C5.1.1 - Energy change in endothermic + exothermic reaction

• Energy is conserved in chemical reactions. The amount of energy in the universe at the end of a chemical reaction is the same as before the reaction takes place. If a reaction transfers energy to the surroundings the product molecules must have less energy than the reactants, by the amount transferred.
• An endothermic reaction is one in which heat energy is taken in (absorbed) from its surroundings, causing the temperature of the surroundings to decrease.

  This decrease in temperature can be measured with a thermometer and used to calculate the amount of heat energy transferred to the system. One example of an endothermic reaction is the melting of ice in a glass of water. The ice absorbs heat energy from the water and the water becomes colder. The surroundings (the water in the glass) have transferred heat energy to the system (the ice). Another example is electrolysis.
• An exothermic reaction is a reaction which releases heat energy. The heat energy released by the system is transferred from the system into the surroundings causing the temperature of the surroundings to increase. The temperature rise of the surroundings can be measured using a thermometer and used to calculate the amount of heat energy transferred. Examples of exothermic reactions include:

  • Combustion reactions
  • Neutralisation reactions
  • Reaction between water and calcium oxide