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
2025-2027 syllabus, in our words · A2 LevelWhat the syllabus expects you to be able to do in this topic, in plain English. For the exact wording, see the official syllabus (PDF).
- Explain the meaning of each of these terms and use them: rate constant, rate equation, half-life, order of reaction and overall order, intermediate, and rate-determining step.
- (a) Use, and show understanding of, rate equations of the type rate = k[A]ᵐ[B]ⁿ, with m and n each equal to 0, 1 or 2. (b) Deduce reaction orders from graphs of concentration against time, or from experimental results obtained with the initial rates method and the half-life method. (c) Interpret experimental results shown graphically, including graphs of concentration against time and of rate against concentration. (d) Use concentration data to calculate an initial rate. (e) Construct the rate equation for a reaction.
- (a) Show understanding that a first-order reaction has a half-life that does not depend on concentration. (b) Use first-order half-lives in calculations.
- Calculate a value of k, the rate constant, for instance (a) from the rate equation together with initial rates; (b) from the half-life, t½, using k = 0.693/t½.
- For a reaction that takes place in several steps: (a) suggest a mechanism that fits both the rate equation and the overall equation; (b) predict the order produced by a given mechanism and rate-determining step; (c) deduce the rate equation for a given reaction from its mechanism and rate-determining step; (d) pick out an intermediate or a catalyst in a given mechanism; (e) use the rate equation and a given mechanism to identify which step is rate-determining.
- Describe qualitatively how a change in temperature affects the rate constant, and so the rate of 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.
- › Always calculate initial rate by drawing a tangent to the curve at the origin (t=0) and determining the gradient.
- › The order of reaction is the power to which a reactant's concentration is raised in the experimental rate equation.
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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