Thermodynamics

Question 1

Enthalpy of formation

Answer 1

when 1 mole of substance is formed from its constituent elements under standard conditions with all substances in their standard states

• exothermic for most substances
  e. g. 2Na(s) + 1/2O2(g) –> Na2O(s)

Question 2

Enthalpy of combustion

Answer 2

when 1 mole of substance undergoes complete combustion in oxygen under standard conditions with all substances in their standard states

• exothermic (-ve)
  e. g. H2(g)+ 1/2O2(g)—> H2O(g)

Question 3

enthalpy of neutralisation

Answer 3

enthalpy change when one mole of water is formed in a reaction between an acid and alkali under standard conditions

• exothermic (-ve)
  e. g. 1/2H2SO4 + NaOH—> 1/2Na2SO4 + H2O

Question 4

First Ionisation energy

Answer 4

enthalpy change when each atom in 1 mole of gaseous atoms loses one electron to form one mole of gaseous 1+ ions

• endothermic (+ve)
  e. g. Mg(g) Mg+(g) + e-

Question 5

Second Ionisation energy

Answer 5

enthalpy change when each ion in 1 mole of gaseous 1+ ions loses one electron to form one mole of gaseous 2+ ions

• endo (+ve)
  e. g. Mg+(g) —> Mg2+(g) + e-

Question 6

First electron affinity

Answer 6

enthalpy change when one atom in one mole of gaseous atoms gains one electron to form one mole of gaseous 1- ions

• exo (-ve) for many non-metals
  e. g. O(g) + e- —> O-(g)

Question 7

Second electron affinity

Answer 7

enthalpy change when each ion in one mole of gaseous 1- ions gains one electron to form one mole of gaseous 2- ions
- endo (+ve) as adding -ve electrons to -ve ion

Question 8

Enthalpy of atomisation

Answer 8

enthalpy change when mole of gaseous atoms is produced from an element in its standard state
- endo (+ve)
1/2I2(s) –> I(g)

Question 9

Hydration enthalpy

Answer 9

enthalpy change when one mole of gaseous ions become hydrated (dissolved in water)
-exo (-ve)
Mg2+ + aq —>Mg2+ (aq)

Question 10

Enthalpy of solution

Answer 10

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 between endo and exo
MgCl2(s) + aq –> Mg2+ (aq) + 2Cl- (aq)

Question 11

Bond dissociation enthalpy

Answer 11

enthalpy change when 1 mole of covalent bonds is broken in the gaseous state
- endo (+ve)
I2(g) –> 2I(g)

Question 12

Lattice enthalpy of formation

Answer 12

enthalpy change when 1 mole of a solid ionic compound is formed from its constituent ions in the gas phase
-exo (-ve)
Mg2+(g) + 2Cl- (g) —> MgCl2 (s)

Question 13

Lattice enthalpy of disassociation

Answer 13

enthalpy change when 1 mole of a solid ionic compound is broken up into its constituent ions in the gas phase
- endo (+ve)
MgCl2 (s) —> Mg2+(g) + 2Cl- (g)

Question 14

Enthalpy of vaporisation

Answer 14

enthalpy change when 1 mole of a liquid is turned to gas
- endo (+ve)
H2O(l) –> H2O (g)

Question 15

Enthalpy of fusion

Answer 15

enthalpy change when 1 mole of a solid is turned into a liquid
-endo (+ve)
Mg(s) —> Mg(l)