Chemical Kinetics — Class 12 MCQs with Answers

Chemical Kinetics — important questions & MCQs with answers (Class 12 Chemistry)

6 solved questions from this chapter's difficulty levels, each with its answer and explanation. Tap an option to check yourself.

0 of 6 answered

1. Q1 of 6Easy

   Rate of reaction is:

   AChange in concentration per unit timeBTotal massCVolume changeDTemperature change
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   Answer: Change in concentration per unit time

   Explanation: Rate is −d[R]/dt for a reactant or +d[P]/dt for a product: concentration change per unit time. Total mass and volume don't change during reaction, and temperature is not what rate measures.

2. Q2 of 6Easy

   Units of rate constant for first-order:

   AunitlessBM·s⁻¹CM⁻¹·s⁻¹Ds⁻¹
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   Answer: s⁻¹

   Explanation: Since rate = k[A]¹, units of k = (mol·L⁻¹·s⁻¹)/(mol·L⁻¹) = s⁻¹ — concentration cancels out, leaving inverse time, unlike second-order k which keeps L·mol⁻¹·s⁻¹.

3. Q3 of 6Medium

   Arrhenius equation: k =

   AA/EaBA·e^(Ea/RT)CA·e^(−Ea/RT)DEa/RT
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   Answer: A·e^(−Ea/RT)

   Explanation: k = A·e⁻ᴱᵃ/ᴿᵀ has a negative exponent because higher Ea suppresses k — a positive exponent would make k grow with activation energy, backwards from collision theory.

4. Q4 of 6Medium

   Assertion (A): For a zero order reaction, the half-life is directly proportional to the initial concentration of the reactant. Reason (R): The thermal decomposition of HI on a gold surface is a zero order reaction.

   ABoth A and R are true and R is the correct explanation of ABBoth A and R are true but R is not the correct explanation of ACA is true but R is falseDA is false but R is true
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   Answer: Both A and R are true but R is not the correct explanation of A

   Explanation: Both are true. A follows from the zero order integrated rate law: putting [R] = [R]₀/2 in k = ([R]₀ − [R])/t gives t½ = [R]₀/2k. R only names an example of a zero order reaction; it does not explain why t½ depends on [R]₀.

5. Q5 of 6Hard

   Catalyst:

   AIncreases ΔGBLowers activation energy without changing equilibrium positionCAdds mass to productsDShifts equilibrium
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   Answer: Lowers activation energy without changing equilibrium position

   Explanation: A catalyst opens a lower-Ea pathway for both forward and reverse reactions equally, so it speeds up both directions and leaves the equilibrium position and ΔG unchanged.

6. Q6 of 6Hard

   Assertion (A): For a first order reaction, the time needed for 99% completion is ten times its half-life. Reason (R): For a first order reaction, the time needed for 99% completion is twice the time needed for 90% completion.

   ABoth A and R are true and R is the correct explanation of ABBoth A and R are true but R is not the correct explanation of ACA is true but R is falseDA is false but R is true
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   Answer: A is false but R is true

   Explanation: A is false: t(99%) = (2.303/k) log 100 = 4.606/k and t½ = 0.693/k, so t(99%) is about 6.6 half-lives; it is 99.9% completion (log 1000 = 3, t = 6.909/k) that takes ten half-lives. R is true: t(90%) = (2.303/k) log 10 = 2.303/k, exactly half of 4.606/k.

Chemical Kinetics notes: key concepts (Class 12 Chemistry)

Chemical Kinetics studies reaction rates and their control: rate laws and order (found experimentally, not from stoichiometry), molecularity, integrated rate equations for zero and first order, half-life, the Arrhenius temperature dependence with activation energy, collision theory, and how a catalyst speeds a reaction by lowering Ea without shifting equilibrium.

Rate of reaction

Change in concentration of a reactant or product per unit time: −d[R]/dt or +d[P]/dt. Average rate is over an interval; instantaneous rate is at one instant.

Rate law and rate constant

Rate = k[A]^x[B]^y; k is the rate constant, independent of concentration but dependent on temperature and catalyst.

Order of reaction

Sum of the powers of concentration in the rate law (x + y). Determined EXPERIMENTALLY, can be zero, fractional or negative — never read from the equation.

Molecularity vs order

Molecularity is the number of species colliding in an elementary step (a positive whole number ≤ 3); order is experimental and can be zero or fractional.

Zero-order reaction

Rate is independent of concentration: [A] = [A]₀ − kt. Units of k are mol L⁻¹ s⁻¹, e.g. some surface-catalysed and photochemical reactions.

First-order reaction

Rate ∝ [A]; k = (2.303/t) log([A]₀/[A]). Units of k are s⁻¹, and a plot of log[A] versus t is a straight line.

Half-life

Time for half the reactant to react. For first order t½ = 0.693/k, INDEPENDENT of initial concentration; for zero order t½ = [A]₀/2k.

Arrhenius equation

k = A·e^(−Ea/RT); a higher activation energy Ea or lower temperature gives a smaller k. A is the frequency (pre-exponential) factor.

Activation energy

Minimum extra energy over the average that colliding molecules need to react; it is the barrier between reactants and products via the activated complex.

Effect of temperature

A 10 K rise roughly doubles the rate because the fraction of molecules with energy ≥ Ea increases sharply (temperature coefficient ≈ 2–3).

Collision theory

Reaction needs collisions with sufficient energy (≥ Ea) AND correct orientation; the steric factor P accounts for the orientation requirement.

Catalyst

Provides an alternative path of lower activation energy, speeding forward and reverse reactions equally, so it raises rate but does NOT shift the equilibrium position.