NCERT Solutions for Class 12 Chemistry Chapter 4 Chemical Kinetics in English and Hindi Medium updated for new academic session 2022-2023. Class 12 Chemistry unit 4 intext questions and exercises are explained here in simplified format.

NCERT Solutions for Class 12 Chemistry Chapter 4

Chemical Kinetics

Chemical Kinetics is a branch of Chemistry which deals with chemical reaction, its factors and mechanism. It is closely related to the chemical reaction and physical process.

Rate of a Chemical Reaction
The speed at which a chemical reaction proceeds. It is often expressed in terms of either the concentration (amount per unit volume) of a product that is formed in a unit of time or the concentration of a reactant that is consumed in a unit of time.
Consider a hypothetical reaction, assuming that the volume of the system remains constant.
R ⟶ P

One mole of the reactant R produces one mole of the product P. If [R]₁ and [P]₁ are the concentrations of R and P respectively at time t₁
and [R]₂ and [P]₂ are their concentrations at time t₂ then,
∆t = t₂ – t₁
∆[R] = [R]₂ – [R]₁
∆[P] = [P]₂ – [P]₁
The square brackets in the above expressions are used to express molar concentration.
Rate of disappearance of R = (Decrease in concentration of R) / (Time taken)
= – ∆[R] / ∆t …(1)
Rate of appearance of P = (Increase in concentration of P) / (Time taken)
= + ∆[P] / ∆t …(2)
Since, ∆[R] is a negative quantity (as concentration of reactants is decreasing), it is multiplied with –1 to make the rate of the reaction a positive quantity. Equations 1 and 2 given above represent the average rate of a reaction, rₐᵥ.

Class 12 Chemistry Chapter 4 MCQ

Q1

If the rate of a reaction is expressed by, rate = A [A]² [B], the order of reaction will be

[A]. 2
[B]. 3
[C]. 1
[D]. 0
Q2

The reaction of high molecularity are rare because

[A]. Many body collisions have a low probability.
[B]. Many body collisions are not favoured energetically.
[C]. Activation energy of many body collisions is very large
[D]. Very high concentration is required for such reactions.
Q3

Which of the following is affected by catalyst?

[A]. ∆H
[B]. ∆S
[C]. ∆G
[D]. Eₐ
Q4

A reaction in which reactants (R) are converted into products (P) follows second order kinetics. If concentration of R is increased by four times, what will be the increase in the rate of formation of P?

[A]. 9 times
[B]. 4 times
[C]. 16 times
[D]. 8 times

Factors Influencing Rate of a Reaction

Rate of reaction depends upon the experimental conditions such as concentration of reactants (pressure in case of gases), temperature and catalyst.

FactorsEffect on Reaction Rate
1. Concentration of reactantThe rate of reaction increases with increase in concentration of reactants.
2. Temperature of systemIn general, rate of reaction doubles with 10⁰ rise in temperature.
3. Nature of reactant and productsRate of reaction depends upon the nature of reactants and products.
4. Surface area of reactantsLarger is the surface area of reactants, higher is the rate of reaction.
5. Presence of catalystA catalyst increases the rate of reaction.
6. Presence of lightSome photochemical reactions become faster in the presence of light.

12th Chemistry Unit 4 Multiple Choice Questions

Q5

On increasing the temperature of the reacting system by 10° the rate of reaction almost becomes double. The most appropriate reason for this is

[A]. Activation energy decreases by increases of temperature
[B]. The fraction of molecules having threshold energy increases
[C]. Collision frequency increases
[D]. The value of threshold energy decreases.
Q6

Which among the following is a false statement?

[A]. Rate of zero order reaction is independent of initial concentration of reactant.
[B]. Half-life of a third order reaction is inversely proportional to square of initial concentration of the reactant.
[C]. Molecularity of a reaction may be zero or fraction.
[D]. For a first order reaction, t₁/₂ = 0.693 K
Q7

The dimensions of rate constant of 2nd order reaction involves

[A]. concentration
[B]. concentration and time
[C]. time only
[D]. neither time nor concentration.
Q8

Which of the following statements about the catalyst is true?

[A]. A catalyst accelerates the rate of reaction by bringing down the activation energy.
[B]. A catalyst does not participate in reaction mechanism.
[C]. A catalyst makes the reaction feasible by making ∆G more negative.
[D]. A catalyst makes equilibrium constant more favourable for forward reaction.
Order of a Reaction

the sum of powers of the concentration of the reactants in the rate law expression is called the order of that chemical reaction. Reactions can be first order reaction, second order reaction, pseudo first order reaction etc. depending on the concentration of the reactants.

Zero Order Reactions
Zero order reaction means that the rate of the reaction is proportional to zero power of the concentration of reactants.

The Half-life of a Reaction

The half-life of a reaction is the time in which the concentration of a reactant is reduced to one half of its initial concentration. It is represented as t1/2.

Class 12 Chemistry Chapter 4 Important Questions

What are elementary and complex reactions?

The reactions taking place in one step are called elementary reactions whereas when a sequence of elementary reactions, called mechanism, gives us the product, it is called complex reaction.

Define rate of a reaction.

The rate of a reaction is the change of concentration in any one of the reactants or products per unit time.

Distinguish between rate expression and rate constant of a reaction.

Rate expression is a mathematical expression which denotes the rate of a reaction in terms of molar concentration of reactants.
Rate constant is the rate of the reaction when the concentration of reactants is unity. It is proportionality constant in the rate law and is independent of initial concentrations of the reactants.

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