Define order of reaction with respect to a reactant.

The power to which the concentration of that reactant is raised in the rate equation.

Define rate constant.

The constant of proportionality in the rate equation; it depends on temperature but not on concentration.

What is the effect of changing the concentration of a zero-order reactant?

None: the reactant does not appear in the rate equation.

How is the overall order found?

Add the orders with respect to each reactant.

How are the orders of reaction found?

By experiment, for example from initial rate data, not from the balanced equation.

How does increasing temperature affect k?

It increases k, so the rate increases at the same concentrations.

Rate equations, orders and rate constantsAQA A-Level Chemistry: Flashcards

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What is the rate equation for a reaction of A and B?

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What is the rate equation for a reaction of A and B?: rate = k[A]ᵐ[B]ⁿ
Define order of reaction with respect to a reactant.: The power to which the concentration of that reactant is raised in the rate equation.
Define rate constant.: The constant of proportionality in the rate equation; it depends on temperature but not on concentration.
What are the units of rate?: mol dm⁻³ s⁻¹
What is the effect of doubling the concentration of a first-order reactant?: The rate doubles.
What is the effect of doubling the concentration of a second-order reactant?: The rate quadruples.
What is the effect of changing the concentration of a zero-order reactant?: None: the reactant does not appear in the rate equation.
How is the overall order found?: Add the orders with respect to each reactant.
What are the units of k for a first-order reaction?: s⁻¹
What are the units of k for a second-order reaction?: dm³ mol⁻¹ s⁻¹
What are the units of k for a third-order reaction?: dm⁶ mol⁻² s⁻¹
How are the orders of reaction found?: By experiment, for example from initial rate data, not from the balanced equation.
How does increasing temperature affect k?: It increases k, so the rate increases at the same concentrations.

Exam questions on Rate equations, orders and rate constants

A student investigates the reaction between substances A and B at constant temperature. The reaction has the rate equation rate = k[A][B]², where square brackets show concentrations in mol dm⁻³.
State what is meant by the statement that the reaction is first order with respect to A.2 marks
The initial rate of the reaction between substances P and Q was measured at constant temperature in three experiments. Experiment 1: [P] = 0.10 mol dm⁻³, [Q] = 0.10 mol dm⁻³, initial rate = 2.0 × 10⁻⁴ mol dm⁻³ s⁻¹. Experiment 2: [P] = 0.20 mol dm⁻³, [Q] = 0.10 mol dm⁻³, initial rate = 8.0 × 10⁻⁴ mol dm⁻³ s⁻¹. Experiment 3: [P] = 0.20 mol dm⁻³, [Q] = 0.20 mol dm⁻³, initial rate = 1.6 × 10⁻³ mol dm⁻³ s⁻¹.
Define the term rate constant, k, and state one factor that changes its value.2 marks
In acid solution, propanone reacts with iodine: CH₃COCH₃ + I₂ → CH₃COCH₂I + HI. The initial rate was measured in four experiments, with the concentrations of propanone, H⁺ and I₂ in mol dm⁻³. Experiment 1: 1.00, 0.50, 0.0050, rate = 1.35 × 10⁻⁵ mol dm⁻³ s⁻¹. Experiment 2: 2.00, 0.50, 0.0050, rate = 2.70 × 10⁻⁵. Experiment 3: 1.00, 1.00, 0.0050, rate = 2.70 × 10⁻⁵. Experiment 4: 1.00, 0.50, 0.0100, rate = 1.35 × 10⁻⁵.
Deduce the order of reaction with respect to each of propanone, H⁺ and I₂. Give your reasoning.3 marks

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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).