Heat Energy Changes (Edexcel GCSE Chemistry: Combined Science)

• Chemical reactions occur so that elements can achieve a more stable energy state by gaining a full outer shell of electrons
• This is done by chemical bonding (we have already seen ionic and covalent bonding) where old bonds are broken, and new bonds are formed
• This process involves the transfer of energy into and out of reaction mixtures
• The terms used to describe this are the system (what happens in the chemical reaction) and the surroundings
• The energy comes from the chemical bonds themselves which could be considered as tiny stores of chemical energy
• In the majority of reactions, the energy is in the form of heat energy, although sometimes other types of energy are produced such as light or sound
• The changes in heat can be observed and measured with a thermometer and simple calorimeter

Diagram showing the apparatus for the calorimetry investigation for displacement, dissolving and neutralisation

• The following are some examples of heat changes in reactions
  • Salts dissolving in water:
    • These can either take energy in or give it out

  • Neutralisation reactions:
    • These always give energy out

  • Displacement reactions:
    • These can either take energy in or give it out

  • Precipitation reactions:
    • These always give energy out

• In exothermic reactions energy is given out to the surroundings so the temperature of the surroundings increases
• Combustion, oxidation, and neutralisation reactions are typical exothermic reactions
• Hand warmers used in the wintertime are based on the release of heat from an exothermic reaction
• Self-heating cans of food and drinks such as coffee and hot chocolate also use exothermic reactions in the bases of the containers

Diagram showing the transfer of heat energy outwards from an exothermic reaction

• In endothermic reactions energy is taken in from the surroundings so the temperature of the surroundings decreases
• These types of reactions are much less common than the exothermic reactions
• Electrolysis, thermal decomposition reactions and the first stages of photosynthesis are typical endothermic reactions
• Sports injury treatment often use cold packs based on endothermic reactions to take heat away from a recently injured area to prevent swelling

Diagram showing the transfer of heat energy from the surroundings into an endothermic reaction