3.6 Enthalpy changes for solids and solutions

Hess’s Law

The enthalpy change of a reaction is independent of the route of the reaction, as long as the initial and final conditions are the same.

Standard enthalpy change of combustion

When one mole of a substance is completely burned in oxygen under standard temperature and pressure.

Standard enthalpy change of formation

When one mole of a substance is made from its elements in their standard states, under standard conditions.

Standard enthalpy change of lattice formation

When one mole of an ionic solid is formed from its gaseous ions under standard conditions.

Standard enthalpy change of lattice breaking

Changing one mole of a solid ionic compound into its gaseous ions, under standard conditions.

Standard enthalpy change of atomisation

When one mole of a gaseous atom, under standard conditions, is formed from the element in its standard state. 

Standard first ionisation energy

When one mole of electrons are removed from one mole of gaseous ions.

Electron affinity

When one mole of gaseous atoms gain one mole of electrons to form one mole of gaseous negative ions.

Standard enthalpy change of hydration

enthalpy change when one mole of gaseous ions dissolves in water. 

Standard enthalpy change of solution

When one mole of substance dissolves in an excess of water under standard conditions to form an infinitely dilute solution.

ionic solubility

The solubility of ionic compounds changes depending on the balance of lattice breaking enthalpy and enthalpy of hydration of that substance. This is because the lattice of the first compound has to be broken down into individual ions, then it can be hydrated into aqueous ions in a solution.