Simple rate equations, orders of reaction and rate constants
Cambridge A-Level Chemistry (9701) · Unit 26: Reaction kinetics · 10 flashcards
Simple rate equations, orders of reaction and rate constants is topic 26.1 in the Cambridge A-Level Chemistry (9701) syllabus , positioned in Unit 26 — Reaction kinetics , alongside Homogeneous and heterogeneous catalysts. In one line: The rate equation expresses the relationship between the rate of reaction and the concentration of reactants raised to appropriate powers (orders). The rate constant (k) is the proportionality constant in the rate equation; its value reflects the rate of the reaction at a specific temperature.
Marked as A2 Level: examined at A Level in Paper 4 (A Level Structured Questions) and Paper 5 (Planning, Analysis and Evaluation). It is not tested on the AS-only papers (Papers 1, 2 and 3).
The deck below contains 10 flashcards — 5 definitions, 4 key concepts and 1 calculation — covering the precise wording mark schemes reward. Use the 5 definition cards to lock down command-word answers (define, state), then move on to the concept and calculation cards to handle explain, describe, calculate and compare questions.
The terms 'rate equation' and 'rate constant'
The rate equation expresses the relationship between the rate of reaction and the concentration of reactants raised to appropriate powers (orders). The rate constant (k) is the proportionality constant in the rate equation; its value reflects the rate of the reaction at a specific temperature.
What the Cambridge 9701 syllabus says
Official 2025-2027 spec · A2 LevelThese are the exact learning outcomes Cambridge sets for this topic. The candidate is expected to be able to do each of these on the relevant paper.
- explain and use the terms rate equation, order of reaction, overall order of reaction, rate constant, half-life, rate-determining step and intermediate
- calculate the numerical value of a rate constant, for example by: (a) using the initial rates and the rate equation (b) using the half-life, t½ , and the equation k = 0.693 / t½
- for a multi-step reaction: (a) suggest a reaction mechanism that is consistent with the rate equation and the equation for the overall reaction (b) predict the order that would result from a given reaction mechanism and rate-determining step (c) deduce a rate equation using a given reaction mechanism and rate-determining step for a given reaction (d) identify an intermediate or catalyst from a given reaction mechanism (e) identify the rate determining step from a rate equation and a given reaction mechanism
- describe qualitatively the effect of temperature change on the rate constant and hence the rate of a reaction
Cambridge syllabus keywords to use in your answers
These are the official Cambridge 9701 terms tagged to this section. Mark schemes credit responses that use the exact term — weave them into your answers verbatim rather than paraphrasing.
Tips to avoid common mistakes in Simple rate equations, orders of reaction and rate constants
- › Link the overall order and individual reactant orders to the mechanism; SN2 is suggested by first-order kinetics for both reactants.
- › A full description of heterogeneous catalysis must include adsorption of reactants, bond weakening, reaction, and finally desorption of products.
- › Use a sharp pencil or fine pen to plot points as small crosses (x) for better precision.
- › When explaining rate using collision theory, you must use the phrase 'frequency of collisions' rather than just 'number of collisions'.
- › Always check the units specified in the answer options and convert your time values accordingly before final calculations.
Define the terms 'rate equation' and 'rate constant'.
The rate equation expresses the relationship between the rate of reaction and the concentration of reactants raised to appropriate powers (orders). The rate constant (k) is the proportionality constant in the rate equation; its value reflects the rate of the reaction at a specific temperature.
What is the 'order of reaction' with respect to a specific reactant, and how does it relate to the rate equation?
The order of reaction with respect to a reactant is the power to which the reactant's concentration is raised in the rate equation. It indicates how the rate changes with changing concentration of that specific reactant;
Explain the meaning of 'overall order of reaction'.
The overall order of reaction is the sum of the individual orders of reaction with respect to each reactant in the rate equation.
Describe how to determine the numerical value of a rate constant (k) using initial rates and the rate equation.
Given initial rates for several experiments with varying reactant concentrations, and a known rate equation, substitute the initial rates and concentrations into the rate equation. Solve the resulting equation for 'k'.
Describe how to calculate the rate constant (k) using the half-life (t½) for a first-order reaction.
For a first-order reaction, the rate constant k can be calculated using the equation: k = 0.693 / t½, where t½ is the half-life of the reaction. Make sure time units are consistent.
What is a 'rate-determining step' in a multi-step reaction mechanism?
The rate-determining step is the slowest step in a multi-step reaction mechanism. Because it is the slowest, it determines the overall rate of the reaction, as it acts as a bottleneck.
What is a reaction 'intermediate'?
A reaction intermediate is a species that is formed in one step of a reaction mechanism and consumed in a subsequent step. It does not appear in the overall balanced chemical equation.
Outline the qualitative effect of temperature change on the rate constant (k) and the rate of a reaction.
Increasing the temperature generally increases the rate constant (k) and, consequently, the rate of reaction. This is because a higher temperature provides more molecules with the activation energy needed to react.
Given a multi-step reaction mechanism, how can you identify a catalyst?
A catalyst participates in a reaction mechanism, but is both consumed in an early step and regenerated in a later step. Therefore, a catalyst does not appear in the overall balanced equation.
Explain how a proposed reaction mechanism can be validated (or refuted) by experimental data.
A proposed mechanism is valid if the rate equation derived from the rate-determining step matches the experimentally determined rate equation. If they don't match, the mechanism is incorrect and needs to be revised.
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Simple rate equations, orders of reaction and rate constants sits alongside these A-Level Chemistry decks in the same syllabus unit. Each uses the same spaced-repetition system, so progress in one informs the next.
Key terms covered in this Simple rate equations, orders of reaction and rate constants deck
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