Exothermic and endothermic reactions

Exothermic reaction

A reaction that releases energy to its surroundings, causing the surroundings to get hotter.

Endothermic reaction

A reaction that absorbs energy from its surroundings, causing the surroundings to get colder.

ΔH

The change in enthalpy of a reaction; positive if heat is absorbed, negative if heat is released.

Enthalpy of reaction

The heat energy change (ΔH) that occurs when reactants are converted to products.

Activation energy (Ea)

The minimum energy required for a reaction to occur.

Bond energy

The amount of energy needed to break a chemical bond, equal to the energy released when the bond is formed.

Characteristics of exothermic reactions

Heat is given out, temperature rises, and the container feels hot.

Characteristics of endothermic reactions

Heat energy is absorbed, temperature falls, and the container feels cold.

Example of an exothermic reaction

Neutralization of sodium hydroxide by hydrochloric acid (NaOH + HCl → NaCl + H2O, ΔH = -56.28 kJ mol-1).

Example of an endothermic reaction

The reaction between hydrogen and iodine (H2 + I2 → 2HI, ΔH = +26.50 kJ mol-1).