Exothermic and endothermic reactions
· Energy is always present in a reaction because it is always transferred when chemical bonds are formed or broken.
· Sometimes, the energy is transferred from the reacting chemicals to the surroundings. We call these exothermic reactions. The reaction heats up the surroundings.
· Some reactions, take energy from the surroundings to the reacting chemicals. We call these endothermic reactions. Because they take in energy from their surroundings, these reactions cause a drop in temperature as they happen.
· Fuels burning are an example of an exothermic reaction.
· Neutralisation of acids and alkalis are exothermic.
· Respiration is an example of an exothermic reaction. It reacts sugar and oxygen inside cells to provide energy.
· These reactions are much less common.
· When we dissolve ionic compounds in water, the temperature of the solution drops.
· Thermal decomposition reactions are endothermic.
· The most important example of thermal decomposition is photosynthesis. This reaction uses energy from the sun (surroundings) to turn carbon dioxide and water into sugar and oxygen.
Energy and reversible reactions
· This can be seen when we heat blue copper sulphate crystals. The crystals contain water as part of the lattice formed when they crystallise. We say the copper sulphate drives off water from the crystals to produce anhydrous copper sulphate.
· When we add water to anhydrous copper sulphate we form hydrated copper sulphate. This colour change in the reaction is a useful test for water. This reaction is exothermic.
Energy and equilibrium
We have a closed system when nothing is added or taken away from the reaction mixture. In a closed system the relative amounts of the reactants and products in a revisable reaction depend on the temperature.