15.1 Energy cycles

Question 1

Electron affinity vs first ionisation E

Answer 1

Electron affinity: enthalpy change for 1 mol of (g) atoms attracts 1 mol of e

First ionisation energy: E needed to form the cation

Question 2

Explain lattice enthalpy

Answer 2

Lattice enthalpy: enthalpy change when (g) ions form from 1 mol of solid crystal breaking into (g) ions

Question 3

Explain Born-Haber cycle

Answer 3

Born-Haber cycle: used to explain E changes in ionic compound formation based on Hess’s law (overall formation ΔH equals sum of ΔH of individual steps)

Question 4

Explain Born-Haber cycle steps

Answer 4

1. Atomisation (ΔH_at): ΔH required to produce 1 mol of (g) atoms from standard state

[0.5 bond enthalpy for atoms in molecules]

2. Ionisation (ΔH_I.E.): ΔH occurs on the removal of 1 mol of e from 1 mol of atoms/cations
3. e- affinity (ΔH_e.a.): ΔH 1 mol (g) atoms/anions gains e to from 1 mol of (g) anions
4. Lattice formation (ΔH_L.E.): E required to break the lattice of 1 mol to (g) ions

1+2+3+4 = ΔH_f (direct route)

Question 5

Explain ionic model

Answer 5

Ionic model: assumes that the only interactions are due to electrostatic attractions between oppositely charged ions

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Question 6

What are the lattice enthalpy trends based?

Answer 6

Lattice ΔH increases as the size of cation/anion decreases

Lattice ΔH increases as charge of cation/anion increases

LiF - highest lattice ΔH (small)

CsI - lowest ΔH (large)

Question 7

How is solution enthalpy created?

Answer 7

Solution enthalpy depends of ΔH lattice + ΔH hydration of constituent ions

Water molecules tear lattices apart - E needed - ΔH hydration: occurs when 1 mol of (g) ions is dissolved to form an infinitely dilute solution of 1 mol of (aq) ions

Question 8

Is ΔH hyd exothermic or endothermic?

Answer 8

Exothermic

Question 9

Drawing Born-Haber cycle for NaCl

Answer 9

Question 10

Define salvation

Answer 10

SALVATION: change in enthalpy when 1 mol of (g) atoms added to water to form a solution