Bond Energies

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CognitoBond energies — calculating energy change

Key facts & methods

Bond energy calculations

Bond energies (kJ/mol) are the energy needed to break 1 mole of a specific bond.
Energy required = sum of bond energies of all bonds broken (in reactants).
Energy released = sum of bond energies of all bonds formed (in products).
Overall energy change (ΔH) = energy required to break bonds − energy released on forming bonds.
If ΔH is negative → exothermic. If ΔH is positive → endothermic.

Worked example

H₂ + Cl₂ → 2HCl.
Bonds broken: 1 × H−H (436 kJ/mol) + 1 × Cl−Cl (243 kJ/mol) = 679 kJ.
Bonds formed: 2 × H−Cl (432 kJ/mol each) = 864 kJ.
ΔH = 679 − 864 = −185 kJ/mol.
Negative → energy released → exothermic reaction.

Common bond energies to know

H−H: 436 kJ/mol. Cl−Cl: 243 kJ/mol. H−Cl: 432 kJ/mol.
C−H: 413 kJ/mol. C−C: 347 kJ/mol. C=C: 614 kJ/mol.
C−O: 358 kJ/mol. C=O: 805 kJ/mol. O−H: 464 kJ/mol.
O=O: 498 kJ/mol. N≡N: 945 kJ/mol. N−H: 391 kJ/mol.
Double bonds (C=C, C=O) have higher bond energies than single bonds.

Exam questions — 4 questions · 11 marks · AQA 8462 style

4 marksBond energy calculationAQA 8462 P2 style 🔢 Calculator

Use the bond energies to calculate the overall energy change for: CH₄ + 2O₂ → CO₂ + 2H₂O. Bond energies: C−H = 413, O=O = 498, C=O = 805, O−H = 464 kJ/mol. (4 marks)

Hint: Step 1: list all bonds broken in reactants (CH₄ has 4 C-H, 2O₂ has 2 O=O). Step 2: list all bonds formed in products (CO₂ has 2 C=O, 2H₂O has 4 O-H). Step 3: ΔH = broken − formed.

+40 XP

1 markExothermic vs endothermic from ΔHAQA 8462 style

The bond energy calculation gives ΔH = +50 kJ/mol. What does this tell you?

A The reaction is exothermic — energy is released to the surroundings

B The reaction is endothermic — more energy was needed to break bonds than was released forming new bonds

C The reaction is exothermic because bonds were formed

D The reaction is neither exothermic nor endothermic

3 marksExplain bond energy conceptAQA 8462 style

Explain why bond breaking is endothermic and bond forming is exothermic, and how these two processes determine whether an overall reaction is exothermic or endothermic. (3 marks)

Hint: Breaking = always requires energy. Forming = always releases energy. The difference determines the overall energy change.

+30 XP

Bond energy calculations

Worked example

Common bond energies to know

Module complete! 🎉