Reaction kinetics

Use H2 Chemistry rate data to find orders and rate constants, connect mechanisms to rate laws and explain activation energy and catalysis.

GCE A-Level H2 Chemistry 2027
6 lessons

Start topic

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, link rate equations to mechanisms, 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

Rate Equations, Orders, Rate Constant
Initial Rates Method
Concentration–Time Graphs and Half-life
Activation Energy and Boltzmann Distribution
Mechanisms and Rate-determining Step
Catalysis and Enzymes

Syllabus statements covered

explain and use the terms: rate of reaction; rate equation; order of reaction; rate constant; half-life of a reaction; rate-determining step; 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 and of multi-step processes with a rate-determining step, 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]
construct and use rate equations of the form rate = k[A]m[B]n (limited to simple cases of single-step reactions and of multi-step processes with a rate-determining step, for which m and n are 0, 1 or 2), including: — deducing the order of a reaction by the initial rates method
calculate a rate constant using the initial rates method
devise a suitable experimental technique for studying the rate of a reaction, from given information
construct and use rate equations of the form rate = k[A]m[B]n (limited to simple cases of single-step reactions and of multi-step processes with a rate-determining step, 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
show understanding that the half-life of a first-order reaction is independent of concentration
use the half-life of a first-order reaction in calculations
construct and use rate equations of the form rate = k[A]m[B]n (limited to simple cases of single-step reactions and of multi-step processes with a rate-determining step, for which m and n are 0, 1 or 2), including: — verifying that a suggested reaction mechanism is consistent with the observed kinetics
construct and use rate equations of the form rate = k[A]m[B]n (limited to simple cases of single-step reactions and of multi-step processes with a rate-determining step, for which m and n are 0, 1 or 2), including: — predicting the order that would result from a given reaction mechanism
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 both of 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 has a different mechanism, i.e. one of lower activation energy, giving a larger rate constant
interpret this catalytic effect on a rate constant in terms of the Boltzmann distribution
outline the different modes of action of homogeneous and heterogeneous catalysis, including: — the Haber process
outline the different modes of action of homogeneous and heterogeneous catalysis, including: — the catalytic removal of oxides of nitrogen in the exhaust gases from car engines (see also Section 11.4)
outline the different modes of action of homogeneous and heterogeneous catalysis, including: — the catalytic role of atmospheric oxides of nitrogen in the oxidation of atmospheric sulfur dioxide
outline the different modes of action of homogeneous and heterogeneous catalysis, including: — catalytic role of Fe2+ in the I–/S2O82– reaction
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 and the details of the denaturation process are not required]

Lessons

Work through them in order.

Practise and check

Or choose

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
Find the units of k	divide the rate unit by concentration raised to the overall order	e.g. third order: dm⁶ mol⁻² s⁻¹
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
Time to fall to 1/2ⁿ (first order)	count the half-lives	t = n t_(1/2)
Rate equation from a mechanism	write the rate equation of the slow step	eliminate any intermediate using a fast equilibrium
Effect of temperature	Boltzmann distribution	larger fraction with E ≥ Eₐ and more frequent collisions, so k increases
Effect of a catalyst	alternative mechanism	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 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, and its units depend on the overall order.
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.
Rate-determining step: the slowest step in a mechanism, which controls the overall rate.
Activation energy: the minimum energy that colliding particles need in order to react.
Catalysis: speeding up a reaction by providing an alternative mechanism 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.
Stating the orders but not writing the full rate equation with k, or leaving out the units of k.
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.
Leaving an intermediate in a rate equation derived from a mechanism.
Saying a catalyst changes Δ H or the position of equilibrium.