Chapter 3: Chemical Kinetics Assertion-Reason Questions | chemistry Class 12 CBSE

Assertion-Reason

Easy difficulty • Check the statement and reason

Easy

Question 1

Logic Check

Assertion (A): The rate of a chemical reaction decreases with time during the course of the reaction. Reason (R): As the reaction proceeds, the concentration of reactants decreases, which reduces the rate.

Reasoning Options

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Assertion-Reason

Easy difficulty • Check the statement and reason

Easy

Question 2

Logic Check

Assertion (A): The instantaneous rate of a reaction is determined by drawing a tangent to the concentration-time curve. Reason (R): The slope of the tangent at any point on the curve equals the instantaneous rate at that time.

Reasoning Options

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Assertion-Reason

Medium difficulty • Check the statement and reason

Medium

Question 3

Logic Check

Assertion (A): The rate law of a reaction can always be predicted from the balanced chemical equation. Reason (R): The order of a reaction equals the stoichiometric coefficients of the reactants in all reactions.

Reasoning Options

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Assertion-Reason

Medium difficulty • Check the statement and reason

Medium

Question 4

Logic Check

Assertion (A): Molecularity of a reaction can be zero. Reason (R): Order of a reaction can be zero for reactions in which concentration of reactant does not affect the rate.

Reasoning Options

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Assertion-Reason

Easy difficulty • Check the statement and reason

Easy

Question 5

Logic Check

Assertion (A): For a first order reaction, a plot of ln[R] against time t gives a straight line. Reason (R): The integrated rate equation for first order reactions is ln[R] = -kt + ln[R]₀, which is of the form y = mx + c.

Reasoning Options

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Assertion-Reason

Easy difficulty • Check the statement and reason

Easy

Question 6

Logic Check

Assertion (A): The half-life of a first order reaction is independent of the initial concentration of the reactant. Reason (R): For first order reactions, t½ = 0.693/k, which does not contain the initial concentration term.

Reasoning Options

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Assertion-Reason

Easy difficulty • Check the statement and reason

Easy

Question 7

Logic Check

Assertion (A): The half-life of a zero order reaction depends on the initial concentration. Reason (R): For zero order reactions, t½ = [R]₀/2k, which is directly proportional to the initial concentration.

Reasoning Options

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Assertion-Reason

Medium difficulty • Check the statement and reason

Medium

Question 8

Logic Check

Assertion (A): Decomposition of ammonia on a hot platinum surface at high pressure is a zero order reaction. Reason (R): At high pressure, all active sites on the platinum surface are occupied, so further change in ammonia concentration does not alter the rate.

Reasoning Options

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Assertion-Reason

Easy difficulty • Check the statement and reason

Easy

Question 9

Logic Check

Assertion (A): Increasing the temperature of a reaction always increases the rate constant. Reason (R): Higher temperature increases the fraction of molecules with energy equal to or greater than the activation energy.

Reasoning Options

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Assertion-Reason

Medium difficulty • Check the statement and reason

Medium

Question 10

Logic Check

Assertion (A): A catalyst does not change the equilibrium constant of a reversible reaction. Reason (R): A catalyst catalyses both the forward and backward reactions equally, so the equilibrium position remains unchanged.

Reasoning Options

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Assertion-Reason

Medium difficulty • Check the statement and reason

Medium

Question 11

Logic Check

Assertion (A): Hydrolysis of ethyl acetate in excess water is a pseudo first order reaction. Reason (R): In excess water, the concentration of water remains practically unchanged, so it does not influence the rate.

Reasoning Options

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Assertion-Reason

Hard difficulty • Check the statement and reason

Hard

Question 12

Logic Check

Assertion (A): The order of a complex reaction is given by the molecularity of the overall balanced equation. Reason (R): For complex reactions, the order is determined by the rate-determining (slowest) step, which reflects the actual mechanism.

Reasoning Options

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Assertion-Reason

Hard difficulty • Check the statement and reason

Hard

Question 13

Logic Check

Assertion (A): For a first order reaction, time required for 99% completion is twice the time required for 90% completion. Reason (R): In first order reactions, equal time intervals correspond to equal fractional decreases in concentration.

Reasoning Options

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Assertion-Reason

Medium difficulty • Check the statement and reason

Medium

Question 14

Logic Check

Assertion (A): The rate constant k increases exponentially with temperature according to the Arrhenius equation. Reason (R): k = Ae^(-Ea/RT) shows that as T increases, the exponent -Ea/RT becomes less negative, increasing e^(-Ea/RT).

Reasoning Options

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Assertion-Reason

Easy difficulty • Check the statement and reason

Easy

Question 15

Logic Check

Assertion (A): All collisions between reactant molecules lead to the formation of products. Reason (R): Only effective collisions — those with sufficient energy and proper orientation — result in product formation.

Reasoning Options

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Assertion-Reason

Medium difficulty • Check the statement and reason

Medium

Question 16

Logic Check

Assertion (A): Diamond is thermodynamically unstable and should convert to graphite. Reason (R): The rate of conversion of diamond to graphite is so extremely slow that the change is not perceptible.

Reasoning Options

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Assertion-Reason

Medium difficulty • Check the statement and reason

Medium

Question 17

Logic Check

Assertion (A): A catalyst lowers the activation energy of a reaction. Reason (R): A catalyst changes the Gibbs energy (ΔG) of the reaction, making it more thermodynamically spontaneous.

Reasoning Options

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Assertion-Reason

Hard difficulty • Check the statement and reason

Hard

Question 18

Logic Check

Assertion (A): The Arrhenius factor A is related to collision frequency in the collision theory. Reason (R): In collision theory, Rate = PZ_AB × e^(-Ea/RT), where A corresponds to PZ_AB.

Reasoning Options

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Assertion-Reason

Easy difficulty • Check the statement and reason

Easy

Question 19

Logic Check

Assertion (A): A plot of [R] vs t for a zero order reaction gives a straight line with a negative slope. Reason (R): In zero order reactions, [R] = [R]₀ - kt, which has the form y = c + mx where slope m = -k.

Reasoning Options

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Assertion-Reason

Medium difficulty • Check the statement and reason

Medium

Question 20

Logic Check

Assertion (A): The rate of ionic reactions such as precipitation of AgCl is very fast. Reason (R): Ionic reactions do not require a large activation energy since ions in solution can react directly upon encounter.

Reasoning Options

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Chapter sections

Chapter 3: Chemical Kinetics | Assertion-Reason

Assertion (A): The rate of a chemical reaction decreases with time during the course of the reaction. Reason (R): As the reaction proceeds, the concentration of reactants decreases, which reduces the rate.

Assertion (A): The instantaneous rate of a reaction is determined by drawing a tangent to the concentration-time curve. Reason (R): The slope of the tangent at any point on the curve equals the instantaneous rate at that time.

Assertion (A): The rate law of a reaction can always be predicted from the balanced chemical equation. Reason (R): The order of a reaction equals the stoichiometric coefficients of the reactants in all reactions.

Assertion (A): Molecularity of a reaction can be zero. Reason (R): Order of a reaction can be zero for reactions in which concentration of reactant does not affect the rate.

Assertion (A): For a first order reaction, a plot of ln[R] against time t gives a straight line. Reason (R): The integrated rate equation for first order reactions is ln[R] = -kt + ln[R]₀, which is of the form y = mx + c.

Assertion (A): The half-life of a first order reaction is independent of the initial concentration of the reactant. Reason (R): For first order reactions, t½ = 0.693/k, which does not contain the initial concentration term.

Assertion (A): The half-life of a zero order reaction depends on the initial concentration. Reason (R): For zero order reactions, t½ = [R]₀/2k, which is directly proportional to the initial concentration.

Assertion (A): Decomposition of ammonia on a hot platinum surface at high pressure is a zero order reaction. Reason (R): At high pressure, all active sites on the platinum surface are occupied, so further change in ammonia concentration does not alter the rate.

Assertion (A): Increasing the temperature of a reaction always increases the rate constant. Reason (R): Higher temperature increases the fraction of molecules with energy equal to or greater than the activation energy.

Assertion (A): A catalyst does not change the equilibrium constant of a reversible reaction. Reason (R): A catalyst catalyses both the forward and backward reactions equally, so the equilibrium position remains unchanged.

Assertion (A): Hydrolysis of ethyl acetate in excess water is a pseudo first order reaction. Reason (R): In excess water, the concentration of water remains practically unchanged, so it does not influence the rate.

Assertion (A): The order of a complex reaction is given by the molecularity of the overall balanced equation. Reason (R): For complex reactions, the order is determined by the rate-determining (slowest) step, which reflects the actual mechanism.

Assertion (A): For a first order reaction, time required for 99% completion is twice the time required for 90% completion. Reason (R): In first order reactions, equal time intervals correspond to equal fractional decreases in concentration.

Assertion (A): The rate constant k increases exponentially with temperature according to the Arrhenius equation. Reason (R): k = Ae^(-Ea/RT) shows that as T increases, the exponent -Ea/RT becomes less negative, increasing e^(-Ea/RT).

Assertion (A): All collisions between reactant molecules lead to the formation of products. Reason (R): Only effective collisions — those with sufficient energy and proper orientation — result in product formation.

Assertion (A): Diamond is thermodynamically unstable and should convert to graphite. Reason (R): The rate of conversion of diamond to graphite is so extremely slow that the change is not perceptible.

Assertion (A): A catalyst lowers the activation energy of a reaction. Reason (R): A catalyst changes the Gibbs energy (ΔG) of the reaction, making it more thermodynamically spontaneous.

Assertion (A): The Arrhenius factor A is related to collision frequency in the collision theory. Reason (R): In collision theory, Rate = PZ_AB × e^(-Ea/RT), where A corresponds to PZ_AB.

Assertion (A): A plot of [R] vs t for a zero order reaction gives a straight line with a negative slope. Reason (R): In zero order reactions, [R] = [R]₀ - kt, which has the form y = c + mx where slope m = -k.

Assertion (A): The rate of ionic reactions such as precipitation of AgCl is very fast. Reason (R): Ionic reactions do not require a large activation energy since ions in solution can react directly upon encounter.

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