Reaction kinetics

Determine H1 Chemistry rate equations from data, interpret orders and half-life, and explain activation energy, catalysis and enzyme action.

GCE A-Level H1 Chemistry 2027
5 lessons

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Before you begin

Reaction kinetics is about how fast reactions go and why. The lessons start from experiment: writing rate equations, finding orders by the initial rates method and reading concentration–time graphs and half-lives. They then explain rates with the Boltzmann distribution and activation energy, and finish with how catalysts and enzymes work.

Be comfortable with: solution concentration, collision theory, enthalpy changes and energy profiles, and working with ratios and powers.

Learning goals

Explain and use rate of reaction, rate equation, order of reaction, rate constant, half-life, activation energy and catalysis
Deduce the orders in a rate equation by the initial rates method
Justify whether a reaction is zero or first order from a concentration–time graph
Calculate an initial rate from concentration data using a rate equation
Explain why the half-life of a first-order reaction does not depend on concentration
Explain how changing concentration affects reaction rate using collision frequency
Explain activation energy using the Boltzmann distribution
Explain how temperature affects the rate constant, and so the rate, using the Boltzmann distribution and collision frequency
Explain that a catalyst gives a reaction a pathway of lower activation energy and so a larger rate constant
Interpret a catalyst's effect using the Boltzmann distribution
Outline how heterogeneous catalysis works, using the removal of nitrogen oxides from car exhaust gases
Describe enzymes as protein catalysts with high specificity, explained by the lock-and-key model, and sensitivity to temperature and pH

Syllabus statements covered

explain and use the terms: rate of reaction; rate equation; order of reaction; rate constant; half-life of a reaction; activation energy; catalysis
construct and use rate equations of the form rate = k[A]m[B]n (limited to simple cases of single-step reactions for which m and n are 0, 1 or 2), including: — deducing the order of a reaction by the initial rates method
construct and use rate equations of the form rate = k[A]m[B]n (limited to simple cases of single-step reactions for which m and n are 0, 1 or 2), including: — justifying, for zero- and first-order reactions, the order of reaction from concentration-time graphs
construct and use rate equations of the form rate = k[A]m[B]n (limited to simple cases of single-step reactions for which m and n are 0, 1 or 2), including: — calculating an initial rate using concentration data [integrated forms of rate equations are not required]
show understanding that the half-life of a first-order reaction is independent of concentration
explain qualitatively, in terms of frequency of collisions, the effect of concentration changes on the rate of a reaction
show understanding, including reference to the Boltzmann distribution, of what is meant by the term activation energy
explain qualitatively, in terms of both the Boltzmann distribution and of collision frequency, the effect of temperature change on a rate constant (and hence, on the rate) of a reaction
explain that, in the presence of a catalyst, a reaction follows a different pathway, i.e. one of lower activation energy, giving a larger rate constant
interpret this catalytic effect in terms of the Boltzmann distribution
outline the mode of action of heterogeneous catalysis, as exemplified by the catalytic removal of oxides of nitrogen in the exhaust gases from car engines
describe enzymes as protein molecules that act as biological catalysts with high specificity (in the reactions that they catalyse and in their choice of substrates as exemplified by the lock-and-key model), temperature sensitivity and pH sensitivity [Knowledge of the levels of structure of proteins is not required. Details of the denaturation process will be discussed in 9 (m).]

Lessons

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Topic reference

Quick reference

Question	First move	What to show
Find an order from initial rates	compare two experiments in which only one concentration changes	rate₂/rate₁ = ([A]₂/[A]₁)^m, then the order
Write the rate equation	use the orders found from data	rate = k[A]^m[B]ⁿ, including k
Effect of changing concentrations	raise each concentration factor to its order	multiply the factors together
Calculate an initial rate	substitute k and the concentrations	each concentration raised to its order; units of rate
Order from a concentration–time graph	check whether the graph is straight or whether successive half-lives are equal	straight line: zero order; constant half-life: first order
Effect of temperature	Boltzmann distribution	larger fraction with E ≥ Eₐ and more frequent collisions, so k increases
Effect of a catalyst	alternative pathway	lower Eₐ, larger fraction with E ≥ Eₐ, so k increases

Core knowledge to remember

Rate of reaction: the change in concentration of a reactant or product per unit time.
Rate equation: rate = k[A]^m[B]ⁿ, where the orders m and n (0, 1 or 2) are found by experiment.
Order of reaction with respect to a reactant: the power to which its concentration is raised in the rate equation. The overall order is the sum of the powers.
Rate constant, k: the constant of proportionality in the rate equation. It is fixed at a given temperature.
Half-life: the time taken for the concentration of a reactant to fall to half its value. For a first-order reaction it is independent of concentration.
Activation energy: the minimum energy that colliding particles need in order to react.
Catalysis: speeding up a reaction by providing an alternative pathway with a lower activation energy; the catalyst is regenerated.
Enzymes are protein molecules that act as biological catalysts. They are highly specific and are sensitive to temperature and pH.

Common mistakes

Taking orders from the coefficients in the balanced equation instead of from experimental data.
Comparing two experiments in which more than one concentration changes.
Adding the factors from two reactants instead of multiplying them.
Assuming every reaction has a constant half-life. Only first-order reactions do.
Saying that higher temperature raises the activation energy, or that a catalyst changes the Boltzmann distribution.
Saying a catalyst changes Δ H or is used up.
Saying that a low temperature denatures an enzyme.