lattice enthalpy and born haber

Question 1

Define the term lattice enthalpy.

Answer 1

(The enthalpy change that accompanies)
the formation of one mole of a(n ionic) compound
from its gaseous ions (under standard conditions)

Question 2

Predict the order of melting points for NaBr, KI and RbCl from lowest to highest.

Answer 2

Attraction and ionic size linked:
Greater attraction from smaller ions/closer ions/larger
charge density 
Comparison needed
Energy AND attraction/breaking bonds linked:
More energy/heat to overcome attraction (between ions)
OR
More energy/heat to break (ionic) bonds 

Question 3

Describe and explain the factors that affect the values of lattice enthalpies.

Answer 3

First 2 marks
Decrease in (ionic) size
AND
more negative LE OR more exothermic OR more attraction

Increase in (ionic) charge OR charge density
AND
more negative LE OR more exothermic OR more attraction

———————————————————————————–
Link between LE and attraction
Lattice enthalpy correctly linked to attraction between IONS at
least once

e.g. Greater attraction between ions gives more negative LE

Question 4

Explain the differences between these lattice enthalpies.

Answer 4

Comparison of size of anions
Chloride ion OR Cl– is larger (than F–)
OR Cl– has smaller charge density (than F–) 
Comparison of size AND charge of cations
Mg2+ is smaller (than Na+)
AND
Mg2+ has a greater charge (than Na+) 
Comparison of attraction between ions
F– has greater attraction for Na+ / + ions
AND
Mg2+ has greater attraction for F– / – ions 

Question 5

Explain why it is difficult to predict whether the lattice enthalpy of magnesium sulfide would be more or less exothermic than the lattice enthalpy of sodium oxide.

Answer 5

Comparison of size AND charge of cations
Mg2+ is smaller AND Mg2+ has a greater charge
OR
Mg2+ has a greater charge density

Comparison of size of anions
S2– is larger
OR
S2– has a smaller charge density

Comparison of attraction of a cation and an anion
Mg2+ has stronger attraction OR Na+ has weaker attraction
AND
S2– has weaker attraction OR O2– has stronger attraction


Question 6

A student wanted to determine the lattice enthalpy of sodium carbonate, Na2CO3. Unfortunately this is very difficult to do using a similar Born–Haber cycle to that used for sodium oxide.
Suggest why this is very difficult.

Answer 6

Cycle needs formation of CO32– ions (from C and O)

i.e. NOT breaking up of CO32– ion

Question 7

Why is the enthalpy change of hydration of chloride ions exothermic?

Answer 7

Hydration involves bond forming OR bonds are made

Question 8

The enthalpy change of hydration of magnesium ions is more exothermic than the enthalpy change of hydration of calcium ions.
Explain why.

Answer 8

Magnesium ion OR Mg2+ is smaller
OR Mg2+ has greater charge density 
Mg2+ has a stronger attraction to H2O
OR Mg2+ has a stronger bonding with H2O 

Question 9

When dissolved in water, the enthalpy change of solution of the salt potassium fluoride, KF, is –15 kJ mol–1.The salt rubidium fluoride, RbF, has an enthalpy change of solution in water of –24 kJ mol–1.Suggest reasons for the difference between the enthalpy changes of solution of KF and RbF.

Answer 9

Ionic radius
Potassium ion OR K+ OR K ion is smaller
OR K+ has greater charge density 
Lattice enthalpy
Lattice enthalpy of KF is more negative than RbF 
R+ has greater attraction for F–
H(hydration) of K+ is more negative than Rb+ 
R
enthalpy change of solution lattice

 F–
2
O
K
Hydration enthalpy
∆
O
K+ has greater attraction for H2O
E
Idea that ∆H(solution) is affected more by
enthalpy than by hydration enthalpy 

Question 10

Magnesium chloride has stronger ionic bonds than sodium chloride. Explain why.

Answer 10

Magnesium ion OR Mg2+
has greater charge (than sodium ion OR Na+)
OR Mg2+ has greater charge density

Magnesium ion OR Mg2+ is smaller

Mg2+ has a stronger attraction (than Na+) to Cl– ion
OR
Greater attraction between oppositely charged ions
