Lesson 16.1.4

16.1.4 Reaction mechanisms and the rate-determining step Quiz: Pearson Edexcel Chemistry, Unit 16

20 questions

Written by Revision Ninja · Last reviewed 11 October 2026

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Lesson 16.1.4, Reaction mechanisms and the rate-determining step: 20 multiple choice questions for the Pearson Edexcel Chemistry (9CH0), Unit 16: Kinetics II, written with Revision Ninja.

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The 20 questions

  1. What is a rate-determining step?

    • The step that produces the final product of the overall equation
    • The fastest step in a mechanism, which uses up all the intermediates
    • The first step in a reaction mechanism, which always involves a catalyst
    • The slowest step in a reaction mechanism, which controls the overall rate of the reaction

  2. Which species can appear in the rate equation?

    • Only the reactant that has the largest stoichiometric coefficient
    • Only intermediates formed in the fast steps after the rate-determining step
    • Only the products of the overall reaction
    • Reactants and any catalyst involved in the rate-determining step or in steps before it

  3. Why can an intermediate not appear in the overall rate equation?

    • It is made and used up within the mechanism, so it is not a reactant in the overall equation
    • It is formed after the rate-determining step, so it cannot affect the rate of the overall reaction at all
    • It is always a gas, so it escapes from the reaction vessel before the concentration can be measured
    • It is only present in the products at the end, so it cannot react with any other species in the mixture

  4. In the acid-catalysed iodination of propanone, what is the order with respect to iodine?

    • 0
    • 2
    • 1
    • -1

  5. What is the order with respect to H+ in the acid-catalysed iodination of propanone?

    • 1/2
    • 2
    • 0
    • 1

  6. Which halogenoalkane hydrolysis has the rate equation rate = k[halogenoalkane]?

    • SN2 hydrolysis of a secondary halogenoalkane, where the rate depends on two reactants
    • SN2 hydrolysis of a tertiary halogenoalkane, where the rate depends on the hydroxide ion
    • SN1 hydrolysis of a tertiary halogenoalkane, where the rate depends only on the halogenoalkane
    • SN2 hydrolysis of a primary halogenoalkane, where the rate depends on the hydroxide ion

  7. For 2NO + O2 -> 2NO2 with rate = k[NO]^2[O2], what is the overall order?

    • 1
    • 3
    • 2
    • 4

  8. A mechanism has step 1 (slow): A + B -> X, and step 2 (fast): X + C -> D. Which rate equation is consistent?

    • rate = k[A][B][C]
    • rate = k[A][B]
    • rate = k[X][C]
    • rate = k[D]

  9. A rate equation is rate = k[NO2]^2 for the overall reaction 2NO2 -> N2O4. Which description of the rate-determining step fits?

    • The rate-determining step involves N2O4, which is the product of the reaction
    • Termolecular: three NO2 molecules collide in the rate-determining step
    • Bimolecular: two NO2 molecules collide in the rate-determining step
    • Unimolecular: one NO2 molecule decomposes in the rate-determining step

  10. The overall equation is 2A + B -> C and experiments show rate = k[A]^2. Which conclusion about the rate-determining step is best?

    • The rate-determining step involves two molecules of A and no B
    • The rate-determining step involves only the product C
    • The rate-determining step involves one molecule of A and one molecule of B
    • The rate-determining step involves one molecule of B and one molecule of A

  11. For SN2 hydrolysis of a primary halogenoalkane in a single step, which rate equation applies?

    • rate = k[halogenoalkane]
    • rate = k[OH-]
    • rate = k[halogenoalkane]^2
    • rate = k[halogenoalkane][OH-]

  12. In the SN1 mechanism, which species is involved in the slow rate-determining step?

    • The carbocation and the hydroxide ion, which react in the slowest step
    • Only the halogenoalkane, which breaks down to form a carbocation and a halide ion
    • The halogenoalkane and hydroxide ions colliding together
    • Only the hydroxide ion, which attacks the carbocation

  13. A reaction profile shows several steps. Which step is usually the rate-determining step?

    • The step with the lowest activation energy, since it is the fastest step
    • The step with the highest activation energy, since it is the slowest step
    • The step that releases the most energy, since it controls the rate
    • The final step that forms the overall products of the equation

  14. When does a catalyst appear in the rate equation?

    • When the catalyst takes part in the rate-determining step or in a step before it
    • When the catalyst is present in the overall equation as a reactant
    • When the catalyst is added in a large excess compared with the reactants
    • When the catalyst is a solid that is not in the same phase as the reactants

  15. Why does the overall stoichiometric equation not always predict the rate equation?

    • Catalysts always change the stoichiometry of the reaction
    • The rate equation is always determined by the products rather than the reactants
    • Rate depends on the slowest step, which may involve fewer or different species than the overall equation
    • Rate always depends on the total number of moles of reactants in the balanced equation

  16. In the acid-catalysed iodination of propanone, the rate does not depend on [I2]. What does this indicate?

    • I2 is the only reactant that controls the speed of the enol formation in the first and slowest step
    • I2 is not involved in the rate-determining step, so it reacts in a later fast step
    • I2 forms an intermediate that is itself the rate-determining species in the first and slowest step
    • I2 is the catalyst, so it appears in the overall equation twice and is regenerated at the end

  17. A rate equation is rate = k[NO]^2[H2]. Which mechanism is consistent with this and the overall equation 2NO + 2H2 -> N2 + 2H2O?

    • Step 1 (slow): H2 + H2 -> H4; Step 2 (fast): H4 + 2NO -> N2 + 2H2O
    • Step 1 (slow): NO + H2 -> N2O + H2O; Step 2 (fast): NO + H2 -> N2 + H2O
    • Step 1 (slow): 2NO + 2H2 -> N2 + 2H2O, with no intermediate formed
    • Step 1 (slow): 2NO + H2 -> N2O + H2O; Step 2 (fast): N2O + H2 -> N2 + H2O

  18. Doubling [OH-] has no effect on the rate of hydrolysis of 2-bromo-2-methylpropane. What does this suggest?

    • The reaction is catalysed by hydroxide ions, which are regenerated at the end
    • The reaction has a termolecular rate-determining step involving two hydroxide ions
    • The reaction is SN1, since the rate-determining step involves only the halogenoalkane
    • The reaction is SN2, since hydroxide ions attack in the rate-determining step

  19. Rate = k[A][B]^2 is observed. Which two-step mechanism is most consistent with this rate equation?

    • Step 1 (slow): A + B -> X; Step 2 (fast): X + B -> products
    • Step 1 (slow): A + B + B -> products, in a single termolecular step
    • Step 1 (fast): A + B -> X; Step 2 (slow): X + A -> products
    • Step 1 (fast equilibrium): B + B <=> B2; Step 2 (slow): B2 + A -> products

  20. In a reaction mechanism, what is an intermediate?

    • A catalyst that is regenerated at the end of every step and is never used up in the mechanism
    • A species formed in one step of the mechanism and consumed in a later step
    • A species that appears in the overall equation as a product formed in the final step of the mechanism
    • A reactant that is in excess and is never consumed in any step of the reaction mechanism

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