Topic 15.1: Energetics – Energy cycles
The concept of the energy change in a single step reaction being equivalent to the summation of smaller steps can be applied to changes
involving ionic compounds.
• Understanding: Representative equations (eg M+(g) → M+(aq)) can be used for enthalpy/energy of hydration, ionization, atomization,
electron affinity, lattice, covalent bond and solution.
▪ Ionization enthalpy: enthalpy change that occurs in the removal of 1 mol of electrons from 1 mol of atoms in gaseous state
• Representative equation: M (g) → M+ (g) + e-
• Enthalpy change: exothermic; for most atoms
▪ Electron affinity: enthalpy change on the addition of 1 mol of electrons from 1 mol of atoms in gaseous state
• Representative equation: X (g) + e- → X- (g)
• Enthalpy change: endothermic; for most atoms
▪ Atomization enthalpy: enthalpy change that occurs on the formation of 1 mol separate gaseous atoms of an element in standard state
• Representative equation: M (s) → M (g) | M (l) → M (g) | ½ X2 (g) → X (g)
• Enthalpy change: endothermic
▪ Hydration enthalpy: enthalpy change that occurs when 1 mol of gaseous ions are added to water to form dilute solution
• Representative equation: M+ (g) → M+ (aq) | X- (g) → X- (aq)
• Enthalpy change: endothermic
▪ Solvation enthalpy: enthalpy change that occurs when 1 mol of gaseous ions are added to solvent (not water) to form dilute solution
• Representative equation: M+ (g) → M+ (aq) | X- (g) → X- (aq)
• Enthalpy change: endothermic
▪ Solution enthalpy: enthalpy change that occurs when 1 mol of ionic substance is dissolved in large excess of pure solvent
• Representative equation: MX (s) → M+ (aq) + X- (aq)
• Enthalpy change: can be exothermic or endothermic
▪ Lattice enthalpy: enthalpy change that occurs on the formation of 1 mol of gaseous ions from the solid lattice
• Representative equation: MX (s) → M+ (g) + X- (g)
• Enthalpy change: endothermic (exothermic when reversed)
• Understanding: Enthalpy of solution, hydration enthalpy and lattice enthalpy are related in an energy cycle.
• Applications and skills: Construction of energy cycles from hydration, lattice and solution enthalpy. For example dissolution of solid
NaOH or NH4Cl in water.
▪ Relationship of solution, hydration and lattice enthalpy
• Levels: solid state (lowest), aqueous state (medium), gaseous state (highest)
• ∆Hsol = ∆Hhyd(cations) + ∆Hhyd(anions) – ∆HLe
• ∆Hhyd = ∆HLe + ∆Hsol
• ∆HLe = ∆Hhyd(cations) + ∆Hhyd(anions) – ∆Hsol
• Applications and skills: Construction of Born-Haber cycles for group 1 and 2 oxides and chlorides.
▪ Born-Haber cycle: cycle that combines enthalpy changes associated with different steps in the formation of ionic compounds
▪ Construction of Born-Haber cycle for metal chlorides
• Metals: need to be atomized into gaseous atoms, then
ionized to create gaseous ions of required charge
▪ ∆Hat: endothermic
▪ ∆HIE1: endothermic
▪ ∆HIE2: may be required for group 2 metals
• Chlorides: need to be atomized into separate gaseous
atoms, then ionized to create 1- charge gaseous ions
▪ ∆Hat: endothermic
▪ ∆Hea: exothermic
• ∆Hf = ∆Hat + ∆HIE + ∆Hea - ∆HLE
The concept of the energy change in a single step reaction being equivalent to the summation of smaller steps can be applied to changes
involving ionic compounds.
• Understanding: Representative equations (eg M+(g) → M+(aq)) can be used for enthalpy/energy of hydration, ionization, atomization,
electron affinity, lattice, covalent bond and solution.
▪ Ionization enthalpy: enthalpy change that occurs in the removal of 1 mol of electrons from 1 mol of atoms in gaseous state
• Representative equation: M (g) → M+ (g) + e-
• Enthalpy change: exothermic; for most atoms
▪ Electron affinity: enthalpy change on the addition of 1 mol of electrons from 1 mol of atoms in gaseous state
• Representative equation: X (g) + e- → X- (g)
• Enthalpy change: endothermic; for most atoms
▪ Atomization enthalpy: enthalpy change that occurs on the formation of 1 mol separate gaseous atoms of an element in standard state
• Representative equation: M (s) → M (g) | M (l) → M (g) | ½ X2 (g) → X (g)
• Enthalpy change: endothermic
▪ Hydration enthalpy: enthalpy change that occurs when 1 mol of gaseous ions are added to water to form dilute solution
• Representative equation: M+ (g) → M+ (aq) | X- (g) → X- (aq)
• Enthalpy change: endothermic
▪ Solvation enthalpy: enthalpy change that occurs when 1 mol of gaseous ions are added to solvent (not water) to form dilute solution
• Representative equation: M+ (g) → M+ (aq) | X- (g) → X- (aq)
• Enthalpy change: endothermic
▪ Solution enthalpy: enthalpy change that occurs when 1 mol of ionic substance is dissolved in large excess of pure solvent
• Representative equation: MX (s) → M+ (aq) + X- (aq)
• Enthalpy change: can be exothermic or endothermic
▪ Lattice enthalpy: enthalpy change that occurs on the formation of 1 mol of gaseous ions from the solid lattice
• Representative equation: MX (s) → M+ (g) + X- (g)
• Enthalpy change: endothermic (exothermic when reversed)
• Understanding: Enthalpy of solution, hydration enthalpy and lattice enthalpy are related in an energy cycle.
• Applications and skills: Construction of energy cycles from hydration, lattice and solution enthalpy. For example dissolution of solid
NaOH or NH4Cl in water.
▪ Relationship of solution, hydration and lattice enthalpy
• Levels: solid state (lowest), aqueous state (medium), gaseous state (highest)
• ∆Hsol = ∆Hhyd(cations) + ∆Hhyd(anions) – ∆HLe
• ∆Hhyd = ∆HLe + ∆Hsol
• ∆HLe = ∆Hhyd(cations) + ∆Hhyd(anions) – ∆Hsol
• Applications and skills: Construction of Born-Haber cycles for group 1 and 2 oxides and chlorides.
▪ Born-Haber cycle: cycle that combines enthalpy changes associated with different steps in the formation of ionic compounds
▪ Construction of Born-Haber cycle for metal chlorides
• Metals: need to be atomized into gaseous atoms, then
ionized to create gaseous ions of required charge
▪ ∆Hat: endothermic
▪ ∆HIE1: endothermic
▪ ∆HIE2: may be required for group 2 metals
• Chlorides: need to be atomized into separate gaseous
atoms, then ionized to create 1- charge gaseous ions
▪ ∆Hat: endothermic
▪ ∆Hea: exothermic
• ∆Hf = ∆Hat + ∆HIE + ∆Hea - ∆HLE
