Enthalpy Definitions

Question 1

Enthalpy of formation

Answer 1

Enthaply chnage when one mole of a substance is formed from its constituent elements with all substances in thier standard state. (Exothemric for most substances)
2Na(s) + 1/2O2 (g) –> Na2O(s)

Question 2

Enthalpy of combustion

Answer 2

Enthalpy change when one mole of a substance undergoes complete combustion in oxygen with all substances in standard states (Exothermic)
H2(g) + 1/2O2(g) –> H20(g)

Question 3

1st Ionisation energy

Answer 3

The first ionisation energy is the enthalpy change when one mole of gaseous loses one electron per atom to produce gaseous 1- ions. (Endothermic)
Mg(g) –> Mg+(g) + e-

Question 4

2nd Ionisation energy

Answer 4

The second ionisation energy is when one mole of gaseous 2+ ion is produced from one mole of 1+ (Endothermic)
Mg1+(g) –> Mg2+(g) + e-

Question 5

1st Electron affinity

Answer 5

The first electron affinity is the enthalpy chnage when one mole of gaseous 1- ions gains one electron per ion to produce gaseous 2- ions.
(Exothermic)
O(g) + e- –> O-(g)

Question 6

2nd Electron affinity

Answer 6

The second electron affinity is the enthalpy change when one mole og gaseous 1- ions gains one electron per ion to produce 2- ions
(Endothermic as adding -ve electron to +ve ion)
O-(g) e- –> O2-(g)

Question 7

Enthalpy of atomisation

Answer 7

Enthalpy chnage when one mole of gaseous atoms is produced from an element in its normal state
(Endo themric)
1/2 I2(s)–> 2I (g)

Question 8

Hydration enthaply

Answer 8

Enthalpy change when one mole of gaseous ions become hydrated (disovled in water
(Exothemric)
Mg2+(g) + aq –> Mg2+ (aq)

Question 9

Enthalpy of solution

Answer 9

Enthalpy change when one mole of an ionic solid dissovles in an amount of water large enough so that the dissolved ions are well seperated and do not interact with eachother.
(Varies)
MgCl2(s) +aq –> Mg2+(aq) + 2Cl-(aq)

Question 10

Bond dissociation enthalpy

Answer 10

Enthalpy chnage when one mole of covalent bonds is broken in the gaseous state. (Endothermic)
I2(g) –> 2I(g)

Question 11

Lattice enthalpy of formation

Answer 11

Enthalpy change when one mole of a solid ionic compund is formed into its ocnstituent ions in the gas phase (Exothemic)
Mg2+(g) = 2Cl-(g) –> MgCl2(s)

Question 12

Larrice enthalpy of disociation

Answer 12

Enthalpy chnage when one mole of a solid ionic compund is broken up into its constituent ions in the gas phase. (Endothermic)
MgCl2(s) –> Mg2+(g) + 2Cl-(g)