4 Energetics Exam-style questions. AQA Chemistry

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1 1 Amended from AQA Chemistry Unit 2 Chemistry in Action CHEM2 January 2011 (Question 9) a A student carried out a laboratory experiment to determine the enthalpy change when a sample of butan-1-ol was burnt. The student found that the temperature of 175 g of water increased by 8.0 C when mol of pure butan-1-ol was burnt in air and the heat produced was used to warm the water. Use the student s results to calculate a value, in kj mol 1, for the enthalpy change when one mole of butan-1-ol is burnt. Give your answer to an appropriate number of significant figures. (The specific heat capacity of water is 4.18 J K 1 g 1 ) b i Give the meaning of the term standard enthalpy of combustion. This resource sheet may have been changed from the original 1

2 c Use the standard enthalpy of formation data from the table and the equation for the combustion of butan-1-ol to calculate a value for the standard enthalpy of combustion of butan-1-ol. CH 3 CH 2 CH 2 CH 2 OH(l) O 2 (g) CO 2 (g) H 2 O(l) ΔH f ο / kj mol CH 3 CH 2 CH 2 CH 2 OH(l) + 6O 2 (g) 4CO 2 (g) + 5H 2 O(l) d The student repeated the experiment described in part a and obtained an experimental value for the enthalpy of combustion for each alcohol in this series. These experimental values were then compared with calculated values from standard enthalpies of formation, as shown in Figure 1.1. Figure 1.1 This resource sheet may have been changed from the original 2

3 i In terms of bonds broken and bonds formed, explain why the calculated values of enthalpies of combustion of these alcohols, when plotted against M r, follow a straight line. (2 marks) ii Give two reasons why the values obtained by the student doing the experiment are lower than the calculated values using the enthalpy of formation data. (2 marks) This resource sheet may have been changed from the original 3

4 2 Amended from AQA Chemistry Unit 2 Chemistry in Action CHEM2 January 2012 (Question 4) Iron is extracted from iron(iii) oxide using carbon at a high temperature. a At a high temperature, carbon undergoes combustion when it reacts with oxygen. i Suggest why it is not possible to measure the enthalpy change directly for the following combustion reaction. C(s, graphite) O2 (g) CO(g) ii State Hess s Law. iii State the meaning of the term standard enthalpy of combustion. This resource sheet may have been changed from the original 4

5 b Use the standard enthalpies of formation in the table below and the equation to calculate a value for the standard enthalpy change for the extraction of iron using carbon monoxide. Fe 2 O 3 (s) CO(g) Fe(l) CO 2 (g) f H ο / kj mol Fe 2 O 3 (s) + 3CO(g) 2Fe(l) +3CO 2 (g) c i Write an equation for the reaction that represents the standard enthalpy of formation of carbon dioxide. Include state symbols in your answer. ii State why the value quoted in part c for the standard enthalpy of formation of CO 2 (g) is the same as the value for the standard enthalpy of combustion of carbon. This resource sheet may have been changed from the original 5

6 3 Amended from AQA Chemistry Unit 2 Chemistry in Action CHEM2 June 2009 (Question 2) Barium can be extracted from barium oxide (BaO) in a process using aluminium. A mixture of powdered barium oxide and powdered aluminium is heated strongly. The equation for this extraction process is shown below. 3BaO(s) + 2Al(s) 3Ba(s) + Al 2 O 3 (s) Some standard enthalpies of formation are given in the table below. Substance BaO(s) Al 2 O 3 (s) H f ο / kj mol a State what is meant by the term standard enthalpy of formation. b State why the standard enthalpy of formation of barium and that of aluminium are both zero. c Use the data to calculate the standard enthalpy change for the reaction shown by the equation above. This resource sheet may have been changed from the original 6

7 4 Amended from AQA Chemistry Unit 2 Chemistry in Action CHEM2 May 2012 (Question 2) Methanol (CH 3 OH) is an important fuel that can be synthesised from carbon dioxide. a The table shows some standard enthalpies of formation. CO 2 (g) H 2 (g) CH 3 OH(g) H 2 O(g) H f Ɵ / kj mol i Use these standard enthalpies of formation to calculate a value for the standard enthalpy change of this synthesis. CO 2 (g) + 3H 2 (g) CH 3 OH + H 2 O(g) ii State why the standard enthalpy of formation for hydrogen gas is zero. This resource sheet may have been changed from the original 7

8 b A student carried out an experiment to determine the enthalpy change when a sample of methanol was burnt. The student found that the temperature of 140 g of water increased by 7.5 C when mol of methanol was burnt in air and the heat produced was used to warm the water. Use the student s results to calculate a value, in kj mol 1, for the enthalpy change when one mole of methanol was burnt. Give your answer to an appropriate number of significant figures. (The specific heat capacity of water is 4.18 J K 1 g 1 ). This resource sheet may have been changed from the original 8

9 5 Amended from AQA Chemistry Unit 2 Foundation Physical and Inorganic Chemistry CHM2 June 2003 (Question 4) a Write an equation for the complete combustion of propanone, C 3 H 6 O, to form carbon dioxide and water. b In a laboratory experiment, 1.45 g of propanone were burnt completely in oxygen. The heat from this combustion was used to raise the temperature of 100 g of water from K to K. i Calculate the number of moles of propanone in the 1.45 g. (2 marks) ii Calculate the heat energy required to raise the temperature of 100 g of water from K to K. (The specific heat capacity of water is 4.18 J K 1 g 1.) (2 marks) iii Hence, calculate a value, in kj mol 1, for the enthalpy of combustion of propanone. c In a similar experiment, the enthalpy of combustion of butanone, C 4 H 8 O, was found to be 1290 kj mol 1. A data book value for the same reaction is ΔH ο c = 2430 kj mol 1. i Suggest one reason why the experimental value is very different from the data book value. This resource sheet may have been changed from the original 9

10 ii This data book value of ΔH c ο for butanone ( 2430 kj mol 1 ) refers to the formation of carbon dioxide gas and water in the gaseous state. How would this value differ if it referred to the formation of water in the liquid state? Explain your answer. Difference Explanation This resource sheet may have been changed from the original 10

11 6 From AQA Chemistry Unit 2 Foundation Physical and Inorganic Chemistry CHM2 June 2005 (Question 1) a Explain the meaning of the terms mean bond enthalpy and standard enthalpy of formation. Mean bond enthalpy Standard enthalpy of formation (5 marks) b Some mean bond enthalpies are given below. Bond N H N N N N H O O O Mean bond enthalpy / kj mol Use these data to calculate the enthalpy change for the following gas-phase reaction between hydrazine, N 2 H 4, and hydrogen peroxide, H 2 O 2. c Some standard enthalpies of formation are given below. N 2 H 4 (g) H 2 O 2 (g) H 2 O(g) H ο f / kj mol These data can be used to calculate the enthalpy change for the reaction in part b. N 2 H 4 (g) + 2H 2 O 2 (g) N 2 (g) + 4H 2 O(g) This resource sheet may have been changed from the original 11

12 i State the value of H f ο for N 2 (g). ii Use the H f ο values from the table to calculate the enthalpy change for this reaction. (4 marks) d Explain why the value obtained in part b is different from that obtained in part c ii. This resource sheet may have been changed from the original 12