7.4 Bond Energy Calculations

A free GCSE Chemistry lesson for AQA (8462) 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 in terms of bond energies
• Use bond energies to calculate the overall energy change for a reaction
• Work backwards from an energy change to find an unknown bond energy
• Scale an energy change to a given number of moles or mass

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.

Breaking and making bonds (Higher tier)

During a chemical reaction, bonds in the reactants are broken and new bonds in the products are made.

• Breaking bonds needs energy, so it is endothermic.
• Making bonds releases energy, so it is exothermic.
• If more energy is released making bonds than is taken in breaking bonds, 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 C–O; Energy (kJ/mol) 358

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: draw or 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. Remember to 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.

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

Energy for a given amount (Higher tier)

An energy change in kJ/mol is for the amounts in the balanced equation. For the combustion of methane above, −818 kJ is released when 1 mol of CH₄ burns. To find the energy for another amount, find the number of moles first.

Worked example: energy released by a mass of fuel

How much energy is released when 4.0 g of methane burns? (Mr CH₄ = 16)
Moles of CH₄ = mass ÷ Mr = 4.0 ÷ 16 = 0.25 mol
Energy released = 0.25 × 818 = 204.5 kJ

Answer: about 205 kJ released

Exam tip: If an equation makes 2 mol of product, the energy change per mole of product is half the value for the equation. Always check which amount the kJ/mol value refers to.

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 number of molecules in the balanced equation.
• Missing the sign or units: always write e.g. −185 kJ/mol.

Revision questions

Try these, then check your answers below each one.

Question 1. (Higher tier) Which statement is correct? (Choose from: Breaking bonds is endothermic and making bonds is exothermic, Breaking bonds is exothermic and making bonds is endothermic, Both breaking and making bonds are exothermic, Both breaking and making bonds are endothermic.)

Answer: Breaking bonds is endothermic and making bonds is exothermic

Working: Breaking bonds needs energy (endothermic); forming bonds releases energy (exothermic).

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

Answer: -9 kJ/mol

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

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

Answer: False

Working: False. Negative (broken − made < 0) means exothermic.

Related lessons

• Previous lesson: 7.3 Reaction Profiles
• Next lesson: 7.5 Chemical Cells and Batteries
• In this chapter: 7.1 Exothermic and Endothermic Reactions
• In this chapter: 7.2 Using Energy Transfers from Reactions
• In this chapter: 7.6 Fuel Cells

Continue in the full course

This lesson is section 7.4 (AGCCH0704) of our free GCSE Chemistry 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 Chemistry course.