Enthalpy Changes and Calculations - Chapter 6 Review

Standard Enthalpy Change of Reaction

• ΔH°r

• Umbrella term for other Enthalpy Changes

• When all the reactants react to form the products

• Under Standard Conditions

• H2 (g) + ½ O2 (g) → H2O (l) ΔH°r = -287 kJ/mol

Standard Enthalpy Change of Formation

• ΔH°f

• When 1 mol of a compound is formed

• C (graphite) + 2S (s) → CS2 (l) ΔH°f [CS2 (l)] = 98.7 kJ/mol

Standard Enthalpy Change of Combustion

• ΔH°c

• When 1 mol of a compound is burnt in excess oxygen

• S (s) + O2 (g) → SO2 (g) ΔH°c [S (s)] = -296.8 kJ/mol

• Equivalent to the Standard Enthalpy Change of Formation (i.e. SO2)

Standard Enthalpy Change of Neutralization

• ΔH°n

• When 1 mol of water is formed

• Acid + Alkali reaction

• HCl (aq) + NaOH (aq) → H20 (l) + NaCl (aq) ΔH°n = -57.1 kJ/mol

• For any acid-alkali reaction the ionic equation is:

  • H+ (aq) + [OH]- (aq) → H2O (l)

Standard Enthalpy Change of Solution

• ΔH°sol

• When 1 mol of solute is dissolved in a solvent

• NaOH (s) + aqueous solvent → NaOH (aq)

• Forms an Infinitely Dilute Solution:

  • Solution that doesn’t produce further ΔH when more solvent is added

Standard Enthalpy Change of Atomisation

• ΔH°at

• When 1 mol of gaseous atom is formed from its element

• ½ H2 (g) → H (g) ΔH°at [ ½ H2 (g)] = 218 kJ/mol

Standard Enthalpy Change of Hydration of an Anhydrous Salt

• ΔH°

• When 1 mol of a hydrated salt is formed

• Na2S2O3 (s) + 5 H2O (l) → Na2S2O3H2O (s) ΔH° = -55.0 kJ/mol