Chemical Energetics (A Level Only) (CIE A Level Chemistry)

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

Table 5.1

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]

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

Na2O (s) is less negative than K2O (s)	Na2O (s) is the same as K2O (s)	Na2O (s) is more negative than K2O (s)

[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 

[1]

[1]

[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%.

This question is about enthalpy change of hydration.

Explain what is meant by the term enthalpy change of hydration, ΔH^θ_hyd?

[1]

The enthalpy change of solution for MgCl₂, (MgCl₂(s)), is represented by the following equation.

MgCl₂(s) + aq → Mg²⁺(aq) + 2Cl ^– (aq)

Describe the simple apparatus you could use, and the measurements you would make, in order to determine a value for (MgCl₂(s)) in the laboratory.

Use the data in Table 4.1 to calculate the value for the enthalpy change of solution for magnesium chloride. 

Magnesium reacts with fluorine to form magnesium fluoride. 

Explain what is meant by the term first electron affinity. 

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

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 sulfide 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.

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.

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. 

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.

A Group 1 metal, M, formed an ionic compound with chlorine.

The enthalpy of lattice dissociation for this compound is +773 kJ mol^-1 and the hydration enthalpy of a chloride ion is -363 kJ mol^-1. 

The enthalpy of solution of the Group 1 chloride is +4 kJ mol^-1. 

Using the information in Table 4.1, identify the Group 1 ion, M⁺. Show your working.

The same Group 1 metal from part (a) forms an ionic lattice with another halide ion. This new ionic compound has an enthalpy of lattice formation of -705 kJ mol^-1. 

Using M to represent the Group 1 metal, suggest a formula for the new ionic lattice and explain your answer.

The enthalpy of hydration becomes less exothermic as you go down Group 1. Using the values in part (a):