Energy transfer is a key concept in chemical reactions, where energy moves between the system (the reaction itself) and the surroundings (everything outside the reaction). According to the principle of conservation of energy:

This means that during a chemical reaction, energy within the system will either be absorbed from or released to the surroundings. If the reaction loses energy, that energy is transferred to the surroundings, often causing a change in temperature.

By measuring how the temperature changes, we can tell whether a reaction is exothermic or endothermic.

Activation energy The minimum amount of energy needed for bonds to break

• On a reaction profile, it's the difference between reactants and the highest point on the curve.
• (If energy is less than activation energy then there's not enough energy to start, so the reaction so doesn't happen).