Bond Energy Calculations [H]

This lesson covers Higher Tier content for AQA GCSE Chemistry. You will learn what bond energies are, how to use bond energy data to calculate the overall energy change of a reaction, and how to determine whether a reaction is exothermic or endothermic from bond energy values. This is a quantitative skill that is frequently tested in the Higher Tier exam papers.

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What Is Bond Energy?

Bond energy (also called bond enthalpy or bond dissociation energy) is the amount of energy needed to break one mole of a particular type of covalent bond. It is measured in kJ/mol (kilojoules per mole).

The same amount of energy is released when the same type of bond is formed.

Bond	Bond Energy (kJ/mol)
H-H	436
O=O	498
O-H	463
C-H	413
C=O	799
C-O	358
N-H	391
N=N	945
Cl-Cl	242
H-Cl	431
C-C	347
C=C	614
H-Br	366
H-F	568
Br-Br	193

Exam Tip: You will be given bond energy values in the exam — you do NOT need to memorise them. However, you MUST understand how to use them in calculations. Practise the method until it is automatic.

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The Principle of Bond Energy Calculations

The overall energy change of a reaction can be calculated using this principle:

Overall energy change = Energy needed to break bonds (in reactants) - Energy released when forming bonds (in products)

Or expressed as a formula:

Energy change = Total bond energies broken - Total bond energies formed

Step	Action
Step 1	Draw out the structural formulae of all reactants and products showing all bonds
Step 2	List all the bonds broken in the reactants
Step 3	Add up the total energy needed to break all bonds in the reactants
Step 4	List all the bonds formed in the products
Step 5	Add up the total energy released when forming all bonds in the products
Step 6	Calculate: Energy change = Energy in (bonds broken) - Energy out (bonds formed)

Exam Tip: If the answer is NEGATIVE, the reaction is EXOTHERMIC (more energy released than taken in). If the answer is POSITIVE, the reaction is ENDOTHERMIC (more energy taken in than released).

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Worked Example 1: Combustion of Hydrogen

Reaction: 2H2 + O2 --> 2H2O

Step 1: Identify all bonds broken in the reactants

Bond	Number	Bond Energy (kJ/mol)	Total (kJ)
H-H	2	436	872
O=O	1	498	498
Total energy to break bonds			1370

Step 2: Identify all bonds formed in the products

Bond	Number	Bond Energy (kJ/mol)	Total (kJ)
O-H	4	463	1852
Total energy to form bonds			1852

Step 3: Calculate the overall energy change

Energy change = 1370 - 1852 = -482 kJ/mol

The answer is negative, so the reaction is exothermic. This makes sense because combustion reactions always release energy.

graph TD
    A["Bonds Broken (Reactants)"] --> B["2 x H-H = 872 kJ"]
    A --> C["1 x O=O = 498 kJ"]
    B --> D["Total In = 1370 kJ"]
    C --> D
    E["Bonds Formed (Products)"] --> F["4 x O-H = 1852 kJ"]
    F --> G["Total Out = 1852 kJ"]
    D --> H["Energy Change = 1370 - 1852 = -482 kJ"]
    G --> H
    H --> I["EXOTHERMIC (negative value)"]

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Worked Example 2: Reaction of Nitrogen and Hydrogen (Haber Process)

Reaction: N2 + 3H2 --> 2NH3

Step 1: Identify all bonds broken in the reactants

Bond	Number	Bond Energy (kJ/mol)	Total (kJ)
N=N (triple bond)	1	945	945
H-H	3	436	1308
Total energy to break bonds			2253

Step 2: Identify all bonds formed in the products

Bond	Number	Bond Energy (kJ/mol)	Total (kJ)
N-H	6	391	2346
Total energy to form bonds			2346

Step 3: Calculate the overall energy change

Energy change = 2253 - 2346 = -93 kJ/mol

The answer is negative, so the Haber process is exothermic.

Exam Tip: When counting bonds in NH3, remember that each molecule of NH3 has 3 N-H bonds. Since the equation makes 2NH3, there are 2 x 3 = 6 N-H bonds formed in total. Always use the balanced equation to count bonds correctly.

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Worked Example 3: Decomposition of Water (Endothermic)

Reaction: 2H2O --> 2H2 + O2

Step 1: Bonds broken in reactants

Bond	Number	Bond Energy (kJ/mol)	Total (kJ)
O-H	4	463	1852
Total energy to break bonds			1852

Step 2: Bonds formed in products

Bond	Number	Bond Energy (kJ/mol)	Total (kJ)
H-H	2	436	872
O=O	1	498	498
Total energy to form bonds			1370

Step 3: Calculate

Energy change = 1852 - 1370 = +482 kJ/mol

The answer is positive, so the reaction is endothermic. This is the reverse of combustion of hydrogen, and the energy change is equal but opposite — exactly as expected.

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Common Mistakes in Bond Energy Calculations