Unit Overview

Chemical energetics

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

1. Unit at a glance

This unit builds step-by-step understanding of energy changes that occur during chemical reactions. You will start by learning how to define and measure different types of enthalpy change, before moving to the core principle of Hess' law, which lets you calculate unknown enthalpy values even when they cannot be measured directly. The final topic connects bond breaking and bond forming to overall reaction enthalpy, explaining why some reactions release energy and others absorb it.

This unit contains the following core sub-topics:

Enthalpy changes

Learn to define, classify, and calculate different types of standard enthalpy change from experimental data.

★★⏱ 10 min

Hess' law

Apply Hess' law to construct enthalpy cycles and calculate unknown enthalpy changes for reactions.

★★★⏱ 12 min

Bond enthalpies

Use average bond enthalpy values to calculate reaction enthalpy and compare relative bond strengths.

★★⏱ 8 min

2. Common Pitfalls

Wrong move:

Reversing the sign convention for bond breaking and making

Why:

This leads to incorrect final values for reaction enthalpy calculated from bond enthalpies

Correct move:

Remember: bond breaking is endothermic ( $+ Δ H$ ), bond making is exothermic ( $- Δ H$ )

Wrong move:

Ignoring reaction stoichiometry in Hess' law calculations

Why:

Enthalpy change scales with moles of reactant, so unadjusted values give incorrect results

Correct move:

Always multiply enthalpy values by the molar coefficients in your target reaction equation

3. Quick Reference Cheatsheet

Concept	Key Formula/Rule
Experimental enthalpy change	$q = m c Δ T$ , $Δ H = - q / n$ (per mole of limiting reactant)
Hess' Law Statement	Total enthalpy change is independent of the reaction path taken
Enthalpy from formation values	$Δ H_{r} = \sum Δ H_{f} (products) - \sum Δ H_{f} (reactants)$
Enthalpy from combustion values	$Δ H_{r} = \sum Δ H_{c} (reactants) - \sum Δ H_{c} (products)$
Enthalpy from bond enthalpies	$Δ H_{r} = \sum (bonds broken) - \sum (bonds formed)$

What's Next

Begin your study of this unit by working through the first sub-topic on enthalpy changes, which lays the foundational definitions and calculations needed for the rest of the unit. After you complete all three sub-topics here, you can move on to the next unit in the CIE A-Level Chemistry syllabus.