1.5
Practice questions
c. The reaction between aluminium oxide and carbon to form aluminium and carbon monoxide does not occur to a significant extent until the temperature reaches a value about 1000 K above that of the answer to part b. Give one reason for this.
c. The standard enthalpies of hydration of the Mg2+ and the Cl− ions are −1 920 kJ mol−1 and −364 kJ mol−1, respectively. Use this information and data from the table in part a to calculate the enthalpy of solution of magnesium chloride.
d. State the method used to reduce aluminium oxide on an industrial scale. Give the essential conditions for this industrial process.
d. The standard enthalpy of solution of ammonium chloride, NH4+Cl−, is +15 kJ mol−1. i. Explain why ammonium chloride dissolves spontaneously in water even though this process is endothermic. ii. A 2.0 g sample of ammonium chloride is dissolved in 50 g of water. Both substances are initially at 20 °C. Calculate the temperature change and the final temperature of the solution. Assume that the specific heat capacity of the solution is 4.2 J K−1 g−1.
AQA June 2007 Unit 5 Question 2 3. The sketch graph (Figure Q2) shows how the entropy of a sample of water varies with temperature.
Entropy
AQA June 2007 Unit 5 Question 1 2. Data for the following reaction, which represents the reduction of aluminium oxide by carbon, are shown in Table Q2. Al2O3(s) + 3C(s) → 2Al(s) + 3CO(g)
Substance Al2O2(s)
∆Hf(kJ mol-1)
S(J K-1 mol-1)
−1669
51
C(s)
0
6
Al(s)
0
28
−111
198
CO(s) Table Q2
a. Calculate the values of ΔHϴ, ΔSϴ and ΔGϴ for the above reaction at 298 K and suggest why this reaction is not feasible at 298 K. b. Calculate the temperature above which this reaction is feasible. (If you have been unable to calculate values for ΔHϴ and ΔSϴ in part a you may assume that they are +906 kJ mol−1 and +394 J K−1 mol−1 respectively. These are not the correct values.)
0
0
T1
T2
Temperature (K) Figure Q2
a. Suggest why the entropy of water is zero at 0 K. b. What change of state occurs at temperature T1? c. Explain why the entropy change, ΔS, at temperature T2 is much larger than that at temperature T1. d. It requires 3.49 kJ of heat energy to convert 1.53 g of liquid water into steam at 373 K and 100 kPa. i. Use these data to calculate the enthalpy change, ΔH, when 1.00 mol of liquid water forms 1.00 mol of steam at 373 K and 100 kPa. ii. Write an expression showing the relationship between free-energy change, ΔG, enthalpy change, ΔH, and entropy change, ΔS. iii. For the conversion of liquid water into steam at 373 K and 100 kPa, ΔG = 0 kJ mol−1.
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