GCSE Combined Science (AQA)

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Bond Energy Calculations

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Energy changes Exothermic and endothermic reactions

Bond Energy Calculations

9:09 Bond Energies
Spec 4.5.1.3
Previous Energy in Bonds Next Properties of Hydrocarbons
  • Energy change in a reaction can be calculated using bond energies.
  • Bond energy is the amount of energy needed to break one mole of a given covalent bond.
  • Energy is needed to break bonds.
  • Example: 412 kJ to break one mole of C-H bonds, 799 kJ to break one mole of C=O double bonds.
  • Total energy needed to break bonds in a molecule is the sum of bond energies for all bonds in the molecule.
  • Example: Methane (CH4) has 4 C-H bonds, total energy to break = 4 x 412 kJ = 1,648 kJ.
  • Example: Carbon dioxide (CO2) has 2 C=O double bonds, total energy to break = 2 x 799 kJ = 1,598 kJ.
  • Energy released when forming a bond is the same as the energy needed to break the bond.
  • Total energy released when forming a molecule is the sum of energy released when each bond forms.
  • Example: Methane formation releases 1,648 kJ, CO2 formation releases 1,598 kJ.
  • During reactions, reactant bonds are broken and product bonds are formed.
  • Overall energy change = energy needed to break reactant bonds - energy released when forming product bonds.
  • Exothermic reaction: less energy needed to break reactant bonds than released forming product bonds.
  • Example: H2 + Br2 -> 2 HBr, energy change = 629 kJ (breaking) - 732 kJ (forming) = -103 kJ (exothermic).
  • Endothermic reaction: more energy needed to break reactant bonds than released forming product bonds.
  • Example: Reverse of HBr formation, energy change = 732 kJ (breaking) - 629 kJ (forming) = +103 kJ (endothermic).
  • Example calculation: 2 H2 + O2 -> 2 H2O.
  • Step 1: Energy needed to break reactant bonds = 2 x 432 kJ (H-H) + 496 kJ (O=O) = 1,360 kJ.
  • Step 2: Energy released forming product bonds = 2 x 2 x 463 kJ (H-O) = 1,852 kJ.
  • Step 3: Energy change = 1,360 kJ - 1,852 kJ = -492 kJ.
  • Step 4: Negative energy change indicates exothermic reaction.
  • Exothermic reaction: more energy released by bond formation than taken in to break bonds, energy released to surroundings.

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