Energy cannot be created or destroyed in a chemical reaction. The total amount of energy after the reaction is the same as before it. Energy is only shared out differently between the chemicals and the surroundings. If a reaction transfers energy to the surroundings, the products must have less energy than the reactants, by exactly the amount transferred.
An exothermic reaction transfers energy to the surroundings, so the temperature of the surroundings goes up. Combustion, many oxidation reactions and neutralisation are exothermic. Self-heating cans and hand warmers use exothermic reactions, because the energy released warms the can contents or your hands.
An endothermic reaction takes in energy from the surroundings, so the temperature of the surroundings goes down. Thermal decompositions and the reaction between citric acid and sodium hydrogencarbonate are endothermic. Some sports injury packs use an endothermic reaction to make a cold pack that cools the injury. A drawback is that the pack can only be used once.
Required practical 10 investigates the variables that affect the temperature change when solutions react, for example an acid with a metal or a carbonate, a neutralisation, or a metal displacement. Use a polystyrene cup with a lid to reduce energy loss. Measure the same volume of solution each time and record the starting temperature. Stir, then record the highest or lowest temperature reached. You only need the temperature change, not a calculation of the energy change.