Thermodynamics (OCR A Level Chemistry)

This question is about fluorine and the associated energy changes when it is reacted with magnesium to form magnesium fluoride.

The table below gives some values of standard enthalpy changes. Use these values to answer the questions.

This question is about lithium fluoride.

Complete the Born–Haber cycle for lithium fluoride by adding the missing species on the lines.

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

This question is about the energy changes represented in a Born-Haber cycle when forming silver chloride.

This question focuses on the difference between the theoretical enthalpy values and experimental values of ionic substances. A perfect ionic model can be used to calculate a theoretical value for the enthalpy of lattice dissociation.

This question looks at the enthalpy steps of calcium sulfide and its Born-Haber cycle.

This question is about enthalpy changes in solution.

This question focuses on the enthalpy of hydration of fluoride compounds.

The enthalpy of hydration of the fluoride and chloride ions are -524 kJ mol^-1 and -364 kJ mol^-1. Explain why the value for the enthalpy of hydration for the fluoride ion is more negative than that for the chloride ion. You must refer to the attractive forces involved in your answer.

Use the enthalpy values below to suggest why there is a difference between the hydration values of calcium ions and sodium ions.

The table below gives data for the ionic compound calcium bromide, CaBr₂. 

The table below shows some solution and hydration enthalpies.

This question looks at how the entropy change of water varies with temperature.

Standard entropies can be used to calculate the entropy change of a reaction, ΔS. For example, for the reaction between nitrogen monoxide and oxygen which is shown below. 

The contact process is a method used by industries to form sulfur trioxide, by reacting sulfur dioxide and oxygen together over a vanadium (V) oxide catalyst.

The equation for this reaction is shown below: 

2SO₂ (g) + O₂ (g) ⇌ 2SO₃ (g)

This question is about the enthalpy of solution of sodium chloride.

If the value of enthalpy of solution of sodium chloride is +4 kJ mol^-1, explain why the free energy change for dissolving sodium chloride in water is negative, despite the enthalpy change being a positive value.

Calcium carbonate thermally decomposes to form calcium oxide and carbon dioxide, as shown below:

CaCO₃ (s) → CaO (s) + CO₂ (g)

The enthalpy change of the above reaction is ΔH = +178 kJ mol^-1 and the entropy change is ΔS = +161 J K^-1 mol^-1. 

Calculate the temperature at which the free-energy change, ΔG, for this process is zero.

Some ionic compounds such as potassium chloride, KCl, will dissolve in water at room temperature in an endothermic process. 

Diamond and graphite are both allotropes of carbon. The table below shows the data for the conversion of graphite into diamond. Use this data to calculate values for ΔH and ΔS for the reaction. Use these values to explain why this reaction is not feasible under standard pressure at any temperature.

Carbon (graphite) → Carbon (diamond)

Carbon dioxide and hydrogen can react to form methanol. 

CO₂ (g) + 3H₂ (g) ⇌ CH₃OH (g) + H₂O (g)

Data for enthalpy of formation and entropy is shown in the table.

Calculate the Gibbs free-energy change (ΔG), in kJ mol^–1, for this reaction at 790 K.

Explain whether this reaction is feasible at 790 K.

Use your values of ΔH and ΔS from part (a) to calculate the temperature above which this reaction is feasible.

Explain why the reaction in part a) has a negative entropy change. 

This question is about Born-Haber cycles. 

Draw a fully labelled Born-Haber cycle for the formation of solid potassium sulfide, K₂S, from its elements. 

Include state symbols for all species involved. 

Use the data to calculate the lattice enthalpy of K₂S.

If lithium replaced potassium in the lattice, what would you expect to happen to the value of the lattice enthalpy?

Explain your answer.   

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

This question is about the feasibility of a reaction. 

Calcium carbonate can be thermally decomposed to make calcium oxide. 

Table 1 below shows the standard entropies of each substance. 

Explain how the conditions can be changed for the reaction in a) to make the reaction feasible.

The evaporation of water is an endothermic reaction but also a spontaneous reaction.

Explain why.

Refer to the entropy change that occurs and the change in the arrangement of water molecules. 

Use the information in the table to calculate the temperature above which the reaction between hydrogen and oxygen to form gaseous water is not feasible.

Explain what would happen to the sample of gaseous water if it was heated to a higher temperature than that calculated in part (d). 

This question is about lattice enthalpy. 

Use the following data in Table 1 to calculate the lattice enthalpy of caesium oxide. 

A different Group 1 metal forms 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 the table, identify the Group 1 ion.

The same Group 1 metal from part (b) 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 (b):

i) Explain why the enthalpy of hydration of Group 1 ions is negative.

ii) Explain why the enthalpies of hydration become less negative as you go down the group.

Strontium is used as a red colouring agent in fireworks as it provides a very intense red colour.

Use the data in Table 1 to calculate the atomisation energy for chlorine in strontium chloride. 

Using information in part (a) suggest a value for the lattice enthalpy of formation of rubidium chloride.

Explain your choice of value.