Energy changes in chemical reactionsEnergy and chemical bonds
Exothermic reactions transfer energy to the surroundings and the temperature of the surroundings increases. Endothermic reactions take in energy and the temperature of the surroundings decreases.
bonds in the reactantA substance that reacts together with another substance to form products during a chemical reaction. are broken
new bonds are made in the productA substance formed in a chemical reaction.
Energy is absorbed to break bonds. Bond-breaking is an endothermic process.
Energy is released when new bonds form. Bond-making is an exothermic process.
Whether a reaction is endothermic or exothermic depends on the difference between the energy needed to break bonds and the energy released when new bonds form.