2.2e Lattice enthalpy of potassium iodide KI a group 1 metal halide

Problem solving using other cycles (initially not using enthalpy level diagrams)

• ΔH enthalpy abbreviations used for the Born-Haber Cycle of potassium iodide

  • f = enthalpy of formation

  • atom = atomisation energy

  • BE = bond enthalpy

  • IE = ionisation energy

  • LE = lattice enthalpy expressed exothermically i.e. from free gaseous ions to ionic crystals.

  • elec.affin = electron affinity

• Each cycle involves 6–8 enthalpy values, of which you must know all of them except one!

• You can then calculate the unknown enthalpy value by substitution and simple algebraic rearrangement.

• No numerical values are shown on all Born–Haber cycle diagrams, but some are shown on selected enthalpy level diagrams.

K(s)	+	1/2I2(s)	ΔHθf(KI)	K+I–(s)
ΔHθatom(K)		ΔHθatom(I2)			ΔHθLE(KI)
		I(g) +  e-	ΔHelec. affin.(I)	I–(g) +
K(g)	ΔHθ1st IE(K)				K+(g) +  e-
The Born–Haber Cycle for the formation of a potassium iodide, an ionic halide salt

From Hess's Law

and therefore on rearranging gives the lattice enthalpy of potassium iodide

ΔH^θ_f(KI) = ΔH^θ_atom(K) + ΔH^θ_atom(I₂) + ΔH^θ_{1st IE}(K) + ΔH^θ_elec.affin.(I) + ΔH^θ_LE(KI)

-ΔH^θ_LE(KI) = ΔH^θ_atom(K) + ΔH^θ_atom(I₂) + ΔH^θ_{1st IE}(K) + ΔH^θ_elec.affin.(I) - ΔH^θ_f(KI)

and be very careful of the signs in the algebra as well as the enthalpy values!

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Other lattice enthalpies that can be calculated with the potassium iodide Born-Haber Cycle using an enthalpy level diagram

A general Born-Haber Cycle for a group 1 metal halide salt using an enthalpy level diagram to calculate the lattice enthalpy.

ΔH_f^θ(MX) = standard enthalpy of formation of group 1 halide salt e.g. iodide  (↓ exothermic)

ΔH^θ_at(M) = standard enthalpy of atomisation of the group 1 metal (↑ endothermic)

 ΔH^θ_atom(X₂) = standard enthalpy of atomisation of the halogen e.g. iodine (↑ endothermic)

 ΔH^θ_ea.(X) = standard enthalpy of the electron affinity of the halogen e.g. iodine (↓ exothermic)

ΔH^θ_{1st IE}(M) = standard enthalpy of the 1st ionisation of the group 1 metal (↑ endothermic)

ΔH^θ_LE(MX) = lattice enthalpy of the halide salt e.g. iodide (↑ endothermic)

route B = route A

ΔH_f^θ(MX) + ΔH^θ_LE(MX) = ΔH^θ_at(M) + ΔH^θ_atom(X) + ΔH^θ_{1st IE}(M) +  ΔH^θ_ea.(X)

Solve for unknown e.g. lattice enthalpy of potassium iodide and watch the algebraic and enthalpy signs and the state of the halogen (g/l/s) and be very careful of the signs in the algebra as well as the enthalpy values!

 ΔH^θ_LE(MX) = ΔH^θ_at(M) + ΔH^θ_atom(X) + ΔH^θ_{1st IE}(M) +  ΔH^θ_ea.(X) - ΔH_f^θ(MX)

Using I₂(s), all the iodides from group I metals, lithium iodide LiI, sodium iodide NaI, rubidium iodide RbI, caesium iodide CsI and francium iodide FrI.

Hence you can calculate the lattice enthalpy of any group 1 metal halide assuming it is a purely ionic compound.

See also the Born-Haber Cycle for Group 1 chlorides and Group 1 bromides

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Born-Haber Cycle and Lattice Enthalpy INDEX

Energetics–Thermochemistry–Thermodynamics Notes INDEX

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How to draw the Born-Haber Cycle for potassium iodide, how to calculate the lattice enthalpy for potassium iodide from a Born-Haber Cycle, a full explanation of the terms and enthalpy values of the Born-Haber Cycle for potassium iodide, what do I need to know about the Born-Haber Cycle of potassium iodide for AQA, Edexcel, OCR, Salters, CIE, WJEC Eduqas & CCEA  A-level chemistry, US grades 11-12 K12 AP Honors chemistry courses, how to use enthalpies of formation, ionisation, atomisation, electron affinity and lattice enthalpy to problem solve the Born-Haber Cycle for potassium iodide.