CBSE Class 12 Chemistry Chemical Kinetics Board Exam Notes

KEY CONCEPTS

This is differential rate equation.
Order of reaction:‐ For the reaction a A + b B → c C + d D Rate = K[A]x [B]y
x + y = order of reaction (where x & y may or may not be equal to stoichiometric coefficient)

Note:‐
i. if x + y = 1, reaction is called I order reaction.
ii. if x + y = 2, reaction is called II order reaction.
iii. if x + y = 3, reaction is called III order reaction.
iv. if x + y = 0, reaction is called zero order reaction.
v. if x + y = fraction , reaction is called fractional order reaction.

5. Units of rate constants and graph between rate and conc. of reactant

6. Integrated rate equation for zero order and first order reaction
Note:‐ 1. For zero order reaction t1/2 α conc. of reactant.
2. For I order reaction t1/2 is independent of conc of reactant.
3. t α [conc]1-n whe n = order of reaction.

IMPORTANT DEFINITIONS
1 Pseudo first order reaction:‐ A bimolecular reaction, in which one reactant is present in large excess and rate of reaction is independent of its concentration. Such a reaction is called pseudo first molecular reaction.
Eg. CH3COOC2H5 + H2O →CH3COOH + C2H5OH molecularity of reaction is two and order of reaction is one.

2. Molecularity of reaction:‐The number of reacting species (atoms, ions or molecules) taking part in an elementary reaction, which must collide simultaneously in order to bring about a chemical reaction is called molecularity of a reaction.

Types of reaction based on molecularity

This is differential rate equation.
Order of reaction:‐ For the reaction a A + b B → c C + d D Rate = K[A]x [B]y
x + y = order of reaction (where x & y may or may not be equal to stoichiometric coefficient)

Note:‐
i. if x + y = 1, reaction is called I order reaction.
ii. if x + y = 2, reaction is called II order reaction.
iii. if x + y = 3, reaction is called III order reaction.
iv. if x + y = 0, reaction is called zero order reaction.
v. if x + y = fraction , reaction is called fractional order reaction.

5. Units of rate constants and graph between rate and conc. of reactant

6. Integrated rate equation for zero order and first order reaction
Note:‐ 1. For zero order reaction t1/2 α conc. of reactant.
2. For I order reaction t1/2 is independent of conc of reactant.
3. t α [conc]^1-n whe n = order of reaction.

IMPORTANT DEFINITIONS
1 Pseudo first order reaction:‐ A bimolecular reaction, in which one reactant is present in large excess and rate of reaction is independent of its concentration. Such a reaction is called pseudo first molecular reaction.
Eg. CH3COOC2H5 + H2O →CH3COOH + C2H5OH molecularity of reaction is two and order of reaction is one.

2. Molecularity of reaction:‐The number of reacting species (atoms, ions or molecules) taking part in an elementary reaction, which must collide simultaneously in order to bring about a chemical reaction is called molecularity of a reaction.

Types of reaction based on molecularity

NOTE:‐
i. It is theoretical value.
ii. It cannot be zero or a non integer.
iii. molecularity greater than three is not observed.
iv. molecularity is applicable only for elementary reactions. For complex reaction molecularity has no meaning.

3. Order of reaction:‐ the power to which conc. term of a reactant is raised in rate law expression is called order w.r.t. that reactant. The sum of powers of conc. of all reactants in rate law expression is called o v e r a l l order of reaction.
for the reaction a A + b B → c C + d D
                               Rate = K[A]x [B]y
                              x + y = order of reaction.

NOTE:‐
i. Order of a reaction is an experimental quantity
ii. It can be zero and even a fraction
iii. Order is applicable to elementary as well as complex reactions

4. Elementary step:‐ Many reactions complete in a number of steps. Each individual step of a reaction is called elementary step.

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