Kinetics: 2 - Enthalpy of solution & entropy

1. (a) Draw a labelled Hess’s law cycle to show how the lattice energy and the enthalpies of hydration are related to the enthalpy of solution of an ionic compound, MX(s).

(b) How are the enthalpy values used to suggest whether MX(s) is soluble in water

2. The following data relate to the thermodynamic changes which occur when Group 2 hydroxides dissolve in water.

Enthalpy of hydration of Mg2+ –1890 kJ mol–1 Enthalpy of hydration of Ba2+ –1275 kJ mol–1

Enthalpy of hydration of OH– –550 kJ mol–1 Lattice energy of Mg(OH)2 –2995 kJ mol–1

Lattice energy of Ba(OH)2 –2320 kJ mol–1

(a) (i) Define the term enthalpy of hydration.

(ii) Explain why this enthalpy change is always exothermic.

(b) Why is the lattice energy of magnesium hydroxide more exothermic than that of barium hydroxide?

(c) (i) Draw a labeled Hess’s law cycle to show how the lattice energy and the enthalpies of hydration are related to the enthalpy of solution of magnesium hydroxide, Mg(OH)2(s).

(ii) Use your cycle and the data to calculate the enthalpy of solution of magnesium hydroxide. Include a sign and units with your answer.

(d) Use the data to explain how the solubility of barium hydroxide compares with that of magnesium hydroxide.

3. The table below gives some information about the sulphates of two of the elements in Group2

Formula of sulphate Lattice energy / kJmol-1 Hydration enthalpy of cation /kJmol-1 Solubility/mol dm-3

MgSO4 -2874 -1920 1.83

BaSO4 -2374 -1360 9.4 x10-6

Comment on the trend in the solubility of these sulphates in relation to the lattice energies and hydration enthalpies as shown in the table.

4. Draw a Hess’s law cycle for dissolving calcium chloride .Use it together with the data given below to calculate the ∆H hydration of the chloride ion, Cl-

Enthalpy change / kJmol-1

∆H hyd(Ca2+(g)) -1650

∆Hlatt (CaCl2(s)) -2237

∆Hsolu(CaCl2(s)) -83

5. Dissolving Sodium chloride

NaCl(s) → Na+(aq) + Cl-(aq) At 298K ΔH = +3.9 kJ/mol

ΔSsyst = +39.2 J K-1 mol-1

(a)Explain why dissolving NaCl is an endothermic process.(b)Explain why ΔSsyst positive.

(c) Show that dissolving NaCl is feasible at 298K.

Department of science / Advanced Level GCE, Grade 12- 2012