Energy is conserved in chemical reactions. The total amount of energy in the universe at the end of a reaction is the same as it was before. Energy is only moved between the chemicals and their 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 increases. Combustion, many oxidation reactions and neutralisation are exothermic. Self-heating cans and hand warmers use exothermic reactions.
An endothermic reaction takes in energy from the surroundings, so the temperature of the surroundings decreases. Thermal decompositions and the reaction of citric acid with sodium hydrogencarbonate are endothermic. Some sports injury packs use an endothermic reaction to cool an injury.
Required practical 4: you can investigate temperature changes in reacting solutions, such as an acid with a metal or a carbonate, a neutralisation, or a displacement reaction. Use a lid and an insulated cup, and measure the volume and starting temperature of each solution. Stir, then record the highest or lowest temperature reached. Keep every other variable the same each time. A rise in temperature shows an exothermic reaction and a fall shows an endothermic reaction.