Free Reaction Kinetics MCQs with Answers

11. On an energy profile diagram for an exothermic reaction

• A. the products lie at a lower energy level than the reactants
• B. the products lie above the reactants
• C. there is no activation energy barrier
• D. the activation energy is always zero

Explanation: An exothermic reaction releases energy to the surroundings, so the products sit lower and the enthalpy change is negative, but the curve still rises to a peak first because the activation barrier must be crossed. Confusing the barrier height with the overall enthalpy change is the classic error when reading these diagrams. An endothermic profile has the products higher than the reactants.

12. The units of the rate constant k for a first order reaction are

• A. mol dm-3 s-1
• B. s-1
• C. mol-1 dm3 s-1
• D. mol-2 dm6 s-1

Explanation: For a first order reaction rate equals k times concentration, so k must have units of rate divided by concentration, which reduces to reciprocal seconds. The units of k therefore change with the order, being mol dm-3 s-1 for zero order and mol-1 dm3 s-1 for second order. Checking the units is a quick way to confirm that an order has been deduced correctly.

13. The value of the rate constant k for a given reaction changes when

• A. the concentration of the reactants is increased
• B. the volume of the vessel is changed
• C. the temperature is changed
• D. more product is removed

Explanation: The rate constant is constant only at a fixed temperature; the Arrhenius equation shows it rising sharply as temperature increases or as activation energy falls, which is also why a catalyst increases k. Changing concentration changes the rate but leaves k untouched, since the concentration term is already written separately in the rate equation. This distinction between rate and rate constant is examined frequently.

14. The half life of a first order reaction

• A. doubles as the reaction proceeds
• B. depends on the initial concentration
• C. is constant and independent of the initial concentration
• D. is always one second

Explanation: For first order kinetics the time taken for the concentration to fall by half is fixed, so successive halvings take equal times, and this is precisely why radioactive decay is described by a half life. The relationship is half life equals 0.693 divided by k. For zero and second order reactions the half life does depend on the starting concentration.

15. Powdering a solid reactant increases the rate of reaction because it increases the

• A. concentration of the solid
• B. temperature of the mixture
• C. activation energy
• D. surface area available for collisions

Explanation: A reaction between a solid and a fluid can only occur at the exposed surface, so grinding the solid exposes far more particles and the collision frequency rises in proportion. This is why a fine dust of flour or coal can explode while a solid lump merely burns. The concentration of a pure solid has no meaning, and neither the temperature nor the activation energy is affected.

16. The rate determining step of a multi step reaction is

• A. the fastest step
• B. the slowest step, which controls the overall rate
• C. always the first step
• D. the step with the largest number of molecules

Explanation: An overall reaction can proceed no faster than its slowest stage, in the same way that a road is only as clear as its narrowest point, so the rate equation reflects the species involved up to and including that step. This is why the experimentally measured order often differs from the coefficients in the overall equation. The slow step is the one with the highest activation energy.

17. In the reaction between marble chips and hydrochloric acid, the rate can be followed by measuring

• A. the mass of the flask, which falls as carbon dioxide escapes
• B. the colour of the acid
• C. the temperature of the marble
• D. the mass of the flask, which rises as gas is produced

Explanation: Carbon dioxide leaves the open flask, so the total mass decreases steadily and the loss per unit time is a direct measure of the rate. The alternative method is to collect the gas in a syringe and record its volume at fixed intervals. Mass could never increase, since nothing is entering the flask from outside.

18. Which statement about a catalyst is INCORRECT?

• A. It remains chemically unchanged at the end of the reaction
• B. It is needed only in a small amount
• C. It shifts the equilibrium towards the products
• D. It lowers the activation energy of both the forward and reverse reactions

Explanation: Because a catalyst lowers the barrier equally in both directions, the two rates rise by the same factor and the equilibrium position and the value of Kc are completely unaffected. What changes is how quickly equilibrium is reached. The other three statements are all true, and the small quantity needed follows from the catalyst being regenerated in each cycle.

19. For a reaction A gives products, doubling the concentration of A leaves the rate unchanged. The reaction is

• A. first order in A
• B. second order in A
• C. third order in A
• D. zero order in A

Explanation: If concentration has no effect on rate then the exponent of that term must be zero, since anything raised to the power zero equals one. Had the rate doubled the reaction would be first order, and had it quadrupled it would be second order, which is the reasoning used to find orders experimentally. Zero order behaviour usually signals a saturated catalyst or enzyme.

20. The Arrhenius equation shows that the rate constant increases when

• A. the activation energy increases
• B. the temperature falls
• C. the activation energy decreases or the temperature rises
• D. the concentration of reactants rises

Explanation: The equation contains an exponential term in which activation energy appears with a negative sign and temperature in the denominator, so lowering the barrier or raising the temperature both increase k, and the dependence is exponential rather than linear. This is why a catalyst and a temperature rise produce such large changes in rate. Concentration does not appear in the equation at all.