Lattice Energy & Born-Haber Cycles (Cambridge (CIE) A Level Chemistry): Exam Questions

Lattice energies are always negative showing that they represent exothermic changes.

i)  Explain what is meant by lattice energy.

[2]

ii) Explain why lattice energy is an exothermic process.

[1]

Table 5.1

i) Use relevant data from Table 5.1 to calculate the lattice energy, , of potassium oxide, K₂O (s). Show your working.

, of K₂O (s) = ............................................. kJ mol^–1 [3]

ii) State how Na₂O (s) differs from K₂O (s). Indicate this by placing one tick () in the appropriate box in Table 5.2.

Table 5.2

Explain your answer.

[1]

One use of pure crystals of lithium fluoride are in X-ray monochromators. 

The Born- Haber cycle for lithium chloride is shown in Fig. 2.1 

Fig. 2.1 

i) Define the term enthalpy of atomisation.

[1]

ii) Explain why the enthalpy of atomisation of fluorine is positive.

[1]

iii) Complete the Born-Haber cycle for lithium fluoride in Fig 2.1 by adding the missing species on the lines.

[2]

Use the data in the Table 2.1 and your completed Born–Haber cycle from part (a) to answer the questions below.

Table 2.1

i) Calculate the enthalpy of lattice formation of lithium fluoride.

[2]

ii) Explain and justify how the enthalpy of lattice formation of LiBr compares with that of LiF.

You must refer to the size of the ions in your answer.

[3]

This question is about enthalpy changes in solution.

i) Write the equation for the process showing the enthalpy of solution of potassium fluoride. Include state symbols in your answer.

[1]

ii) Use the data in Table 2.2 to calculate the standard enthalpy of solution of potassium fluoride.

[2]

Table 2.2

Explain why the value for the enthalpy of hydration, ΔH^θ_hyd, of Group 1 ions increases from lithium to caesium.

Calcium chloride, CaCl₂, is an important industrial chemical used in refrigeration plants, for de-icing roads and for giving greater strength to concrete.

Use an equation to show what is meant by the lattice energy of calcium chloride.

Explain how the lattice energies of the following salts compare in magnitude with that of calcium chloride. 

• calcium fluoride

• calcium sulfide 

Use the data in Table 3.1 to calculate the lattice energy of CaCl₂.

Table 3.1

When a solution of CaCl₂ is added to a solution of the dicarboxylic acid, malonic acid, the salt calcium malonate is precipitated as a white solid.

The solid has the following composition by mass: Ca, 28.2%; C, 25.2%; H, 1.4%; O, 45.2%.

i) Calculate the empirical formula of calcium malonate.

[2]

ii) Suggest the structural formula of malonic acid.

[1]

Magnesium reacts with fluorine to form magnesium fluoride. 

Explain what is meant by the term first electron affinity. 

i) Write an equation for the second electron affinity of fluorine.

[1]

ii) Suggest why the second electron affinity for fluorine is endothermic. 

[1]

Determine the electron affinity of a fluorine atom, ΔH^θ_EA using Table 5.1.

[3]

Table 5.1

Explain the trend in the first electron affinities going from fluorine to iodine. 

Potassium sulfide is a reagent used in analytical chemistry and pharmaceutical preparations. 

Draw a fully labelled Born-Haber cycle for the formation of solid potassium from its elements. 

Include state symbols for all species involved.

Use the data in Table 1.1 to calculate the lattice enthalpy of potassium sulfide, ΔH^θ_latt. Show your working.

Table 1.1

ΔH^θ_latt = ................................. kJ mol^-1

Explain the difference in values between the first and second electron affinities of sulfur. 

Magnesium oxide is often used in optical applications due to its light-reflecting properties in crystal form.

Write an equation to represent the lattice energy of MgO.

Use the data in Table 2.1, to calculate a value for the second ionisation energy of magnesium. ΔH^θ_ie. Show your working.

Table 2.1

second ionisation energy of Mg = .............................. kJ mol^-1

Magnesium oxide is generally insoluble in water whereas calcium oxide is sparingly soluble. Explain why there is no enthalpy of solution data for calcium oxide.

The incomplete Born-Haber cycle for sodium selenide is shown below in Fig. 3.1.

Write the equations for processes 1, 2 and 3. 

Fig. 3.1

1 ............................................................

2 ............................................................

3 ............................................................

If sulfur is used as opposed to selenium in the lattice, suggest the change in value of the lattice energy, ΔH^θ_latt. Explain your answer.

Change .....................................

Explanatiion .......................................................................................

                    ........................................................................................

Use the data in Table 3.1 to calculate the lattice energy of aluminium oxide, ΔH^θ_latt. Show your working.

Table 3.1

ΔH^θ_latt = .................................... kJ mol^-1