Rate-determining step and reaction mechanismsAQA A-Level Chemistry: Flashcards

What these 12 flashcards ask

What is the rate-determining step?
What is an intermediate?
What must the steps of a mechanism add up to?
What does first order with respect to a species show about the mechanism?
What does zero order with respect to a reactant show?
What does second order with respect to a species show?
How do you write a rate equation from a mechanism?
Why can the rate equation not be written from the overall equation?
If a species appears in the rate equation but not the overall equation, what is it?
What effect does speeding up a fast step have on the overall rate?
Does a mechanism that fits the rate equation prove the mechanism?
In the hydrolysis of (CH₃)₃CBr, why is hydroxide zero order?

Exam questions on Rate-determining step and reaction mechanisms

The reaction NO₂(g) + CO(g) → NO(g) + CO₂(g) has the experimentally determined rate equation rate = k[NO₂]². A two-step mechanism is proposed. Step 1 (slow): 2NO₂ → NO₃ + NO. Step 2 (fast): NO₃ + CO → NO₂ + CO₂.
Explain why CO does not appear in the rate equation even though it is a reactant.2 marks
In acidic solution, propanone reacts with iodine: CH₃COCH₃ + I₂ → CH₃COCH₂I + HI. The experimentally determined rate equation is rate = k[CH₃COCH₃][H⁺]. The reaction is zero order with respect to iodine. Hydrogen ions are not used up.
Explain why H⁺ appears in the rate equation although it does not appear in the overall equation.2 marks
Two halogenoalkanes are hydrolysed by aqueous sodium hydroxide at constant temperature. For 2-bromo-2-methylpropane, (CH₃)₃CBr, the reaction is first order with respect to the halogenoalkane and zero order with respect to hydroxide ions. For bromoethane, CH₃CH₂Br, the reaction is first order with respect to the halogenoalkane and first order with respect to hydroxide ions.
Use the orders of reaction for 2-bromo-2-methylpropane to explain what they show about its mechanism.3 marks

Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).