1) (10 marks)
(a) Two important thermodynamic functions are enthalpy and entropy. Discuss how each of these functions is related to a chemical reaction. Explain how they may be combined to give a third function that will predict the spontaneity of a chemical reaction. (5 marks)
(b) For the reaction
MgCO3(s) --> MgO(s) + CO2(g)
make a reasoned prediction of the sign of ?SQ and explain your answer. (2 marks)
(c) The decomposition of calcium carbonate is represented by the equation
CaCO3(s) --> CaO(s) + CO2(g)
The process is known to be non spontaneous at room temperature but spontaneous at high temperature.
i. Make a reasoned prediction of the sign of ?HQ assuming its value does not change greatly with temperature. Explain your answer.
ii. Would the value of the equilibrium constant for the process at room temperature be greater than or less than or equal to zero? Explain your answer. (3 marks)
2) (a) Discuss with a relevant example the meaning of ?HQ the standard enthalpy change of a
reaction. What is the significance of ?HQ when considering the feasibility of a reaction?
(b) Describe in some detail an experiment to measure the enthalpy change associated with the reaction of aqueous sodium hydroxide and hydrochloric acid. Include descriptions or labelled sketches of any relevant experimental set ups and indicate the type of experimental data to be collected. Show how a numerical value of ?HQ may be calculated from the experimental data. (10 marks)
(c) For the reaction between aqueous sodium hydroxide and hydrochloric acid the value of ?HQ is 55.9 kJ mol-1. When the experiment is repeated using ethanoic acid in place of hydrochloric acid the value of ?HQ is found to be 54.6 kJ mol-1. Explain the difference in ?HQ values for the two experiments. (2 marks)
(d) For the reaction H+(aq) + OH-(aq) Ô H2O(l) ?SQ is known to be 80.5 J×mol-1×K-1. Comment on the value of ?SQ. Predict the sign expected for ?GQ for this reaction and explain your reasoning. Would you predict the value of the equilibrium constant for this reaction to be greater than less than or equal to one? Explain your reasoning. (6 marks)
4) Hydrogen is produced commercially by the reaction of a hydrocarbon with steam at high temperatures in the presence of a catalyst to produce carbon dioxide and hydrogen. Some heats of formation are presented below.
COMPOUND ?Hf (kJ mol-1)
(a) Write a balanced equation for the reaction of propane (C3H8) with steam. (2 marks)
(b) i. Why is it NOT necessary to give a ?Hf value for H2(g)?
ii. What is the enthalpy change for the reaction in which one mole of propane combines completely with steam?
iii. Is the reaction exothermic or endothermic? Explain your answer. (5 marks)
(c) The ?Hf value for H2O(l) is -286 kJ mol-1. Account for this difference. (1 mark)
(d) Why is the catalyst added in this commercial process? How does it achieve its effect?
5) Given the following thermodynamic information:
heat of combustion of C(s) - 392.9 kJ mol-1
heat of combustion of H2(g) - 285.5 kJ mol-1
heat of formation of methanol(l) - 249.5 kJ mol-1
heat of crystallization of H2O(l) - 334.4 J g -1
heat capacity of H2O(l) - 4.18 J g-1 K-1
(a) i. Write a balanced thermochemical equation for the combustion of methanol.
ii. Calculate the heat of formation of methanol (in kJ mol-1) (4 marks)
(b) 8.0 g of methanol was burned in excess oxygen. Calculate the enthalpy change which accompanied this process. (2 marks)
(c) i. To what temperature would 500g of liquid water at 10oC be raised by the
combustion of 8.0g of methanol assuming complete thermal transfer?
ii. What mass of ice at a temperature of 0oC could be completely melted by the combustion of 8.0g of methanol assuming complete thermal transfer?
6) (a) Explain the following observations.
i. The reaction Cl2(g) Ô 2Cl(g) is endothermic but occurs spontaneously at very high temperatures.
ii. The process Na(g) Ô Na+(g) is endothermic but the process Cl(g) + e- à Cl-(g) is exothermic.
iii. The reaction H2(g) + I2(s) Ô 2HI(g) is endothermic but the reaction H2(g) + I2(g) à 2HI(g) is exothermic. (7 marks)
(b) It has been stated that 'reversible reactions tend to proceed spontaneously in the direction which is accompanied by the evolution of heat'. Discuss this statement critically using relevant examples to support your answer. (3 marks)
7) Hydrazine (N2H4) will react vigorously with oxygen gas to produce nitrogen gas and liquid water according to the equation:
N2H4(l) + O2(g) --> N2(g) + 2H2O(l)
(a) The reaction of 0.210g of hydrazine with a stoichiometric amount of oxygen is found to release 4.10 kJ of heat energy. (4 marks)
i. What is the sign associated with the enthalpy change of this process?
ii. Calculate the enthalpy of combustion of hydrazine in kJ×mol-1.
(b) The enthalpy of formation of hydrazine (?Hf°) is represented by 2H2(g) + N2(g) --> N2H4(l) (6 marks)
i. Write an equation to represent the enthalpy of formation of H2O(l)
ii. Using the equation from (b) i. and the original equation for the combustion of hydrazine show how they may be combined to give a third equation which represents the enthalpy of formation of hydrazine.
iii. If the enthalpy of formation of water is known to be –286 kJ×mol-1 calculate a numerical value for the enthalpy of formation of hydrazine.
(c) If hydrazine were oxidised according to the equation
N2H4(l) + 3O2(g) --> 2NO2(g) + 2H2O(l)
would the enthalpy change for this reaction be greater than or less than the enthalpy
change for the reaction which produces nitrogen gas as a product? Explain your
reasoning . (hint: ?Hf° (NO2) = +33 kJ×mol-1)
8) (a) (6 marks)
i. Using water as an example explain the differences between the three states of matter. Your answer should refer to the forces between the particles their energies and the distances which separate them in each phase.
ii. Each change of phase is accompanied by an enthalpy change. Give the names by which these enthalpy changes are known and state whether or not they are endothermic or exothermic. Remember that for each phase change there are two possible enthalpy changes – one in each direction.
(a) As a chemical reaction proceeds energy is often absorbed or released. (10 marks)
i. The terms endothermic and exothermic are frequently applied to such reactions. Explain what each of these terms means and give a chemical reaction as an example of each.