Enthalpy definition

Question 1

Enthalpy of formation (∆Hf):

Answer 1

Enthalpy change when one mole of a substance is formed from its constituent elements with all substances in their standard states

Exothermic (-ve) for most substances

e.g. Na2O(s)
2 Na(s) + ½ O2(g) → Na2O(s)

Question 2

Enthalpy of combustion (∆Hc)

Answer 2

Enthalpy change when one mole of a substance undergoes complete combustion in oxygen with all substances in standard states

Exothermic (-ve)

e.g. hydrogen
H2(g) + ½ O2(g) → H2O(g)

Question 3

First Ionisation enthalpy (∆Hi)

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.

Endothermic (+ve)

e.g. magnesium
Mg(g) → Mg+(g) + e-

Question 4

Second Ionisation enthalpy (∆Hi)

Answer 4

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

Endothermic (+ve)

e.g. magnesium
Mg+(g) → Mg2+(g) + e-

Question 5

First electron affinity (∆Hea)

Answer 5

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

Exothermic (-ve) for many non-metals

e.g. oxygen
O(g) + e- → O-(g)

Question 6

Second electron affinity (∆Hea)

Answer 6

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.

Endothermic (+ve) as adding –ve electron to –ve ion

e.g. oxygen
O-(g) + e- → O2-(g)

Question 7

Enthalpy of atomisation (∆Ha)

Answer 7

Enthalpy change when one mole of gaseous atoms is
produced from an element in its normal state.

Endothermic (+ve)

e.g. iodine
½ I2(s) → I(g)

Question 8

Hydration enthalpy (∆Hhyd)

Answer 8

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

Exothermic (-ve)

e.g. magnesium ions
Mg2+(g) + aq → Mg2+(aq)

Question 9

Enthalpy of solution (∆Hsol)

Answer 9

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

Varies

e.g. magnesium chloride
MgCl2(s) + aq → Mg2+(aq) + 2 Cl-(aq)

Question 10

Bond dissociation enthalpy (∆Hdis)

Answer 10

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

Endothermic (+ve)

e.g. I-I
bond I2(g) → 2 I(g)

Question 11

Lattice enthalpy of formation (∆HL)

Answer 11

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

Exothermic (-ve)

e.g. magnesium chloride
Mg2+(g) + 2 Cl-(g) → MgCl2(s)

Question 12

Lattice enthalpy of disociation (∆HL)

Answer 12

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

Endothermic (+ve)

e.g. magnesium chloride MgCl2(s) → Mg2+(g) + 2 Cl-(g)