C6. Energy changes in chemical reactions

Exothermic - heat is given offEndothermic - heat is taken off

Change in heat to the water = mass of water x specific heat capacity of water x rise in temperature in water 

Two stages of a chemical reactionStage 1: Energy needed to break bonds Stage 2: Energy released to form bonds Chemical energy => Heat (thermal energy) and vice versa 

• When chemical reactions occur, as well as the formation of the products - the chemical change, there is also a heat energy change which can often be detected as a temperature change. • This means the products have a different energy content than the original reactants  • If the products contain less energy than the reactants, heat is released or  given out to the surroundings and the change is called exothermic. The temperature of the system will be observed to rise in an exothermic change.

• If the products contain more energy than the reactants, heat is taken in or absorbed from the surroundings and the change is called endothermic. If the change can take place spontaneously, the temperature of the reacting system will fall but, as is more likely, the reactants must be heated to speed up the reaction and provide the absorbed heat.

• The difference between the energy levels of the reactants and products gives the overall energy change for the reaction • the heat change is called the enthalpy change is denoted by delta H, ΔH. o ΔH is negative (-ve) for exothermic reactions i.e. heat energy is given out and lost from the system to the surroundings which warm up. o ΔH is positive (+ve) for endothermic reactions i.e. heat energy is gained by the system and taken in from the surroundings which cool down OR, as is more likely, the system is heated to provide the energy needed to effect the change.

Energetics of a Reaction