Enthalpy Change Definitions

Question 1

Enthalpy of formation

ΔH f

Answer 1

Enthalpy change when one mole of a substance is formed from its constituent elements in their standard states under standard conditions, all reactants and products being in their standard states

Question 2

Enthalpy of combustion

ΔH c

Answer 2

Enthalpy change when one mole of a substance undergoes complete combustion in oxygen under standard conditions, all reactants and products being in their standard state

Question 3

Ionisation enthalpy

ΔH i

Answer 3

The first ionisation energy is the enthalpy change when one mole of gaseous atoms loses one electron per atom to produce gaseous 1+ ions

The second ionisation energy is when one mole of gaseous 2+ ions is produced from one mole of 1+ ions

Question 4

Electron affinity

ΔH ea

Answer 4

The first electron affinity is the enthalpy change when one mole of gaseous atoms gains one electron per atom to produce gaseous 1- ions

The second electron affinity is the enthalpy change when one mole of gaseous 1- ions gains one electron per ion to produce gaseous 2- ions

Question 5

Enthalpy of atomisation

ΔH a

Answer 5

Enthalpy change when one mole of gaseous atoms is produced from an element under stand conditions, all reactants and products bring in their standard states

Question 6

Hydration enthalpy

ΔH hyd

Answer 6

Enthalpy change when one mole of gaseous ions become hydrated (dissolved in water)

This always gives out energy (exothermic, -ve)

Question 7

Enthalpy of solution

ΔH sol

Answer 7

Enthalpy change when one mole of an ionic solid dissolves in an amount of water large enough so that the dissolved ions are well separated and do not interact with each other

Question 8

Bond dissociation enthalpy

ΔH dis

Answer 8

Enthalpy change when one mole of covalent bonds is broken in the gaseous state

This is breaking a bond so always requires energy (endothermic, +ve)

Question 9

Lattice enthalpy of formation

ΔH L

Answer 9

Enthalpy change when one mole of a solid ionic compound is formed into its constituent ions in the gas phase

Bonds are bring formed, so ΔH is -ve

Sometimes lattice enthalpy is defined as lattice enthalpy of dissociation when the ionic compound is split into its ions (this is +ve as bonds are broken)