Chemical kinetics (First test)

For each question choose the answer you consider to be the best.

1. Which are correct units for the rate of a reaction?

A. mol dm^-3

B. mol dm^-3 s^-1

C. mol s^-1

D. mol dm^-3 s

2. Which is a correct statement about the rate of a chemical reaction?

A. It can only be determined experimentally.

B. It can be deduced from the chemical equation for the reaction.

C. It can be predicted from the sign and value of the enthalpy change, ∆H, for the reaction.

D. It only depends on the nature of the reactants not on their concentration.

3. A student wished to follow the rate of the reaction between calcium carbonate and hydrochloric acid. Which of the following methods would not work?

A. Measuring the volume of carbon dioxide gas evolved over time.

B. Measuring the change in mass of the contents of the flask over time.

C. Measuring the change in colour of the contents of the flask over time.

D. Measuring the change in pH of the contents of the flask over time.

4. Which of the following are the advantages of using a pH probe connected to a data logger to read the pH of a reaction over time rather than mIB Docs (2) Teamally using a pH meter to take the readings?

I. The readings can be taken at precise regular intervals.

II. Regular readings can be taken at very short time intervals.

III. Regular readings can be taken at very long time intervals.

A. I and II only

B. I and III only

C. II and III only

D. I, II and III

5. 100 cm³ of 1.0 mol dm^-3 sulfuric acid is added to an excess of sodium carbonate at 298 K. Assuming the sample of sodium carbonate is kept constant and remains in excess which will not increase the initial rate of the reaction?

6. Equal masses of excess magnesium metal were added to separate solutions of hydrochloric acid. The volume of hydrogen evolved was measured at regular time intervals and the results presented graphically. Which curve best represents the results obtained for the different solutions of hydrochloric acid shown below?

7. Which explains why increasing the temperature increases the rate of a chemical reaction?

A. More reactant particles possess energy greater than or equal to the activation energy.

B. The activation energy decreases at higher temperatures.

C. Reactant particles travel faster at higher temperatures.

D. Reactant particles are more likely to be in the gaseous state at higher temperatures.

8. What is the function of solid vanadium pentoxide when it is used as a catalyst in the Contact process?

A. It moves the position of equilibrium in favour of more products.

B. It increases the initial rate of reaction.

C. It lowers the enthalpy change for the reaction.

D. It increases the yield of products formed.

9. Which is a correct statement concerning the catalysed and uncatalysed pathways for a particular reaction?

A. The enthalpy change of the catalysed reaction is equal to the enthalpy change of the uncatalysed reaction.

B. The enthalpy change of the catalysed reaction is lower than the enthalpy change of the uncatalysed reaction.

C. The enthalpy change of the uncatalysed reaction is lower than the enthalpy change of the catalysed reaction.

D. The activation energy of the uncatalysed reaction is lower than the activation energy of the catalysed reaction.

10. Catalysts provide an alternative pathway for a chemical reaction. What is true about this alternative pathway?

A. It provides a lower enthalpy change for the reaction.

B. It provides a higher activation energy for the reaction.

C. It lowers the temperature of the reaction.

D. It enables the position of equilibrium to be reached faster.

11. The rate equation for a nucleophilic substitution reaction is

rate = k[CH₃CH₂Cl][OH⁻]

What are the correct units for k?

A. mol² dm^-6 s^-1

B. mol^-2 dm⁶ s^-1

C. mol^-1 dm³ s^-1

D. mol dm^-3 s^-1

12. The rate expression for the chemical decomposition of a compound S is rate = k[S]

Which of the following shows the correct intervals for the successive half-lives of this reaction (in minutes)?

A. 0.1, 0.2, 0.4, 0.8

B. 0.2, 0.2, 0.2, 0.2

C. 0.8, 0.4, 0.2, 0.1

D. 0.1, 0.2, 0.3 , 0.4

13. Which best describes the rate-determining step in a reaction mechanism that consists of a series of separate steps?

A. The step with the highest activation energy.

B. The step that is the most exothermic.

C. The step involving the smallest number of reactant particles.

D. The step with the smallest change in entropy.

14. A proposed mechanism for a reaction is

AB₂(g) + AB₂(g) → A₂B₄(g)

A₂B₄(g) → A₂(g) + 2B₂(g)

A₂(g) + B₂(g) → 2AB(g)

Which is the correct equation for the overall reaction taking place?

A. 2AB₂(g) → 2AB(g) + B₂(g)

B. A₂B₄(g) → 2AB(g) + B₂(g)

C. A₂(g) + B₂(g) → 2AB(g)

D. 2AB₂(g) + A₂(g) →4AB(g)

15. A particular reaction mechanism involves four separate steps. The value for the rate of reaction for each step is shown in the table.

Which step is the rate-determining step?

A. 1

B. 2

C. 3

D. 4

16. The accepted mechanism for the reaction between nitrogen dioxide and fluorine to form NO₂F(g) is

NO₂(g) + F₂(g) → NO₂F(g) + F(g)

NO₂(g) + F(g) → NO₂F(g)

The first step is the rate-determining step.
What is the correct rate expression for this reaction?

A. rate = k[NO₂(g)]²[F₂(g)]

B. rate = k[NO₂(g)]²[F(g)]

C. rate = k[NO₂ (g)][F(g)]

D. rate = k[NO₂(g)][F₂(g)]

17. Three separate experiments were carried out to measure the initial rate of the reaction between A and B. The table shows the relevant data.

What is the overall order of the reaction?

A. 1

B. 2

C. 3

D. 4

18. Iodate ions and iodide ions undergo a redox reaction in acid solution.

IO₃^-(aq) + 5I^-(aq) + 6H⁺(aq) → 3I₂(aq) + 3H₂O(l)

The rate expression for this reaction is

rate = k[IO₃^- (aq)] x [I^-(aq)] x [H⁺(aq)]²

Which is a correct statement about this reaction?

A. The units for the rate constant could be mol^-2 dm⁶ s^-1.

B. Doubling the hydrogen ion concentration will have the same effect as doubling both [IO₃^-(aq)] and [I^-(aq)] at the same time.

C. The reaction is third order overall.

D. The reaction is second order with respect to iodate ions.

19. When the temperature of a reaction is increased what happens to the activation energy, E_a, and the rate constant, k, for the reaction?

A. E_a increases and k does not change.

B. E_a does not change and k increases.

C. Both E_a and k increase.

D. Neither E_a nor k change.

20. The activation energy for a reaction can be obtained by taking the gradient of a straight line graph involving the rate constant, k, and the temperature, T. Which graph should be plotted to obtain the straight line?

A. k against T

B. 1/k against ln T

C. ln k against 1/T

D. ln k against T

Answers

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