Energetics II Definitions - Chemistry A Level

Standard lattice energy

Energy change when 1 mole of an ionic solid is formed from its constituent gaseous ions under standard conditions. e.g. Na⁺ (g) + F^- (g) → NaF(s)

Standard enthalpy of atomisation

Enthalpy change when 1 mole of gaseous atoms is formed from the elements in its standard states. Always endothermic. e.g. Na(s) → Na(g)

First electron affinity

The enthalpy change that takes place when one electron is added to each atom in one mole of gaseous atoms to form one mole of gaseous 1- ions (could be asked for successive electron affinities). e.g. S(g) → S^-(g)

Enthalpy change of hydration

The enthalpy change when 1 mole of a gaseous ion is completely dissolved in water under standard conditions.

e.g. Na⁺(g) → Na⁺(aq)

Enthalpy change of solution

Enthalpy change when 1 mole of ionic solid completely dissolves in water under standard conditions to form an infinitely dilute solution e.g. KCl(s) → K⁺(aq) + Cl^-(aq)

Standard conditions

100 kPa, 298 K, 1 mol dm^-3 concentrations of ions

Exothermic

Heat given off; −ve value of enthalpy change.

Endothermic

Heat absorbed; +ve value of enthalpy change.

Standard enthalpy change of reaction

Enthalpy change when reaction occurs in the molar quantities shown in the chemical equation under standard conditions.

eg. 2Na(s) + H₂SO₄ → Na₂SO₄(s) + H₂(g)

Standard enthalpy change of formation

Enthalpy change when 1 mole of a compound is formed from its elements in their standard states under standard conditions. eg. H₂ (g) + ½ O₂ (g) → H₂O (l)

Standard enthalpy of combustion

Enthalpy change when 1 mole of a substance is completely burned in oxygen under standard conditions. eg. C₂H₆(g)+ 3.5 O₂(g) → 2CO₂ (g) + 3H₂O (l)

Standard enthalpy of neutralisation

Enthalpy change when an acid and alkali react together under standard conditions to form 1 mole of water. eg. NaOH(s) + HCl(aq) → H₂O(l) + NaCl(aq)

Hess’s law

The total enthalpy change is independent of the reaction pathway taken

Equation for calorimetry calculations

heat change Q = mC ΔT, where m = mass, C = specific heat capacity, ΔT = change in temperature.

Specific heat capacity

The amount of energy needed to raise a temperature of 1 g of a substance by 1 degree.

Mean bond enthalpy

average amount of energy needed to break a specific type of bond, measured over a variety of different molecules.