Standard Enthalpy Change (DP IB Chemistry): Revision Note

Standard enthalpy change

• The standard enthalpy change for a chemical reaction, ΔH^Ꝋ_, refers to the heat transferred at constant pressure under standard conditions and states

• These standard conditions are:

  • A pressure of 100 kPa

  • A concentration of 1 mol dm^-3 for all solutions

  • Each substance involved in the reaction is in its standard state (solid, gas or liquid)

• Temperature is not part of the definition of standard state, but a temperature of 298.15 K is usually given as the specified temperature

• To show that a reaction has been carried out under standard conditions, the symbol ⦵ is used

  • E.g. ΔH^Ꝋ = the standard enthalpy change

Standard enthalpies

• The standard enthalpy changes that are commonly used in energy calculations are:

Standard enthalpy of reaction

• The enthalpy change when the reactants in the stoichiometric equation react to give the products under standard conditions

• Symbol = ΔH^Ꝋ_r

• Can be exothermic or endothermic

Standard enthalpy of formation

• The enthalpy change when one mole of a compound is formed from its elements under standard conditions

• Symbol = ΔH^Ꝋ_f

• Can be exothermic or endothermic

Standard enthalpy of combustion

• The enthalpy change when one mole of a substance is burnt in excess oxygen under standard conditions

• Symbol = ΔH^Ꝋ_c

• Always exothermic

Standard enthalpy of neutralisation

• The enthalpy change when one mole of water is formed by reacting an acid and alkali under standard conditions

• Symbol = ΔH^Ꝋ_neut

• Always exothermic