Exothermic and endothermic reactions

5.1.1 Energy transfer during exothermic and endothermic reactions

5.1.2 Reaction profiles

5.1.3 The energy change of reactions (HT only)


  • Eg thermal decomposition, citric acid + NaHCO3
  • Take in energy from surroundings
  • Temp ↓
  • Products more energy than reactants
  • Energy taken to break bonds more than energy released when making bonds


  • Combustion (O2 + heat), neutralisation
  • Gives out energy to surroundings
  • Temp ↑
  • Products less energy than reactants
  • Energy released to make bonds more than energy taken when breaking bonds

Activation energy (Ea) – min energy required to start a reaction

Enthalpy change = ∑ reactants – ∑ products

Describe an experiment that the student could do to prove that this reaction is exothermic.

To gain full marks in this question you should write your ideas in good English. Put them into a sensible order and use the correct scientific words. (6)

  1. Pour cold water into a beaker
  2. Take temp of water at the start using a thermometer
  3. Pour soup power into beaker of cold water and mix it with a glass rod
  4. Take temp of the solution with a thermometer
  5. The temp increases which indicate that it’s an exothermic reaction

How to reduce heat loss? (1)

  • Add a lid / use insulation

Suggest why it’s important to stir the chemicals thoroughly (1)

  • To make sure all substances are fully reacted

Why scientists’ conclusion immediately accepted? (2)

  • They had proof
  • Experiment can be repeated

How reliability of work confirmed? (1)

Experiment was repeated

Describe overall reaction (2)

  • Net energy transfer
  • Exo/endo reaction

Size and strength

  • Cl atoms have fewer electron energy levels / shells
  • Cl atoms form stronger bonds
  • Cl-Cl bond stronger than Br-Br
  • C-Cl bond stronger than C-Br

Energy required

  • More energy required to break bonds with Cl
  • More energy given out when making bonds with Cl
  • Overall energy change depends on size of energy changes


  • If C-Cl bond changes less → exothermic
  • If C-Cl bond changes more → endothermic
  • Can’t tell how overall energy change will differ as don’t know which changes more

X – energy required to break bonds of water (activation energy)

Y – Energy released when hydrogen + oxygen formed

Z: 1856-1370=486 kJ

Overall, reaction is endothermic