• To fairly compare the changes in enthalpy between reactions, all reactions should be carried out under standard conditions
• These standard conditions are:
  • A?pressure?of 101 kPa
  • A?temperature?of 298 K (25?^oC)
  • Each substance involved in the reaction is in its normal physical state (solid, gas or liquid)
• To show that a reaction has been carried out under standard conditions, the symbol ? is used
  • ΔH^??= the standard enthalpy change
• These are a number of key definitions for common language relating to enthalpy change that all chemists need to know

Enthalpy definitions table

Worked example: Calculating the enthalpy change of reaction of water

Answer

Since two moles of water molecules are formed in the question above, the energy released is simply:

ΔHr^??= 2 mol x (-286 kJ mol^-1)= -572 kJ mol^-1

Worked example: Calculating the enthalpy change of formation

Answer

Since two moles of Fe2O3?(s) are formed the total change in enthalpy for the reaction above is:

ΔHf^??=? 2 x ( -824.2 kJ mol^-1)= - 1648 kJ

Worked example: Calculating enthalpy changes

Answer

Answer 1:?ΔHr^?

Answer 2:?ΔHr^??as one mole of CO₂?is formed from its elements in standard state and ΔHc^??as one mole of carbon is burnt in oxygen

Answer 3:?ΔHneut^??as one mole of water is formed from the reaction of an acid and alkali