Topic 13: Further Energetics

Question 1

Enthalpy of formation

Answer 1

• Enthalpy change when 1 mole of a substance is formed from its constituent elements which are in their standard states (standard conditions)
  e. g 2Na(s) + 1/2O2(g) —> Na2O(s)

EXO (-ve) most substances

Question 2

Enthalpy of Combustion

Answer 2

• Enthalpy change when 1 mole of a substance is completely burned in excess oxygen with all substances in standard states and conditions
  e. g H2(g) + 1/2O2(g) —> H2O(l)

Exothermic (-ve)

Question 3

Enthalpy change of Neutralisation

Answer 3

• Enthalpy change when 1 mole of water is formed when an acid and an alkali react under standard conditions

H+ + OH- —> H2O

EXOthermic (-ve)

Question 4

1st Ionisation energy

Answer 4

• Enthalpy change when each atom in 1 mole of gaseous atoms loses an electron to form 1 mole of 1+ gaseous ions

Na(g) —> Na+(g) + e-

ENDOthermic (+ve)

Question 5

2nd Ionisation energy

Answer 5

• Enthalpy change when each ion in 1 mole of gaseous 1+ ions loses an electron forming one mole of gaseous 2+ ions

Mg+(g) —> Mg2+(g) + e-

ENDOthermic (+ve)

Question 6

First & second Electron affinity

Answer 6

(1st) - Enthalpy change when each atom of 1 mole of gaseous atoms gains 1 electron forming 1 mole of 1- gaseous ions
(2nd) - Enthalpy change when each ion in 1 mole of gaseous 1- ions gains 1 electron forming 1 mole of gaseous 2- ions

O(g) + e- —> O-(g)
O-(g) + e- —> O2-(g)

(1st) EXOthermic (-ve) for many non-metals
(2nd) ENDOthermic (+ve)

Question 7

Enthalpy of atomisation

Answer 7

• Enthalpy change when 1 mole of gaseous atoms is produced from an element in its standard state

1/2I2(s) —> I(g)

ENDOthermic (+ve)

Question 8

Enthalpy of Hydration

Answer 8

Enthalpy change when 1 mole of gaseous ions become hydrated (+(aq)) (dissolved in water)

EXOthermic (-ve)

Mg2+(g) + (aq) —> Mg2+(aq)

Question 9

Enthalpy of solution

Answer 9

Enthalpy change when 1 mole of an IONIC SOLID dissolves in an amount of water that separates the dissolved ions so that they don’t interact with each other

Exo or Endo

MgCl2(s) + (aq) —> Mg2+(aq) + 2Cl-(aq)

Question 10

Bond dissociation enethalpy

Answer 10

Enthalpy change when 1 mole of a covalent bond is broken in the gaseous state

ENDOthermic

I2(g) —> 2I(g)

Question 11

Lattice enthalpy of formation

Answer 11

Enthalpy change when 1 mole of a SOLID IONIC compound is formed from its GASEOUS constituent elements

Question 12

Lattice enthalpy of dissociation

Answer 12

Enthalpy change when 1 mole of a SOLID IONIC compound is broken up into its constituent ions in the gaseous state

ENDOthermic (+ve)

NaCl (s) –> Na+(g) + Cl-(g)

Question 13

Enthalpy of vaporisation

Answer 13

Enthalpy change when one mole of a liquid is turned into gas

Endothermic (+ve)

H2O(l) —> H2O(g)

Question 14

Enthalpy of fusion

Answer 14

Enthalpy change when one mole of a solid is turned into a liquid

ENDOthermic (+ve)

Mg(s) —> Mg(l)