60 free MCQs on Chemical Kinetics with worked answers and explanations. Study the speed of reactions and what affects it: concentration, temperature, and catalysts. Covers rate law, order of reaction, the Arrhenius equation, and reaction mechanisms step by step.
Take the timed Chemical Kinetics chapterwise test →Below are 60 practice questions on Chemical Kinetics, sorted Easy → Hard. Tap “Show answer & explanation” under any question to check yourself. Want the full theory first? Read the Chemical Kinetics notes.
First-order decay curve: concentration halves every t½, and that half-life is constant - a defining test for first-order kinetics.
Q1.
Reaction rate is measured as:
Answer: B. Change in concentration over time
Why: Rate = change in concentration of reactant or product per unit time.
Q2.
Increasing temperature generally:
Answer: C. Increases reaction rate
Why: Higher temperature increases kinetic energy, so more molecules exceed the activation energy barrier.
Q3.
A catalyst increases reaction rate by:
Answer: B. Lowering activation energy
Why: A catalyst provides an alternative pathway with a lower activation energy barrier.
Q4.
The order of a reaction is:
Answer: B. Determined experimentally
Why: Order is determined from experimental rate data, not from the stoichiometric coefficients.
Q5.
For a first-order reaction, the unit of rate constant k is:
Answer: C. s⁻¹
Why: For first-order reactions, k has units of s⁻¹ (reciprocal time).
Q6.
The minimum energy required for a reaction to occur is called:
Answer: B. Activation energy
Why: Activation energy (Ea) is the minimum energy needed to reach the transition state.
Q7.
The half-life of a first-order reaction:
Answer: B. Is independent of initial concentration
Why: For first-order reactions, t½ = 0.693/k, which is constant and independent of concentration.
Q8.
For a zero-order reaction, the rate is:
Answer: C. Independent of concentration
Why: For zero-order reactions, rate = k; the rate is constant regardless of reactant concentration.
Q9.
The rate law for a reaction must be determined by:
Answer: C. Experimental data
Why: Rate laws must be determined experimentally; they cannot be predicted from stoichiometry.
Q10.
Which factor does NOT change the equilibrium constant K of a reaction?
Answer: D. Adding a catalyst
Why: A catalyst does not change K since it speeds up both forward and reverse reactions equally.
Q11.
The branch of chemistry that deals with the rates of reactions is:
Answer: B. chemical kinetics
Why: Chemical kinetics studies how fast reactions occur and the factors affecting them.
Q12.
The rate of a reaction is the change in concentration of a reactant or product per unit:
Answer: B. time
Why: Rate = change in concentration ÷ time, with units such as mol L⁻¹ s⁻¹.
Q13.
The minimum energy that colliding molecules must possess in order to react is the:
Answer: B. activation energy
Why: Only molecules with energy ≥ the activation energy can cross the barrier and react.
Q14.
Raising the temperature generally causes the rate of a reaction to:
Answer: B. increase
Why: Higher temperature gives more molecules the activation energy and increases collision frequency, so the rate rises.
Q15.
A substance that increases a reaction rate while itself remaining unchanged at the end is a:
Answer: B. catalyst
Why: A catalyst provides a lower-energy pathway and is regenerated, so it is not consumed overall.
Q16.
The usual units for the rate of a reaction are:
Answer: B. mol L⁻¹ s⁻¹
Why: Rate is a change of concentration with time, so its units are mol L⁻¹ s⁻¹.
Q17.
For a first-order reaction, the half-life is ___ the initial concentration:
Answer: B. independent of
Why: The half-life of a first-order reaction, t½ = 0.693/k, does not depend on the starting concentration.
Q18.
The sum of the powers of the concentration terms in the experimental rate law is the:
Answer: B. order of reaction
Why: The overall order is the sum of the exponents of the reactant concentrations in the rate law.
Q19.
Which of these does NOT normally affect the rate of a reaction?
Answer: D. colour of the flask
Why: Temperature, catalyst and concentration affect rate; the colour of the container does not.
Q20.
A reaction whose rate is independent of the concentration of the reactant is of:
Answer: B. zero order
Why: A zero-order reaction proceeds at a constant rate regardless of reactant concentration.
Q21.
The Arrhenius equation k = Ae<sup>-Ea/RT</sup> shows that k:
Answer: D. Both b and c are correct
Why: k increases exponentially with temperature and decreases as activation energy Ea increases.
Q22.
For rate = k[A][B], if [A] doubles and [B] is halved, the rate:
Answer: C. Stays the same
Why: Rate = k × (2[A]) × (½[B]) = k[A][B] = unchanged. The two changes cancel each other.
Q23.
The half-life of a first-order reaction with k = 0.1 min⁻¹ is:
Answer: A. 6.93 min
Why: t½ = 0.693/k = 0.693/0.1 = 6.93 min. For first-order reactions, t½ is constant.
Q24.
For a second-order reaction, the unit of rate constant k is:
Answer: C. L mol⁻¹ s⁻¹
Why: For order n, units of k are L<sup>n-1</sup> mol<sup>1-n</sup> s⁻¹. For n = 2: L mol⁻¹ s⁻¹.
Q25.
The molecularity of the elementary reaction 2NO + O₂ → 2NO₂ is:
Answer: C. 3
Why: Molecularity = total reactant molecules in the elementary step = 2 (NO) + 1 (O₂) = 3.
Q26.
A catalyst changes all of these EXCEPT:
Answer: D. Equilibrium constant K
Why: A catalyst lowers Ea and increases k and rate, but cannot change the equilibrium constant K.
Q27.
In a pseudo-first-order reaction, one reactant is present in:
Answer: A. Large excess
Why: When one reactant is in huge excess, its concentration barely changes, making the reaction appear first-order.
Q28.
Activation energy can be calculated from the slope of the graph of:
Answer: B. log k vs 1/T
Why: From Arrhenius: log k = log A - Ea/(2.303RT). Slope of log k vs 1/T equals -Ea/(2.303R).
Q29.
A reaction is found to be of zero order with respect to a reactant. What does this indicate about the rate of the reaction?
Answer: A. The rate stays constant and does not depend on the concentration of that reactant
Why: For a zero-order reaction with respect to a reactant, the rate is independent of that reactant's concentration and remains constant over time.
Q30.
For a reaction with rate = k[A]<sup>2</sup>[B], what happens to the rate if the concentration of both A and B are doubled?
Answer: D. The rate increases by a factor of 8
Why: Rate is proportional to [A]<sup>2</sup>[B]; doubling both gives (2)<sup>2</sup> x (2) = 8 times the original rate.
Q31.
The rate constant of a zero-order reaction has units of:
Answer: B. mol L⁻¹ s⁻¹
Why: For a zero-order reaction rate = k, so k carries the units of rate: mol L⁻¹ s⁻¹.
Q32.
The rate constant of a first-order reaction has units of:
Answer: A. s⁻¹
Why: For first order, rate = k[A], so k has units of s⁻¹ (independent of concentration).
Q33.
For a first-order reaction, the half-life t½ in terms of the rate constant k is:
Answer: A. 0.693/k
Why: t½ = 0.693/k for a first-order reaction, independent of the starting concentration.
Q34.
The number of reacting species taking part in an elementary reaction step is its:
Answer: B. molecularity
Why: Molecularity is the number of species colliding in an elementary reaction; it is always a whole number.
Q35.
According to the Arrhenius equation k = A e<sup>−Ea/RT</sup>, increasing the activation energy Ea:
Answer: B. decreases k
Why: A larger Ea makes the exponential term smaller, so the rate constant k decreases.
Q36.
A plot of ln k against 1/T (the Arrhenius plot) is a straight line with slope:
Answer: A. −Ea/R
Why: Taking logs of the Arrhenius equation gives ln k = ln A − Ea/RT, so the slope is −Ea/R.
Q37.
As a typical reaction proceeds with time, its rate generally:
Answer: B. decreases
Why: As reactants are used up their concentrations fall, so the rate decreases with time.
Q38.
For the first-order reaction 2N₂O₅ → 4NO₂ + O₂, doubling the concentration of N₂O₅ changes the rate by a factor of:
Answer: B. 2
Why: For a first-order reaction, rate ∝ [N₂O₅], so doubling the concentration doubles the rate.
Q39.
The temperature coefficient of a reaction is the factor by which its rate increases for every rise of:
Answer: B. 10 °C
Why: The temperature coefficient is the ratio of rate constants for a 10 °C (10 K) rise, usually about 2–3.
Q40.
A catalyst increases the reaction rate by providing a pathway of:
Answer: B. lower activation energy
Why: A catalyst offers an alternative route with a lower activation energy, so more molecules can react.
Q41.
A first-order reaction has k = 0.231 min⁻¹. The time for 90% completion is:
Answer: A. 10 min
Why: t = (2.303/k) × log([A]₀/[A]) = (2.303/0.231) × log(100/10) = 9.97 × 1 ≈ 10 min.
Q42.
The integrated rate law for a second-order reaction (rate = k[A]²) is:
Answer: B. 1/[A] = 1/[A]₀ + kt
Why: For second-order: 1/[A] = 1/[A]₀ + kt. A plot of 1/[A] vs time gives a straight line with slope k.
Q43.
Concentration of A falls from 0.4 M to 0.2 M in 20 min, and from 0.2 M to 0.1 M in another 20 min. The order is:
Answer: B. First
Why: The half-life is constant (20 min) regardless of concentration. This is a hallmark of first-order kinetics.
Q44.
A catalyst lowers activation energy without changing:
Answer: C. Enthalpy change ΔH
Why: A catalyst affects Ea, k, and rate. Thermodynamic quantities like ΔH are unchanged since start and end states are the same.
Q45.
The pre-exponential factor A in the Arrhenius equation represents:
Answer: B. Collision frequency and steric factor
Why: A = collision frequency × orientation probability (steric factor). It accounts for how often molecules collide with the right orientation.
Q46.
For a zero-order reaction A → B with [A]₀ = 0.5 M and k = 0.05 M/s, total reaction time is:
Answer: A. 10 s
Why: For zero-order: t = [A]₀/k = 0.5/0.05 = 10 s. The reaction goes to completion at this time.
Q47.
For a reaction with k₁ = 0.005 s⁻¹ at 300 K and k₂ = 0.020 s⁻¹ at 320 K, the activation energy is approximately:
Answer: B. 55 kJ/mol
Why: Ea = 2.303R × (T₁T₂/(T₂-T₁)) × log(k₂/k₁) = 2.303×8.314×(300×320/20)×log(4) ≈ 55,300 J/mol ≈ 55 kJ/mol.
Q48.
For a reaction, a graph of log(rate) vs log[A] gives a straight line with slope 2. The order with respect to A is:
Answer: C. 2
Why: log(rate) = log k + n × log[A]. The slope of log(rate) vs log[A] equals the order n. Slope 2 means second-order.
Q49.
A reaction has a rate constant of 4 x 10<sup>-3</sup> s<sup>-1</sup> at 300 K and 1.6 x 10<sup>-2</sup> s<sup>-1</sup> at 340 K. Using the Arrhenius equation, the activation energy is closest to which value (R = 8.314 J/K/mol)?
Answer: B. 29.4 kJ/mol
Why: Using log(k<sub>2</sub>/k<sub>1</sub>) = Ea/(2.303R) x (1/T<sub>1</sub> - 1/T<sub>2</sub>), with k<sub>2</sub>/k<sub>1</sub> = 4, T<sub>1</sub> = 300 K, T<sub>2</sub> = 340 K, solving gives Ea approximately 29.4 kJ/mol.
Q50.
For a first-order reaction, a graph of ln[A] versus time gives a straight line. If the line has a slope of -0.0231 min<sup>-1</sup>, what is the half-life of the reaction?
Answer: C. 30 min
Why: Slope = -k, so k = 0.0231 min<sup>-1</sup>; t(1/2) = 0.693/k = 0.693/0.0231 = 30 min.
Q51.
For a first-order reaction, the time needed for 75% completion is ___ the half-life:
Answer: B. twice
Why: 75% completion leaves 25% (one quarter), which corresponds to two half-lives.
Q52.
The half-life of a zero-order reaction is given by:
Answer: A. [A₀]/2k
Why: For a zero-order reaction t½ = [A₀]/2k, so it is directly proportional to the initial concentration.
Q53.
The rate of a reaction doubles when the temperature rises from 300 K to 310 K. Its temperature coefficient is about:
Answer: B. 2
Why: A doubling of rate for a 10 K rise corresponds to a temperature coefficient of roughly 2.
Q54.
For a reaction with the rate law rate = k[A][B]², the overall order is:
Answer: C. 3
Why: Overall order = 1 + 2 = 3.
Q55.
In the Arrhenius equation k = A e<sup>−Ea/RT</sup>, the term A is called the:
Answer: B. pre-exponential factor
Why: A is the pre-exponential (frequency) factor, related to the frequency and orientation of collisions.
Q56.
A first-order reaction has a rate constant k = 0.0693 min⁻¹. Its half-life is:
Answer: B. 10 min
Why: t½ = 0.693/k = 0.693/0.0693 = 10 minutes.
Q57.
The molecularity of a reaction can never be:
Answer: D. zero or a fraction
Why: Molecularity is a count of colliding species, so it must be a positive whole number - never zero or a fraction.
Q58.
For a reaction A → products, doubling [A] quadruples the rate. The order with respect to A is:
Answer: B. 2
Why: rate ∝ [A]ⁿ; if doubling gives a fourfold rise then 2ⁿ = 4, so n = 2.
Q59.
The activation energy of a catalysed reaction, compared with the uncatalysed reaction, is:
Answer: B. lower
Why: A catalyst works by lowering the activation energy of the reaction.
Q60.
The integrated rate equation [A] = [A₀] − kt describes a reaction of:
Answer: A. zero order
Why: A linear fall of concentration with time is characteristic of a zero-order reaction.