IB Chemistry HL

Revision Notes

15.1.1 Key Enthalpy Terms

Key Enthalpy Terms

Ionisation energy

  • The ionisation energy (ΔHIE) is the standard enthalpy change that occurs on the removal of 1 mole of electrons from 1 mole of gaseous atoms or positively charged ions
  • Ionisation energy is always endothermic as energy is need to overcome the attraction between an electron and the nucleus
  • The first ionisation energy (ΔHIE1) is the energy required to remove one mole of electrons from 1 mole of gaseous atoms of an element to form 1 mole of 1+ ions in the gaseous phase

ΔHIE1Al (g) → Al+ (g) + e          ΔHIE1ꝋ = +577 kJ mol-1

  • The second ionisation energy (ΔHIE2) is the energy required to remove 1 mole of electrons from 1 mole of gaseous 1+ ions to form 1 mole of 2+ ions in the gaseous phase

ΔHIE2Al+ (g) → Al2+ (g) + e      ΔHIE2= +1820 kJ mol-1

Enthalpy of Atomisation

  • The enthalpy of atomisationHat) is the standard enthalpy change that occurs on the formation of 1 mole of separate gaseous atoms an element in its standard state
  • The ΔHatꝋ is always endothermic as energy is always required to break any bonds between the atoms in the element or to break the element into its gaseous atoms
    • Since this is always an endothermic process, the enthalpy change will always have a positive value

Na (s) → Na (g)           ΔHat = +108 kJ mol -1

½Cl (g) → Cl (g)          ΔHat = +122 kJ mol -1

Electron Affinity

  • The electron affinity (ΔHEA) of an element is the energy change when 1 mole of electrons is gained by 1 mole of gaseous atoms of an element to form 1 mole of gaseous ions under standard conditions
  • For example, the first electron affinity of chlorine is:

Cl (g)+ e → Cl (g)          ΔHEAꝋ = -364 kJ mol-1

  • The first electron affinity is always exothermic as energy is released when electrons are attracted to the atoms
  • However, the second electron affinity of an element can be endothermic as illustrated by oxygen:

O– (g) + e → O2- (g)          ΔHEAꝋ = +844 kJ mol-1

  • This is because a large force of repulsion must be overcome between the negatively charged ion and the second electron requiring a large input of energy

Lattice Enthalpy

  • The lattice enthalpy Hlat) is defined as the standard enthalpy change that occurs on the formation of 1 mole of gaseous ions from the solid lattice
  • The ΔHlatꝋ is always endothermic as energy is always required to break any bonds between the ions in the lattice
    • Since this is always an endothermic process, the enthalpy change will always have a positive value

NaCl (s) → Na+ (g) + Cl (g)     ΔHlat = +790 kJ mol -1

Enthalpy of Solution 

  • The standard enthalpy change of solution (ΔHsolis the enthalpy change when 1 mole of an ionic substance dissolves in sufficient water to form an infinitely dilute solution
  • The symbol (aq) is used to show that the solid is dissolved in sufficient water
  • ΔHsol can be exothermic (negative) or endothermic (positive)

LiBr (s) → LiBr (aq)    ΔHsolꝋ = -48.8 kJ mol -1

KCl (s) → KCI (aq)      ΔHsol= +17.2 kJ mol -1

CaCl2 (s) → CaCl2 (aq)       ΔHsol= -82.8 kJ mol -1

Enthalpy of Hydration

  • The standard enthalpy change of hydration Hhydis the enthalpy change when 1 mole of a specified gaseous ion dissolves in sufficient water to form an infinitely dilute solution

Mg2+ (g) → Mg2+ (aq)   ΔHhyd= -1963 kJ mol -1

Br(g) → Br(aq)          ΔHhyd= -328 kJ mol -1

  • Hydration enthalpies are the measure of the energy that is released when there is an attraction formed between the ions and water molecules
  • Hydration enthalpies are exothermic
  • The term solvation is used in place of hydration if water has been replaced by another solvent
  • When an ionic solid dissolves in water, positive and negative ions are formed
  • Water is a polar molecule with a δ- oxygen (O) atom and δ+ hydrogen (H) atoms which will form ion-dipole attractions with the ions present in the solution
  • The oxygen atom in water will be attracted to the positive ions and the hydrogen atoms will be attracted to the negative ions

Chemical Energetics - Ion-Dipole Bonds, downloadable AS & A Level Chemistry revision notes

The polar water molecules will form ion-dipole bonds with the ions in solution causing the ions to become hydrated 

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