Chemical reactions involve energy changes; during a chemical reaction energy is transferred to or from the surroundings and the temperature changes. For example, when we turn on the gas on our kitchen hob, a chemical reaction, called combustion or simply burning, takes place. Combustion transfers heat to its surroundings, so our food on top of it takes in the heat and cooks. The temperature in our cooking pot rises.
Reactions that transfer energy to their surroundings, like combustion, are exothermic reactions. Neutralisation is another type of exothermic reaction; this is the reaction between acids and alkalis.
The heat energy is released from the atoms bonding to make the products. The reactants actually took in energy in order to break the bonds between them, but this amount of energy was smaller than the energy released when making the products. The temperature of the mixture of an exothermic reaction rises and so does the temperature of the surroundings.
The opposite happens in endothermic reactions. The reactants in endothermic reactions take in more energy to break the bonds between them. This energy is taken from the surroundings. Therefore, the mixture and the surroundings get colder, as the temperature drops. When the bonds in the products form, less energy is released than when reactant bonds break. Electrolysis and thermal decomposition are two types of endothermic reactions.
The following diagram shows the energy changes in the reaction mixtures of exothermic and endothermic reactions.
Study the diagram carefully; this is where it can get tricky. The diagram shows that the reaction energy decreases for exothermic reactions and increases for endothermic. Then how is it possible to have a temperature rise in exothermic reactions and a temperature fall in endothermic? The diagram shows the energy changes on a molecular level. It is true that in exothermic reactions more energy is released when the products form. This energy transfers to the surroundings, so we observe a temperature rise. However, the energy inside the molecules themselves decreases, because it is released to the immediate environment.
Equally, product molecules in endothermic reactions take in a lot of energy when they are formed. This is taken from the surroundings, so a temperature drop is noted.