applicable to solutions only.
Also equal to No. of moles of solute/Vol. of solution in litres or dm^3
The unit of molarity is mol dm^-3. It is commonly abbreviated by the symbol M and is spelled as molar.
MOLALITY = Amount of a a substance (in mol)/Mass of solvent expressed in kg
It is also applicable to solutions only
CONCEPT OF EQUIVALENT
__________________________
"One equivalent of a substance in a reaction is defined as the amount of substance which reacts or liberates 1 mol of electrons or 1 mol of H^+ or 1 mol of OH^- ions.
The equivalent weight of a reagent may vary according to the reaction, but
if considering just acid and base moles and equivalents, then:-
H2SO4 + 2 NaOH -> 2H2O + Na2SO4
The equivalent weight of an acid is that which contains 1.0078 grams of replaceable hydrogen which, in the case of sulfuric acid, would be half the mole weight, because 1 mol of H2SO4 is liberating 2 moles of H^+ ions
In the case of hydrochloric acid, equivalent weight would be the mole weight as 1 mol of HCl liberates 1 mol of H^+ ions.
The equivalent weight of a base is that which contains one replaceable hydroxyl group ( i.e., 17.008g of ionisable hydroxyl ). Thus the equivalent weight of sodium hydroxide ( NaOH ) and potassium hydroxide ( KOH ) would be the mole weight, but for calcium hydroxide ( Ca(OH)2 ) it would be half the mole weight.
Normality
A Normal solution contains one gram equivalent weight ( aka equivalent )
of the reagent in one litre of solution, and is represented by " N ".
Normality = (equivalents of X (solute))/volume of solution in Liters or dm³
Normality = molarity x n (where n = the number of protons exchanged in a reaction).
General topics:
The concept of atoms and molecules;
Dalton's atomic theory;
Mole concept;
Chemical formulae;
Balanced chemical equations;
Calculations (based on mole concept) involving common oxidation-reduction, neutralisation, and displacement reactions;
Concentration in terms of mole fraction, molarity, molality and normality.
---------
Accuracy
Accuracy is a measure of the difference between the true value (the value to be measured) and the value measured by an instrument.
Precision is depends totally on the instrument and it is the difference between measurements of the same dimension made number of times. It is expressed as the difference between one measurement and the arithmetic mean of the number of measurements.
Significant figures: Significant figures in a number are include all the certain digits plus one doubtful digit.
If a number has 4 significant figures or digits, it means 3 of them are certain and fourth one is doubtful.
Rules for determining the number of significant figures
1. All non-zero digits are significant
Decimal place does not determine the number of significant figures.
2. A zero becomes significant in case it comes between two non-zero numbers.
3. The zeros at the beginning of a number are not significant.
For example 0.0004 has only one significant figure.
4. All zeros placed to right of a number are significant. They represent the precision of the measuring scale.
For example 267.000 has six significant figures.
(The precision does not come by writing the number. It comes because the instrument has the ability to read a number certainly up to that level of measurement.)
Rules of calculations involving significant figures
Rule 1. The final result of addition or subtraction should be reported up to the same number of decimal places as are present in the term having the least number of decimal places.
Example- addition of three numbers
6.414
2.3
0.501
------
9.215 is the answer but the answer should be reported up to one decimal place only as 2.3 is the term having least number of decimal places.
Hence correct answer is 9.2
Rule 2. In multiplication or division, the final result should be reported up to the same number of significant figures as are present in the term with the least number of significant figures.
Example: 4.2345*1.25 = 5.293125
The final result should be reported up to three significant figures only as 1.25 has three significant figures. Hence the correct answer to be reported is 5.29.
Rounding off figures of retention of significant figures.
i) If the digit coming after the desired number of significant figures happens to be more than 5, the preceding digit or figure is increased by 1.
ii) If that digit is less than 5, it is neglected and hence the preceding significant figure remains unchanged.
iii) If that digit happens to be 5, the preceding digit is increased by one in case it is odd number. If preceding digit is an even number, it remains the same.
If the problem has number of steps, the rounding off is to be done at the final answer level only.
Chemical classification of matter
1. Element
Further classification: Metals, non-metals, metalloids
2. Compound
Further classification: Inorganic and organic
3. Mixture
Further classification: Homogeneous mixtures - they are called solutions.
Heterogeneous mixtures: They have visible boundaries of separation between the different constituents and they can be easily seen with naked eye.
Laws of chemical combination
a. Law of conservation of mass
During any physical o chemical change, the total mass of the products is equal to the total mass of reactants.
b. Law of constant proportions
A pure chemical compound always contains same elements combined together in the same definite proportion by weight.
c. Law of multiple proportions
When two elements combine to form two or more than two compounds, the weights of one of the elements which combine with a fixed weight of the other, bear a simple whole number ratio.
d. Law of reciprocal proportions
When two different elements combine separately with the same weight of a third element, the ratio in which they do so will be the same or some simple multiple of the ratio in which they combine with each other.
e. Gay Lussac’s law of combing volumes
Under similar conditions of temperature and pressure, whenever gases react together, the volumes of the reacting gases as well as products (if gases) bear a simple whole number ratio.
---------------
Dalton’s atomic theory
To provide theoretical justification to the laws of chemical combination which are experimentally verified, John Dalton postulated a simple theory of matter. The basic postulates of Dalton’s atomic theory are:
a. Matter is made up of extremely small indivisible and indestructible ultimate particles called atoms.
b. Atoms the same element are identical in all respects ie., in shape, size, mass and chemical properties.
c. Atoms of different elements are different in all respects and have different masses and chemical properties.
d. Atom is the smallest unit that takes part in chemical combinations.
d. Atoms of two or more elements combine in a simpler whole number ratio to form compound atoms (molecules).
e. Atoms can neither be created nor destroyed during any physical or chemical change.
f. Chemical reactions involve only combinations, separation or rearrangement of atoms.
Modern atomic theory
As a result of new discoveries made after Dalton developed his postulates, some modifications were done to atomic theory. They are:
1. Atom is no longer considered to be indivisible: It is found that atom is made up of subatomic particles such as electrons, protons and neutrons. We now state how many electrons are there, protons are there in an atom.
2. Atoms of same element may not be similar in all respects. Atoms of same elements have different atomic masses. These different atoms are called isotopes.
3. Atoms of different elements may have similar one or more properties. Atomic mass of calcium and argon (40 a.m.u.) are same. So the property of atomic mass is same for atoms of different elements. Isobars or elements or atoms having the same atomic mass.
4. Atom is the smallest unit which takes part in chemical reactions. Though electrons and protons are there, it is atom which takes part in chemical reactions and electrons exchange takes place between atoms.
5. The ratio in which the different atoms combine may be fixed and integral but may not always be simple. For example in sugar molecule the ratio of C,H and O atoms is 12:22:11, which is not simple.
6. Atom of one elements may be changed into atoms of other element. Transmutation is the process by which atoms one element can be changed inot elements of other elements by subjecting it to alpha rays.
7. The mass of atom can changed into energy. Mass and energy are inconvertible. The equation give for such conversion is E mc². Hence we cannot say that mass is not destructible. But in chemical reactions, atom remains unchanged and its mass is not destroyed to liberate energy.
----------
Mole concept
A mole is measuring unit like for example dozen.
A mole is a collection of 6.022*1023 particles
Stochiometric coefficients or numbers: The numbers which appear before the chemical symbols in a chemical equation.
Chemical equation gives information about moles of various reactants and products. Hence molar masses involved in the reaction and molar masses of products.
Mass percentage of substance in a system
Mole fraction of a substance in a system
--------------
Concentration in terms of mole fraction, molarity, molality and normality.
Mole fraction of a substance in a system: It is the ratio of number of moles one component to the total number of moles (solute and solvent) present in the solution. It is denoted by x.
So mole fraction of solute = Moles of solute/(Moles of solute + Moles of solvent)
Mole fraction of solvent = Moles of solvent/(Moles of solute + Moles of solvent)
MOLARITY = Amount of a substance (in mol)/Volume of solution expressed in dm^3
It is applicable to solutions only.
Also equal to No. of moles of solute/Vol. of solution in litres or dm^3
The unit of molarity is mol dm^-3. It is commonly abbreviated by the symbol M and is spelled as molar.
MOLALITY = Amount of a a substance (in mol)/Mass of solvent expressed in kg
It is also applicable to solutions only
CONCEPT OF EQUIVALENT
__________________________
"One equivalent of a substance in a reaction is defined as the amount of substance which reacts or liberates 1 mol of electrons or 1 mol of H^+ or 1 mol of OH^- ions.
The equivalent weight of a reagent may vary according to the reaction, but
if considering just acid and base moles and equivalents, then:-
H2SO4 + 2 NaOH -> 2H2O + Na2SO4
The equivalent weight of an acid is that which contains 1.0078 grams of replaceable hydrogen which, in the case of sulfuric acid, would be half the mole weight, because 1 mol of H2SO4 is liberating 2 moles of H^+ ions
In the case of hydrochloric acid, equivalent weight would be the mole weight as 1 mol of HCl liberates 1 mol of H^+ ions.
The equivalent weight of a base is that which contains one replaceable hydroxyl group ( i.e., 17.008g of ionisable hydroxyl ). Thus the equivalent weight of sodium hydroxide ( NaOH ) and potassium hydroxide ( KOH ) would be the mole weight, but for calcium hydroxide ( Ca(OH)2 ) it would be half the mole weight.
Normality
A Normal solution contains one gram equivalent weight ( aka equivalent )
of the reagent in one litre of solution, and is represented by " N ".
Normality = (equivalents of X (solute))/volume of solution in Liters or dm³
Normality = molarity x n (where n = the number of protons exchanged in a reaction).
Minggu, 27 Juli 2008
July - December 2008 Revision
I plan to go through each chapter in Chemistry, Physics and Mathematics in revision mode during July-December 2008 apart from reading or studying chapter which I have not read so far.
My thinking is that from 1st January 2009 onwards, the aspirants should focus on memorizing things and a revision during July-Dec at leisurely pace, one chapter per day would help in that. From 1 January 2009, the memorization process should take up three chapters per day.
My thinking is that from 1st January 2009 onwards, the aspirants should focus on memorizing things and a revision during July-Dec at leisurely pace, one chapter per day would help in that. From 1 January 2009, the memorization process should take up three chapters per day.
Carboxylic Acid - July-December Revision
JEE syllabus
Carboxylic acids:
Preparation, properties
Characteristic reactions
formation of esters,
acid chlorides and amides,
ester hydrolysis;
---------
I. Carboxylic acids are the compound containing carboxyl group in their molecules.
-C with a double bond with oxygen and single bond with OH
O
║
C
|
OH
II. These acids can be aliphatic or aromatic.
aliphatic acids:
Formic acid HCOOH
Acetic acid CH-3COOH
Isobutyric acid (Branched)
Aromatic acids
Bezoic acid : H in benzene substituted by COOH. It is the simplest aromatic carboxylic acid.
m-Nitrobenzoic acid: One more H substituted by NO-2
o-Toluic acid (o refers to ortho) Benzoic acid with one more H substituted by CH-3
III. Methods of Preparation of Monocarboxylic Acids:
1. From oxidation of primary alcohols
2. By oxidation of aldehydes and ketones.
3. From hydrolysis of nitriles and cyanides
the nitriles are hydrolysed in dilute acqueous acidic or alkaline medium.
4. From Grignard reagents
The reaction is carried out by bubbling CO2 through the etheral solution of suitable Grignard reagent.
5. By hydrolysis of esters
Hydrolysis of esters with mineral acids or alkalines gives carboxylic acids
6. Carboxylation of alkenes
Heating alkenes with CO and steam under pressure with phospoiric acid at 673 K. This reaction is called Koch reaction.
7. From trihalogen derivatives of hydrocarbons
Hydroysis of 1,1,1,-trihalogen derivatives of alkanes with acqueous KOH.
8. Preparation of aromatic acids from alkyl benzenes
the alkyl side chain of benzene ring can be easily oxidized to carboxylic group with alkalines KMnO4, chromic anhydride or conc. HNO3.
Carboxylic acids:
Preparation, properties
Characteristic reactions
formation of esters,
acid chlorides and amides,
ester hydrolysis;
---------
I. Carboxylic acids are the compound containing carboxyl group in their molecules.
-C with a double bond with oxygen and single bond with OH
O
║
C
|
OH
II. These acids can be aliphatic or aromatic.
aliphatic acids:
Formic acid HCOOH
Acetic acid CH-3COOH
Isobutyric acid (Branched)
Aromatic acids
Bezoic acid : H in benzene substituted by COOH. It is the simplest aromatic carboxylic acid.
m-Nitrobenzoic acid: One more H substituted by NO-2
o-Toluic acid (o refers to ortho) Benzoic acid with one more H substituted by CH-3
III. Methods of Preparation of Monocarboxylic Acids:
1. From oxidation of primary alcohols
2. By oxidation of aldehydes and ketones.
3. From hydrolysis of nitriles and cyanides
the nitriles are hydrolysed in dilute acqueous acidic or alkaline medium.
4. From Grignard reagents
The reaction is carried out by bubbling CO2 through the etheral solution of suitable Grignard reagent.
5. By hydrolysis of esters
Hydrolysis of esters with mineral acids or alkalines gives carboxylic acids
6. Carboxylation of alkenes
Heating alkenes with CO and steam under pressure with phospoiric acid at 673 K. This reaction is called Koch reaction.
7. From trihalogen derivatives of hydrocarbons
Hydroysis of 1,1,1,-trihalogen derivatives of alkanes with acqueous KOH.
8. Preparation of aromatic acids from alkyl benzenes
the alkyl side chain of benzene ring can be easily oxidized to carboxylic group with alkalines KMnO4, chromic anhydride or conc. HNO3.
Langganan:
Postingan (Atom)