Introduction
Bond energies are used to calculate the energy changes that occur during chemical reactions. GCSE Chemistry students need to understand that energy is required to break bonds and that energy is released when new bonds form. Bond energy calculations are a common exam topic and help explain whether reactions are exothermic or endothermic.
Summary
• Bond energies measure the energy required to break chemical bonds.
• Breaking bonds requires energy.
• Forming bonds releases energy.
• Bond energies can be used to calculate overall energy changes.
• Bond energy calculations help identify exothermic and endothermic reactions.
• Bond energies are commonly tested in GCSE exam questions.
What Are Bond Energies?
A bond energy is the amount of energy required to break one mole of a particular bond in gaseous molecules. Different bonds require different amounts of energy to break because some bonds are stronger than others.
For GCSE Chemistry, students are usually provided with a table of bond energies and asked to use the values in calculations. The questions could be to work out the energy change for a specific reaction or it could be the energy change is given and they have to work out an unknown bond energy.
Breaking Bonds And Forming Bonds
Breaking chemical bonds requires energy, making bond breaking an endothermic process. Forming new bonds releases energy, making bond formation an exothermic process.
A useful way to remember this is that energy must be put in to separate atoms, if you were to break a pencil in two you would have to use energy to do this, in the same way as breaking a bond would take energy. Energy is released when new bonds form as the atoms are more stable when bonded then when present as individual atoms.
How To Calculate Energy Changes Using Bond Energies
To calculate the overall energy change of a reaction:
1. If the reaction is not written out fully showing all the bonds then draw out all the compounds
2. Count the number of each type of bond present in the reactants, remember to multiply the number of bonds in a compound but the balancing.
3. Look up the bond energies for the bonds in the reactants and use these to calculate the total energy required to break all bonds in the reactants.
4. Count the number of each type of bond present in the products, remember to multiply the number of bonds in a compound but the balancing.
5. Look up the bond energies for the bonds in the products and use these to calculate the total energy released when all the bonds in the products are formed.
6. Subtract the energy released from the energy required.
Energy Change = Energy Needed To Break Bonds – Energy Released When Bonds Form
This calculation helps explain the overall energy changes seen in exothermic and endothermic reactions.
Worked Example 1
Calculate the energy change when hydrogen reacts with chlorine to form hydrogen chloride using the following bond energies:
| Type of bond | Bond energy / kJ/mol |
| H-H | 436 |
| Cl-Cl | 243 |
| H-Cl | 432 |
1. Show the balanced equation with the bonds
H-H + Cl-Cl → 2H-Cl
2. bonds broken are 1 H-H and 1 Cl-Cl
3. Energy required to break bonds = 436+ 243 = 679
4. bonds broken are 2 H-Cl
5. Energy released when new bonds are formed = 2*432 =864
6. Overall energy change = 679– 864 = -185 kJ/mol
Worked Example 2
Calculate the energy change when hydrogen reacts with oxygen to form water using the following bond energies:
| Type of bond | Bond energy / kJ/mol |
| H-H | 436 |
| O=O | 498 |
| O-H | 464 |
1. Show the balanced equation with the bonds
2H-H + O=O → 2H-O-H
2. bonds broken are 2 H-H and 1 O=O
3. Energy required to break bonds = 2×436 + 498 = 1370
4. bonds broken are 4 O-H (2 O-H per water and 2 water molecules)
5. Energy released when new bonds are formed = 4*464 = 1856
6. Overall energy change = 1370 – 1856 = -486 kJ/mol
Exothermic And Endothermic Reactions
If more energy is released when new bonds form than is required to break the original bonds, the reaction is exothermic. See our exothermic reaction guide for more details on exothermic reactions
If more energy is needed to break bonds than is released when new bonds form, the reaction is endothermic. See our endothermic reaction guide for more details on endothermic reactions
This explains the energy changes seen on reaction profile diagrams. See our reaction profiles guide for more details
Common GCSE Exam Mistakes
• Forgetting that breaking bonds requires energy.
• Forgetting that forming bonds releases energy.
• Mixing up exothermic and endothermic reactions.
• Missing bonds when counting reactants or products.
• Counting the number of bonds incorrectly in either reactants or produces
• Making calculation errors when adding bond energies.
Exam Tips
• Learn the bond energy equation.
• Carefully count every bond shown in the chemical equation.
• Show all working in calculations. You will usually get error carried forward from earlier steps
• Check whether the final answer indicates an exothermic or endothermic reaction.
• Remember that bond energies are usually given in the exam.
How Bond Energies Link To Other GCSE Chemistry Topics
- Energy changes
- Exothermic reactions
- Endothermic reactions
- Reaction profiles
- Activation energy
- Chemical bonding
Final Thoughts
Bond energies help GCSE Chemistry students understand why reactions release or absorb energy. By learning how to calculate energy changes using bond energies, students can better explain exothermic and endothermic reactions and tackle calculation-based exam questions with confidence. For a detailed overview of other topics, see our GCSE Chemistry revision guide.
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