Cambridge International AS & A Level Chemistry – Physical Chemistry

Topic 5
Chemical Energetics

Topic 5 covers the energy changes that accompany chemical reactions: exothermic and endothermic reactions, reaction pathway diagrams and activation energy, standard conditions of 298 K and 101 kPa, the enthalpy changes of reaction, formation, combustion and neutralisation, calculating enthalpy changes from experimental results, bond energies, and Hess’s law energy cycles. Lattice energy and Born-Haber cycles follow at A Level in Topic 23.

Exam Papers

Papers 1 & 2

9701 AS Level

Learning Outcomes

5.1 – 5.2

9 outcomes covered

Topic Parts

5 of 5 live

Revision notes available

Exam Board

Cambridge

9701 (2025 onwards)

Revision Notes

Work through Topic 5 in a structured sequence.

Exo and endo

Enthalpy
Changes

Available

Enthalpy Changes and Standard Conditions

Exothermic and endothermic reactions, reaction pathway diagrams, activation energy and standard conditions.

Standard Enthalpy Changes

The standard enthalpy change definitions and the equation each one refers to.

Calorimetry: Measuring Enthalpy Changes

Calculating ΔH from a temperature change, cooling curve corrections, errors and assumptions.

Hess’s Law and Enthalpy Cycles

Building enthalpy cycles and calculating ΔH from formation and combustion data.

Bond Energies

Bond energy, calculating ΔH from bonds broken and made, and why the answers differ from Hess values.

Start revising

Specification Coverage

Topic 5 Chemical Energetics – Cambridge International AS Level Chemistry

The following Cambridge learning outcomes state what candidates should be able to do. Wording is taken from the Cambridge International AS & A Level Chemistry (9701) syllabus.

5.1 Enthalpy change, ΔH

5.1.1

understand that chemical reactions are accompanied by enthalpy changes and these changes can be exothermic (ΔH is negative) or endothermic (ΔH is positive)

5.1.2

construct and interpret a reaction pathway diagram, in terms of the enthalpy change of the reaction and of the activation energy

5.1.3

define and use the terms:

standard conditions (this syllabus assumes that these are 298 K and 101 kPa) shown by ⦵

enthalpy change with particular reference to: reaction, ΔH_r, formation, ΔH_f, combustion, ΔH_c, neutralisation, ΔH_neut

5.1.4

understand that energy transfers occur during chemical reactions because of the breaking and making of chemical bonds

5.1.5

use bond energies (ΔH positive, i.e. bond breaking) to calculate enthalpy change of reaction, ΔH_r

5.1.6

understand that some bond energies are exact and some bond energies are averages

5.1.7

calculate enthalpy changes from appropriate experimental results, including the use of the relationships q = mcΔT and ΔH = –mcΔT/n

5.2 Hess’s law

5.2.1

apply Hess’s law to construct simple energy cycles

5.2.2

carry out calculations using cycles and relevant energy terms, including:

determining enthalpy changes that cannot be found by direct experiment

use of bond energy data

Topic 5Chemical Energetics

Revision Notes

Topic 5 Chemical Energetics – Cambridge International AS Level Chemistry

5.1 Enthalpy change, ΔH

5.2 Hess’s law

Keep this note — free

Topic 5
Chemical Energetics