Enthalpy Changes

Explain the term exothermic

A reaction that releases heat energy to the surroundings (reactants lose energy).

Explain the term endothermic

A reaction that absorbs heat energy from the surroundings (reactants gain energy).

Explain the term activation energy

The minimum energy required for a chemical reaction to occur by breaking existing bonds.

State standard conditions for enthalpy measurements

Pressure of 100 kPa (1 atm), specified temperature (typically 298 K / 25°C), and concentration of 1.0 mol dm⁻³ for solutions.

Explain standard enthalpy change of reaction (ΔrH°)

The enthalpy change when a reaction occurs in the molar quantities shown in the chemical equation under standard conditions with reactants and products in standard states.

Explain standard enthalpy change of formation (ΔfH°)

The enthalpy change when one mole of a compound is formed from its constituent elements in their standard states under standard conditions.

Explain standard enthalpy change of combustion (ΔcH°)

The enthalpy change when one mole of a substance reacts completely with oxygen under standard conditions with all reactants and products in standard states.

Explain the term average bond enthalpy

The average energy required to break one mole of a specified covalent bond in gaseous molecules by homolytic fission.

Explain standard enthalpy change of neutralisation (ΔneutH°)

The energy change accompanying the reaction of an acid and base to form one mole of H2O(l) under standard conditions.

Explain why an enthalpy change is exothermic using bond energies

Energy released during bond formation in products is greater than energy absorbed during bond breaking in reactants.