1 THERMODYNAMICS
Questions A1. Using data from the table, construct a Born–Haber cycle for: a. KCl(s) to calculate the standard molar enthalpy of formation; define all the terms you use, and explain why each one is either exothermic or endothermic b. KCl2(s) to show why this compound will not be found naturally. A2. Dissolving ammonium chloride in water produces a cooling effect. Draw a suitable enthalpy cycle and use the data below to calculate the enthalpy of solution, ΔsolHϴ[NH4Cl(s)]: ΔlattHϴ[NH4Cl(s)] = 2526 kJ mol−1
A3. Exothermic processes are favoured energetically, yet the above process is endothermic. Explain the idea of entropy and use it to explain why an endothermic process, such as dissolving ammonium chloride, can happen spontaneously. A4. For a 200 g cooling pack, assuming a specific heat capacity of 4.2 J g−1 K−1, calculate the enthalpy change that takes place. A5. Calculate the number of moles of ammonium nitrate that will produce this amount of cooling.
ΔhydHϴ[NH4+(g)] = −307 kJ mol−1 ΔhydHϴ[Cl−(g)] = −381 kJ mol−1.
PRACTICE QUESTIONS 1. Consider the incomplete Born–Haber cycle (Figure Q1) and the data in Table Q1.
Mg2+(g) + 2Cl–(g)
Mg(s) + Cl2(g) MgCl2(s) Figure Q1
Name of standard enthalpy change
Substance to which enthalpy change refers
Value of enthalpy change (kJ mol-1)
Enthalpy of atomisation
chlorine
+121
Enthalpy of atomisation
magnesium
+150
Enthalpy of formation
magnesium chloride
−642
First Ionisation enthalpy
magnesium
+736
Electron affinity
chlorine
−364
Enthalpy of lattice formation
magnesium chloride
−2493
Table Q1
a. Complete the Born–Haber cycle above by writing the appropriate chemical formulae, with state symbols, on the dotted lines. b. Use the cycle and the values given in the table to calculate the second ionisation enthalpy of magnesium.
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