39 exam-style questions on Chemistry · Reaction kinetics, each with the answer and a short explanation. Ten of them are below to try. The chapter test draws ten fresh questions each time and is marked the moment you finish, with your score by topic.
Ten Chemistry · Reaction kinetics questions with answers
Try each one, then open the answer. These ten stay the same; the test draws new ones every time.
1. The rate of a reaction in solution is usually expressed in
A
mol dm−3
B
s−1
C
dm3 mol−1 s−1
D
mol dm−3 s−1
Show answer
Answer: D. Rate = change in concentration ÷ time taken, so its unit is mol dm−3 s−1. mol dm−3 is a concentration, and s−1 is the unit of a first-order rate constant.
2. In a reaction, the activated complex is
A
a stable intermediate that can be isolated and stored
B
the catalyst attached to the reactant molecules
C
the product formed in the slowest step of all
D
an unstable species at the peak of the energy profile
Show answer
Answer: D. The activated complex is the short-lived, high-energy arrangement of atoms at the top of the energy barrier; it can break up to give the products or go back to the reactants. It cannot be isolated.
3. Which of these gas-phase reactions is third order overall?
A
2NO + O2 → 2NO2
B
NO + O3 → NO2 + O2
C
2N2O5 → 4NO2 + O2
D
H2 + I2 → 2HI
Show answer
Answer: A. For 2NO + O2, rate = k[NO]2[O2], an order of 2 + 1 = 3. NO with ozone and H2 with I2 are second order (first order in each reactant), and N2O5 decomposition is first order despite its coefficient of 2.
4. For 2N2O5(g) → 4NO2(g) + O2(g), oxygen is formed at a rate of x mol dm−3 s−1. At what rate is N2O5 used up?
A
x/2
B
x
C
2x
D
4x
Show answer
Answer: C. Two moles of N2O5 are used for every mole of O2 formed, so N2O5 disappears at 2x. 4x is the rate at which NO2 forms.
5. In a gas mixture, many collisions between reactant molecules have more energy than the activation energy, yet they still do not give products. The most likely reason is that these molecules
A
were not suitably oriented when they collided
B
were moving too fast to stay together and react
C
had very different masses from each other
D
collided too often to have time to react
Show answer
Answer: A. An effective collision needs both energy at least equal to Ea and an orientation in which the reacting atoms meet; with the wrong orientation even an energetic collision just bounces apart. This is why only a small fraction of all collisions lead to reaction.
6. For an endothermic reaction, how does the activation energy of the forward reaction compare with that of the reverse reaction?
A
the forward one is larger, by ΔH
B
the reverse one is larger, by ΔH
C
the two are always equal
D
the forward one is twice as large
Show answer
Answer: A. In an endothermic reaction the products lie higher than the reactants, so the climb to the peak is longer from the reactant side: Ea(forward) = Ea(reverse) + ΔH.
7. A second-order reaction of a single reactant A has a half-life of 40 min when [A]0 = 0.20 mol dm−3. What is its half-life at the same temperature when [A]0 = 0.80 mol dm−3?
A
160 min
B
40 min
C
10 min
D
2.5 min
Show answer
Answer: C. For order n, t½ ∝ 1/[A]0n − 1; for second order t½ ∝ 1/[A]0, so four times the starting concentration gives 40 ÷ 4 = 10 min. 40 min would be first order (independent of [A]0) and 160 min zero order (t½ ∝ [A]0).
8. For the decomposition of A, [A] was 0.64 mol dm−3 at the start, 0.32 after 15 min, 0.16 after 30 min and 0.08 after 45 min. What is the order of the reaction?
A
first
B
zero
C
second
D
third
Show answer
Answer: A. [A] halves every 15 min whatever its value, so the half-life is constant, which is the mark of a first-order reaction. A zero-order reaction would lose equal amounts in equal times, and for a second-order reaction each half-life would be twice as long as the one before.
9. When acidified KMnO4 solution is run into warm oxalic acid, the first few drops are decolourised slowly, but later drops are decolourised almost at once. This is because
A
K+ ions released from KMnO4 catalyse the reaction
B
Mn2+ ions produced by the reaction catalyse it
C
excess MnO4− ions catalyse their own reduction
D
the CO2 produced acts as a catalyst for it
Show answer
Answer: B. Mn2+, a product of the reaction, catalyses the oxidation of oxalic acid, so the reaction speeds up as it proceeds: this is autocatalysis. MnO4− is a reactant being used up, and K+ is a spectator ion.
10. A first-order reaction is 75% complete in 30 minutes. What is its half-life?
A
7.5 min
B
22.5 min
C
40 min
D
15 min
Show answer
Answer: D. 75% complete means 25% (one quarter) remains, which takes two half-lives (100% → 50% → 25%). So 2t½ = 30 min and t½ = 15 min.
Chemistry · Reaction kinetics: questions students ask
How many Chemistry · Reaction kinetics MCQs are there?
39, each with the answer and a short explanation, across 2 topics: Rate and rate law, Activation energy and catalysis. Every chapter test draws ten of them, so you can take it many times.
Is the MDCAT Chemistry chapter 7 test free?
Yes. The test, the answers and the explanations are free and need no sign-up. Sign in only if you want your scores kept across devices.
How is the chapter test marked?
The moment you press Finish: your score, the right answer and an explanation for every question, and the ones you got wrong saved in My mistakes to retry.
