What is the standard enthalpy change of neutralisation defined as?

• A. The enthalpy change when a salt hydrates.
• B. The enthalpy change when one mole of water is formed under standard conditions by reaction of an acid with an alkali.
• C. The enthalpy change of atomisation.
• D. The enthalpy change of combustion.

Answer: (B)

The standard enthalpy change of neutralisation is the energy change when one mole of water is formed in an acid–base reaction under standard conditions. It comes from the H+ ions from the acid reacting with OH− ions from the base to produce water, and it is measured per mole of water formed (usually at 298 K and 1 atm, with solutions at 1 M). For strong acid–strong base pairs, this value is consistently about −57 kJ per mole of water because the major energy contribution is the formation of water from H+ and OH− and the ions are fully solvated. With weak acids or bases, the observed neutralisation can be less exothermic because some of the energy goes into additional steps like initial ionisation and solvent reorganisation.

This definition is distinct from the hydration enthalpy of a salt, the atomisation enthalpy (breaking molecules into atoms), or the combustion enthalpy (energy released when a substance burns).