Unit Overview

Further reaction kinetics

CIE A-Level Chemistry· 5 min read 📊 n/a

1. Unit at a glance

This unit progresses from working with experimental rate data to connecting rate laws to the underlying molecular mechanism of a reaction. You will start by learning how to extract order of reaction and build rate equations from raw experimental results.

Next, you will use these rate equations to identify the slowest (rate-determining) step in a multi-step reaction, and deduce the full mechanism. Finally, you will explore how temperature impacts reaction rates via the Arrhenius equation, and practice all common exam calculation types.

Core sub-topics in this unit:

Rate equations and order of reaction

Learn to identify zero, first, and second order reactions and construct rate equations from experimental data

★★⏱ 10 min

Rate-determining step

Deduce full reaction mechanisms by matching the rate equation to the reactants in the rate-determining step

★★★⏱ 8 min

Arrhenius equation

Explore the relationship between activation energy, temperature, and rate constant

★★★⏱ 7 min

Rate constant calculations

Practice calculation methods for rate constants and activation energy from experimental data

★★★⏱ 9 min

2. Common Pitfalls

Wrong move:

Mixing up the overall order of reaction with the order of reaction with respect to individual reactants

Why:

This leads to incorrect rate equations and wrong mechanistic deductions

Correct move:

Always sum the individual orders with respect to each reactant to get the overall order of the reaction

Wrong move:

Assuming the rate-determining step must always be the first step in a reaction mechanism

Why:

This leads to incorrect mechanism proposals that do not match the experimental rate equation

Correct move:

Match the reactants in the rate equation to the species in the rate-determining step regardless of its position in the mechanism

3. Quick Reference Cheatsheet

Concept / Formula	Key Information
General Rate Equation	rate = $k [A]^{m} [B]^{n}$ , where $k$ = rate constant, $m, n$ = order with respect to each reactant
Overall Reaction Order	Sum of the individual orders of reaction: $overall order = m + n + ...$
Rate-Determining Step	Slowest step in a multi-step reaction; the species involved match the order terms in the experimental rate equation
Arrhenius Equation	$k = A e^{- E_{a} / (R T)}$ or $l nk = l n A - \frac{E _{a}}{R T}$ where $E_{a}$ = activation energy, $A$ = pre-exponential factor
Arrhenius Plot Gradient	Gradient of a plot of $l nk$ against $1/ T$ equals $- E_{a} / R$ , used to calculate activation energy

What's Next

Begin with the first sub-topic of this unit to build your understanding of rate laws step by step. Once you complete all sub-topics here, you can progress to the next unit on chemical equilibria, which builds on kinetic concepts to study reversible reactions.