C7.4 Bond Energy Calculations

A free GCSE Combined Science: Trilogy lesson for AQA (8464) from OBA School: (Higher tier) Breaking and making bonds, and calculating the overall energy change of a reaction from bond energies. Watch the video, read the explanation and worked examples, then try the revision questions. You can work through the full lesson, with every video, practice quizzes and printable worksheets with mark schemes, in our free online course.

What you will learn

• Explain that breaking bonds is endothermic and making bonds is exothermic
• Explain why a reaction is exothermic or endothermic using bond energies
• Calculate the overall energy change for a reaction from bond energies
• Work backwards from an energy change to find an unknown bond energy

Key words

• Bond energy: The energy needed to break one mole of a particular covalent bond, in kJ/mol. The same amount is released when that bond forms.
• Endothermic: Taking in energy. Breaking bonds is always endothermic.
• Exothermic: Giving out energy. Making bonds is always exothermic.
• Overall energy change: Energy taken in to break bonds − energy released making bonds. Negative means exothermic; positive means endothermic.

Breaking and making bonds (Higher tier)

• Breaking bonds needs energy, so it is endothermic.
• Making bonds releases energy, so it is exothermic.
• If more energy is released making new bonds than is needed to break the old ones, the reaction is exothermic overall.
• If more energy is needed to break bonds than is released making bonds, the reaction is endothermic overall.

• H–H; Energy (kJ/mol) 436; Bond C–H; Energy (kJ/mol) 413; Bond N≡N; Energy (kJ/mol) 945
• Cl–Cl; Energy (kJ/mol) 243; Bond O=O; Energy (kJ/mol) 498; Bond N–H; Energy (kJ/mol) 391
• H–Cl; Energy (kJ/mol) 432; Bond C=O; Energy (kJ/mol) 805; Bond C–C; Energy (kJ/mol) 347
• Br–Br; Energy (kJ/mol) 193; Bond O–H; Energy (kJ/mol) 464; Bond C=C; Energy (kJ/mol) 612
• H–Br; Energy (kJ/mol) 366; Bond F–F; Energy (kJ/mol) 158; Bond H–F; Energy (kJ/mol) 568

These are average values. In an exam you will always be given the bond energies you need.

Calculating the overall energy change (Higher tier)

Formula: Overall energy change = sum of bond energies of bonds broken − sum of bond energies of bonds made

• Step 1: write the displayed formulae and count every bond in the reactants and the products (multiply by the numbers in the balanced equation).
• Step 2: add up the energy needed to break all the bonds in the reactants.
• Step 3: add up the energy released making all the bonds in the products.
• Step 4: subtract: broken − made. Give the sign and units.

Worked example: hydrogen + chlorine → hydrogen chloride

H₂ + Cl₂ → 2HCl, i.e. H–H + Cl–Cl → 2 H–Cl
Bonds broken: 436 + 243 = 679 kJ/mol
Bonds made: 2 × 432 = 864 kJ/mol
Overall energy change = 679 − 864 = −185 kJ/mol

Answer: −185 kJ/mol; negative, so the reaction is exothermic

A harder example: burning methane (Higher tier)

CH₄ + 2O₂ → CO₂ + 2H₂O. Count the bonds in every molecule.

Worked example: combustion of methane

Bonds broken: 4 × C–H + 2 × O=O = (4 × 413) + (2 × 498) = 1652 + 996 = 2648 kJ/mol
Bonds made: 2 × C=O + 4 × O–H = (2 × 805) + (4 × 464) = 1610 + 1856 = 3466 kJ/mol
Overall energy change = 2648 − 3466 = −818 kJ/mol

Answer: −818 kJ/mol (exothermic)

Exam tip: Each water molecule has two O–H bonds, and CO₂ has two C=O double bonds. Most lost marks come from miscounting bonds.

An endothermic example (Higher tier)

Worked example: decomposing hydrogen bromide

2HBr → H₂ + Br₂, i.e. 2 H–Br → H–H + Br–Br
Bonds broken: 2 × 366 = 732 kJ/mol
Bonds made: 436 + 193 = 629 kJ/mol
Overall energy change = 732 − 629 = +103 kJ/mol

Answer: +103 kJ/mol: more energy is needed to break bonds than is released making bonds, so it is endothermic

Key fact: The reverse of a reaction has the same size of energy change with the opposite sign. So H₂ + Br₂ → 2HBr is −103 kJ/mol.

Working backwards (Higher tier)

Worked example: finding an unknown bond energy

H₂ + F₂ → 2HF. Overall energy change = −542 kJ/mol. H–H = 436, F–F = 158. Find the H–F bond energy.
Bonds broken = 436 + 158 = 594 kJ/mol. So 594 − (2 × H–F) = −542
2 × H–F = 594 + 542 = 1136, so H–F = 1136 ÷ 2 = 568 kJ/mol

Answer: H–F = 568 kJ/mol

Values calculated from bond energies are close to, but not exactly the same as, values measured in experiments, because bond energies are averages taken from many different compounds.

Common mistakes

• Doing “made − broken”. It is broken − made, so a negative answer means exothermic.
• Saying making bonds needs energy. Making bonds releases energy.
• Forgetting to multiply by the numbers in the balanced equation.
• Missing the sign or units: always write, for example, −185 kJ/mol.

Revision questions

Try these, then check your answers below each one.

Question 1. (Higher tier) How do you calculate the overall energy change from bond energies? (Choose from: Energy to break bonds in reactants − energy released making bonds in products, Energy released making bonds − energy to break bonds, Energy to break bonds + energy released making bonds, Energy of products − activation energy.)

Answer: Energy to break bonds in reactants − energy released making bonds in products

Working: Overall energy change = bonds broken − bonds made. If more energy is released making bonds, the answer is negative (exothermic).

Question 2. (Higher tier) H₂ + I₂ → 2HI. Bond energies: H–H = 436, I–I = 151, H–I = 298 kJ/mol. Calculate the overall energy change in kJ/mol.

Answer: -9 kJ/mol

Working: Broken: 436 + 151 = 587. Made: 2 × 298 = 596. Overall = 587 − 596 = −9 kJ/mol (slightly exothermic).

Question 3. (Higher tier) True or false? A positive overall energy change means the reaction is endothermic.

Answer: True

Working: True. More energy is needed to break bonds than is released making bonds, so energy is taken in overall.

Related lessons

• Previous lesson: C7.3 Reaction Profiles
• Next lesson: C7 RP Required Practical: Temperature Changes
• In this chapter: C7.1 Exothermic and Endothermic Reactions
• In this chapter: C7.2 Using Energy Transfers from Reactions

Continue in the full course

This lesson is section C7.4 (AGCSC2404) of our free GCSE Combined Science course for AQA. In the full course you get every video for this section, a self-marking practice quiz, a printable worksheet with a mark scheme, chapter tests, and your progress saved. Go to the full GCSE Combined Science course.