GCSE Chemistry revision

The energy change of reactions

Exothermic and endothermic reactions

AQA 4.5.1.3 Higher Tier only

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Key knowledge

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What Is Bond Energy?

Bond energy is the amount of energy needed to break one mole of a particular covalent bond, measured in kJ/mol.
One mole of bonds equals 6.02 × 10²³ bonds, which is Avogadro's constant.
Bond energy values are always positive because breaking bonds always requires energy input from the surroundings.

1. What Is Bond Energy? AQA

Notes

Bond energy is the amount of energy needed to break one mole of a particular covalent bond, measured in kJ/mol.
One mole of bonds equals 6.02 × 10²³ bonds, which is Avogadro's constant.
Bond energy values are always positive because breaking bonds always requires energy input from the surroundings.

Breaking Bonds — An Endothermic Process

Breaking bonds is an endothermic process because energy must be absorbed from the surroundings.
For example, breaking one mole of H–Cl bonds requires 431 kJ of energy to be supplied.

Forming Bonds — An Exothermic Process

Forming bonds is an exothermic process because energy is released to the surroundings.
For example, forming one mole of H–Cl bonds releases 431 kJ of energy to the surroundings.
The same bond energy value applies to both breaking and forming the same type of bond.

3. Forming Bonds — An Exothermic Process AQA

Notes

Forming bonds is an exothermic process because energy is released to the surroundings.
For example, forming one mole of H–Cl bonds releases 431 kJ of energy to the surroundings.
The same bond energy value applies to both breaking and forming the same type of bond.

Calculating Overall Energy Change

The overall energy change is calculated using: Δ H = Energy required to break bonds - Energy released by forming bonds
A negative overall energy change indicates an exothermic reaction, where more energy is released than absorbed.
A positive overall energy change indicates an endothermic reaction, where more energy is absorbed than released.
In exams, bond energy values are provided in a table, and you must identify all bonds broken and formed from the displayed formula.

4. Calculating Overall Energy Change AQA

Notes

The overall energy change is calculated using: Δ H = Energy required to break bonds - Energy released by forming bonds
A negative overall energy change indicates an exothermic reaction, where more energy is released than absorbed.
A positive overall energy change indicates an endothermic reaction, where more energy is absorbed than released.
In exams, bond energy values are provided in a table, and you must identify all bonds broken and formed from the displayed formula.

Worked Example 1: Hydrogen + Chlorine

In the reaction H₂ + Cl₂ → 2HCl, one H–H bond (436 kJ/mol) and one Cl–Cl bond (242 kJ/mol) are broken, and two H–Cl bonds (431 kJ/mol each) are formed.
The calculation gives (436 + 242) - (2 × 431) = 678 - 862 = -184 kJ/mol, confirming the reaction is exothermic.

5. Worked Example 1: Hydrogen + Chlorine AQA

Notes

In the reaction H₂ + Cl₂ → 2HCl, one H–H bond (436 kJ/mol) and one Cl–Cl bond (242 kJ/mol) are broken, and two H–Cl bonds (431 kJ/mol each) are formed.
The calculation gives (436 + 242) - (2 × 431) = 678 - 862 = -184 kJ/mol, confirming the reaction is exothermic.

Worked Example 2: Nitrogen + Hydrogen (Haber Process)

In the reaction N₂ + 3H₂ → 2NH₃, one N N triple bond (941 kJ/mol) and three H–H bonds (436 kJ/mol each) are broken, while six N–H bonds (391 kJ/mol each) are formed.
The calculation gives (941 + 3 × 436) - (6 × 391) = 2249 - 2346 = -97 kJ/mol, confirming the reaction is exothermic.
Drawing displayed formulae for all reactants and products makes it easier to count every bond broken and formed accurately.

6. Worked Example 2: Nitrogen + Hydrogen (Haber Process) AQA

Notes

In the reaction N₂ + 3H₂ → 2NH₃, one N N triple bond (941 kJ/mol) and three H–H bonds (436 kJ/mol each) are broken, while six N–H bonds (391 kJ/mol each) are formed.
The calculation gives (941 + 3 × 436) - (6 × 391) = 2249 - 2346 = -97 kJ/mol, confirming the reaction is exothermic.
Drawing displayed formulae for all reactants and products makes it easier to count every bond broken and formed accurately.

AQA student objectives

All · Most · Some

Key knowledge

What Is Bond Energy?

Breaking Bonds — An Endothermic Process

Forming Bonds — An Exothermic Process

Calculating Overall Energy Change

Worked Example 1: Hydrogen + Chlorine

Worked Example 2: Nitrogen + Hydrogen (Haber Process)

Top Tips for Bond Energy Questions