CAMOSUN COLLEGE. Chemistry 121 Section 03

Transcription

1 CAMOSUN COLLEGE Chemistry 121 Section 03 Instructor: H. M. Cartwright Page 1 Midterm 2 Examination, March 25 th, 2015, 6.30 p.m. Time: 1 ½ hours Name Student Number Answer all questions on the examination paper itself. If you need extra space, write on the reverse of the paper, indicating clearly within your answer to the question that you have done so. Attempt all the questions. The numbers in square brackets indicate the approximate number of marks available for each part of the question. Do not open the examination paper until you are instructed to do so.

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3 1. (a) What is an exothermic reaction? [1] Page 3 (b) What sign does H have for an exothermic reaction? [1] 2. Explain the difference in thermodynamics between the system and the surroundings. [3] 3. (a) What is a thermochemical equation? [1] (b) What is Hess's law? [2]

4 (c) Calculate H for the formation of acetylene, C 2 H 2, as shown in the reaction 2C(s) + H 2 (g) C 2 H 2 (g) using the following data: [5] Page 4 C 2 H 2 (g) + 2 ½ O 2 (g) 2CO 2 (g) + H 2 O(l) C(s) + O 2 (g) CO 2 (g) H 2 (g) + ½ O 2 (g) H 2 O(l) H = kJ H = kj H = kj (d) Acetylene is burnt in oxy-acetylene torches to give a very high temperature. Taking note of your answer to part (c) above, and the equations given in the question, give reasons why an oxy-acetylene flame reaches such a high temperature. [3]

5 4. Explain what is meant by the following terms which are used in thermochemistry: Page 5 [3] (i) Open system (ii) Closed system (iii) Isolated system 5. (a) What is a state function? [2]

6 (b) For each of the following quantities place a tick to indicate whether the quantity is, or is not, a State Function: [3] Quantity Is a State Function Is not a State Function Page 6 Temperature Work Internal energy Entropy Heat Enthalpy 6. (a) Define the standard enthalpy of formation, H o f, of a compound. [2] (b) Carbon nanotubes are an elemental form of carbon produced by an electrical discharge between carbon electrodes. Which ONE of the following statements about the standard enthalpy of formation of carbon nanotubes is true? [2] (i) H o f is zero because nanotubes are just another elemental form of carbon. (ii) H o f is zero because changing the subscripts of an elemental formula does not change the standard heat of formation. (iii) H o f is not zero because carbon nanotubes are not the most stable form of the element under standard conditions. (iv) H o f is not zero because there is a temperature change when nanotubes are formed.

7 7. For EITHER the 1 st law OR the 2 nd law of thermodynamics: (a) State the chosen law, indicating which law it is. [2] Page 7 (b) Write the relevant equation for the law. [1] (c) Explain the meaning of the symbols in the equation. [2] 8. (a) Explain what is meant by the specific heat of a material. [2]

8 (b) 26 ml of 0.2 M HCl reacted with 8 ml of 0.6 M NaOH in a calorimeter. When the reaction was complete it was found that the temperature had risen by 1.83 K. You may assume that the calorimeter and liquids were all at the same temperature initially. (i) Determine which, if either, of the reagents is present in excess. [2] Page 8 (ii) Assuming that the density of the solutions is 1 g ml -1, the specific heat of the solutions is 4.18 J K -1 g -1 and the heat capacity of the calorimeter is 41 J K -1, calculate the total heat released by the neutralisation. [2] (iii) Calculate the enthalpy of the neutralisation reaction per mole of acid. [2]

9 (iv) In a second experiment the HCl was replaced by 26 ml of 0.2M HNO 3 and the experiment repeated. What should the temperature rise be? Justify your answer. [2] Page 9 (v) In a final experiment the HCl was replaced by 26 ml of 0.2M CH 3 COOH (acetic acid). Should the temperature rise be the same as that in part (iv) or different? Justify your answer. [2] 9. For each of the following processes predict whether the entropy of the system will increase or decrease; briefly explain your reasoning for each one: [3 x 2] O 3 (g) 1 ½ O 2 (g) Liquid sulphur (108 o C) Solid sulphur (108 o C)

10 2HBr(g) Br 2 (g) + H 2 (g) Page (a) What is a spontaneous process in thermodynamics? [2] (b) What thermodynamic parameter is used in thermodynamics to predict the spontaneity of the reaction? [1] (c) Write down an equation for the change in Gibb's Free Energy, G, in terms of changes in entropy and enthalpy. [1] (d) Write a balanced equation for the combustion of liquid dimethyl ether (C 2 H 6 O) in oxygen. [2]

11 (e) What sign should S have for the combustion in part (d)? Justify your answer. [2] Page 11 (f) What sign should H have for the combustion in part (d)? Justify your answer. [2] (g) Taking note of your answers to parts (c), (e) and (f), over what temperature range would this reaction be spontaneous? [2] 11. Halogens can react with one another to form "interhalogen" compounds; the reaction between chlorine and brome is typical: Cl 2 (g) + Br 2 (g) 2ClBr(g) (a) Which ONE of the following statements is true when the reaction has reached equilibrium at a temperature of 80 o C when all species are gases? [2] (i) All reaction has come to a halt. (ii) The amounts of reactant and products are equal. (iii) The amount of reactant is zero. (iv) The concentration of chlorine and bromine is equal. (v) The rates of the forward and back reaction are equal.

12 (b) The normal boiling point of ClBr is 5 o C, of Cl 2 is -34 o C, and of Br 2 is 59 o C. Suppose that the reaction above is carried out at 25 o C; would your conclusion about which of options (i)-(v) is true be different? Explain. Page (a) State Le Chatelier's Principle. [3] (b) Write down the expression for the equilibrium constant K c for each of the following reactions: [4] 2NH 3 (g) N 2 (g) + 3 H 2 (g) CH 3 COOH (aq) CH 3 COO (aq) + H + (aq) CH 3 COOH (aq) + H 2 O (l) CH 3 COO (aq) + H 3 O + (aq)

13 CaCO 3 (s) CaO (s) + CO 2 (g) (c) Predict whether the reaction given below is more likely to be exothermic or more likely to be endothermic (hint: consider the reverse reaction). Justify your choice. [2] Page 13 2CO 2 (g) + 3H 2 O(l) C 2 H 5 OH (l) + 3 O 2 (g) (d) The reaction shown in part (c) above is allowed to run to equilibrium. In which direction would the equilibrium shift if each of the following changes were made: [4 x 2] (i) An increase in the concentration of CO 2? (ii) An increase in the amount of liquid methanol? (iii) An increase in the temperature? (iv) A decrease in the total pressure? (v) The addition of a catalyst?

14 13. Phosphorus trichloride reacts with chlorine in an equilibrium reaction to generate phosphorus pentachloride: Page 14 PCl 3 (g) + Cl 2 (g) PCl 5 (g) (a) Write down the expression for the equilibrium constant for the reaction, K c. [1] (b) At a certain temperature when the reaction had reached equilibrium, the concentrations of the three species were: [PCl 3 ] = 0.42 M [Cl 2 ] = 0.37 M [PCl 5 ] = 0.39M Calculate the value of the equilibrium constant. [1] (c) In a second experiment at the same temperature, the initial concentrations were: [PCl 3 ] = 0.53M [Cl 2 ] = 0.53M [PCl 5 ] = 1.1M. Determine the concentrations of all three species in this second experiment once equilibrium has been achieved. [4] Note: the solutions of the quadratic equation ax 2 + bx + c = 0 are x = b± (b2 4ac) 2a

15 14. Explain the difference between Arrhenius and Bronsted-Lowry acids. Give one example of each type. [4] Page 15