Energetics

Enthalpy of formation (ΔH_f^⦵ )

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

e.g. Na₂O(s)

2 Na(s) + ½ O₂(g) → Na₂O(s)

Enthalpy of combustion (ΔH_c^⦵ )

Enthalpy change when one mole of a substance undergoes complete combustion in oxygen with all substances in their standard states: Exothermic (-)

e.g. hydrogen

H₂(g) + ½ O₂(g) → H₂O(l)

Enthalpy of neutralisation (ΔH_neut^⦵ )

Enthalpy change when 1 mole of water is formed in a reaction between an acid and alkali under standard conditions: Exothermic (-)

e.g. H₂SO₄ + NaOH

½ H₂SO₄(aq) + NaOH(aq) → ½ Na₂SO₄(aq) + H₂O(l)

(First) Ionisation enthalpy (ΔH_IE^⦵ )

Enthalpy change when each atom in one mole of gaseous atoms loses one electron to form one mole of gaseous 1+ ions: Endothermic (+)

e.g. magnesium

Mg(g) → Mg⁺(g) + e^-

(Second) Ionisation enthalpy (ΔH_IE^⦵ )

Enthalpy change when each ion in one mole of gaseous 1+ ions loses one electron to form one mole of gaseous 2+ ions: Endothermic (+)

e.g. magnesium

Mg⁺(g) → Mg²⁺(g) + e^-

(First) Electron affinity (ΔH_EA^⦵ )

Enthalpy change when each atom in one mole of gaseous atoms gains one electron to form one mole of gaseous 1^- ions: Exothermic (-) for many non metals

e.g. oxygen

O(g) + e^- → O^-(g)

(Second) Electron affinity (ΔH_EA^⦵ )

Enthalpy change when each ion in one mole of gaseous 1^- ions gains one electron to form one mole of gaseous 2^- ions: Endothermic (+) as adding negative electron to negative ion

e.g. oxygen

O^- (g) + e^- → O₂^-(g)

Enthalpy of atomisation (ΔH_atm^⦵ )

Enthalpy change when one mole of gaseous atoms is produced from an element in its standard state: Endothermic (+)

e.g. iodine

½ I₂(s) → I(g)

Hydration enthalpy (ΔH_hyd^⦵ )

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

e.g. magnesium ions

Mg²⁺(g) → Mg²⁺(aq)

Enthalpy of solution (ΔH_sol^⦵ )

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

e.g. magnesium chloride

MgCl₂(s) → Mg²⁺(aq) + 2 Cl^-(aq)

Bond dissociation enthalpy (ΔH_BDE^⦵ )

Enthalpy change when one mole of covalent bonds is broken in the gaseous state: Endothermic (+)

e.g. I-I bond

I₂(g) → 2 I(g)

Lattice enthalpy of formation (ΔH_LE^⦵ )

Enthalpy change when one mole of a solid ionic compound is formed from its constituent gaseous ions: Exothermic (-)

e.g. magnesium chloride

Mg²⁺(g) + 2 Cl^- (g) → MgCl²(s)

Lattice enthalpy of dissociation (ΔH_LE^⦵ )

Enthalpy change when one mole of a solid ionic compound is broken up into its constituent gaseous ions: Endothermic (+)

e.g. magnesium chloride

MgCl₂(s) → Mg²⁺(g) + 2 Cl^- (g)

Enthalpy of vaporisation (ΔH_vap^⦵ )

Enthalpy change when one mole of a liquid is turned into a gas: Endothermic (+)

e.g. H₂O(l)

H₂O(l) → H₂O(g)

Enthalpy of fusion (ΔH_fus^⦵ )

Enthalpy change when one mole of a solid is turned into a liquid: Endothermic (+)

e.g. Mg(s)

Mg(s) → Mg(l)

Mean bond enthalpy

• Energy needed to break one mole of covalent bonds in the gaseous state

• Averaged over a range of different compounds/molecules

State the assumptions made during calculations in calorimetry. (3 marks)

• All heat released from the reaction was absorbed by the solution

• Reaction goes to completion

• Specific heat capacity of solution is 4.18 J^-1 g^-1

The enthalpy of combustion determined experimentally is less exothermic than that calculated using enthalpies of formation.

Give four possible reasons for this, other than heat loss. (4 marks)

• Heat escaped to surroundings

• Incomplete combustion of (substance)

• Evaporation of (substance) between weighing and setting light to it

• Didn’t stir to make temperature the same throughout

Define the term enthalpy change

Heat energy change at a constant pressure

Suggest how the students’ method, and the analysis of the results, could be improved in order to determine a more accurate value for the enthalpy of reaction. Justify your suggestions.

Do not refer to the precision of the measuring equipment. Do not change the amounts or the concentration of the chemicals. (6 marks)

Stage 1 : Improved insulation

• Insulate the beaker or use a polystyrene cup or a lid

• To reduce heat loss

Stage 2 : Improved temperature recording

• Record the temperature for a suitable time before adding the metal

• To establish an accurate initial temperature

• Record temperature values at regular time intervals

• To plot the temperature results against time on a graph

Stage 3 : Improved analysis of results

• Extrapolate the cooling back to the point of addition

• To establish a (theoretical) maximum temperature OR temperature change