CHEM J-11 June /01(a)

Transcription

1 CHEM J-11 June /01(a) Combustion of 15.0 g of coal provided sufficient heat to increase the temperature of 7.5 kg of water from 286 K to 298 K. Calculate the amount of heat (in kj) absorbed by the water. The heat capacity of water, C p = 4.2 J K 1 g 1. 3 Assuming that coal is pure carbon, calculate the heat of combustion (in kj mol 1 ) of the coal.

2 CHEM J-11 June /01(a) The standard enthalpy of formation of SO 2 (g) is the enthalpy change that accompanies which reaction? 3 Calculate the standard enthalpy of formation of SO 2 (g) given the following data. SO 2 (g) + 1 /2O 2 (g) SO 3 (g) ΔH = 99 kj mol 1 S(s) + 3 /2O 2 (g) SO 3 (g) ΔH = 396 kj mol 1

3 CHEM J (a) The standard heat of formation of ClF 3 (g) is 159 kj mol 1. Use the bond enthalpies below to calculate the average Cl F bond enthalpy in ClF 3 (g). Bond Cl Cl F F Bond enthalpy / kj mol Explain why this number is different from the average Cl F bond enthalpy estimated for ClF 5 (g) of 151 kj mol 1. Explain the observation that the boiling point of ethanol is much higher than that of dimethyl ether despite these compounds having the same molar mass. 1 compound formula boiling point / C ethanol CH 3 CH 2 OH 78.3 dimethyl ether CH 3 OCH Page Total:

4 CHEM J-11 June /01(a) Page Total:

5 CHEM J-12 June /01(a) Write the equation whose enthalpy change represents the standard enthalpy of formation of NO(g). 3 Given the following data, calculate the standard enthalpy of formation of NO(g). N 2 (g) + 2O 2 (g) 2NO 2 (g) ΔH = 66.6 kj mol 1 2NO(g) + O 2 (g) 2NO 2 (g) ΔH = kj mol 1 Hydrazine, N 2 H 4, burns completely in oxygen to form N 2 (g) and H 2 O(g). Use the bond enthalpies given below to estimate the enthalpy change for this process. 3 Bond Bond enthalpy (kj mol 1 ) Bond Bond enthalpy (kj mol 1 ) N H 391 O=O 498 N N 158 O O 144 N=N 470 O H 463 N N 945 N O 214

6 CHEM J-9 June /01(a) A g piece of copper is heated to 80.0 C before being added to ml of water at 25.0 C. What is the final temperature of the mixture? The specific heat capacity of copper is J g 1 K 1 and the specific heat capacity of water is 4.18 J g 1 K 1. 3

7 CHEM J-12 June /01(a) Use average bond dissociation enthalpies given below to calculate the molar enthalpy change for the following chemical transformation: N 2 (g) + 3H 2 (g) 2NH 3 (g) 6 Bond H H N H N N ΔH / kj mol What is the standard enthalpy of formation, Δ f H, of NH 3 (g)? The standard enthalpy of formation of hydrazine, N 2 H 4 (g) is +96 kj mol 1. Calculate the strength of the N N single bond in hydrazine. Suggest why the N N single bond in hydrazine is much weaker than the N H and H H bonds. Hint: Draw its Lewis structure.

8 CHEM J-9 June /01(a) How much energy is needed to convert 15 g of ice at 0.0 C to water at 60.0 C? The molar heat of fusion of water is kj mol 1 and the specific heat capacity of water is 4.18 J g 1 K 1. 3

9 CHEM J-9 June /01(a) Calculate the standard enthalpy change for the combustion of 1.00 mol of propane gas, C 3 H 8 (g), to CO 2 (g) and H 2 O(l). 3 compound C 3 H 8 (g) CO 2 (g) H 2 O(l) f H / kj mol

10 CHEM J-11 June /01(a) The conversion of SO 2 to SO 3 can occur in the catalytic converters of cars using gasoline containing traces of sulfur compounds. Calculate the enthalpy change of the following reaction. 2SO 2 (g) + O 2 (g) 2SO 3 (g) Data: S(s) + O 2 (g) SO 2 (g) H = kj mol 1 2S(s) + 3O 2 (g) 2SO 3 (g) H = kj mol 1 2

11 CHEM J- 7 June /01(a) Calculate the standard heat of reaction for the following reaction. Data: Zn(s) + 2Cu + (aq) 2Cu(s) + Zn 2+ (aq) o H f = kj mol 1 for Cu + (aq) 2 o H f = kj mol 1 for Zn 2+ (aq)

12 CHEM J-7 June /01(a) A 1.00 g sample of ammonium nitrate, NH 4 NO 3, is decomposed in a bomb calorimeter causing the temperature of the calorimeter to increase by 6.12 K. The heat capacity of the system is 1.23 kj C 1. Describe this process as either endothermic or exothermic. 3 What is the molar heat of decomposition for ammonium nitrate?

13 CHEM J-9 June /01(a) Calculate the standard heat of reaction for the following reaction. Zn(s) + Cu 2+ (aq) Cu(s) + Zn 2+ (aq) 2 Data: H o f = kj mol 1 for Cu 2+ (aq) H o f = kj mol 1 for Zn 2+ (aq)

14 CHEM J-8 June 2005 A sealed L flask at 30 C contains air at a pressure of atm. A 5.00 g sample of liquid water is injected into the flask and the flask heated to a temperature of 150 C, causing the water to vaporise. What is the final pressure in the flask? 3

15 CHEM J-9 June 2005 Consider the following reaction. 2C 8 H 18 (l) + 25O 2 (g) 16CO 2 (g) + 18H 2 O(l) E = kj mol 1 A mixture of C 8 H 18 (10.00 g) and O 2 (30.00 g) is allowed to react. Assuming that the reaction goes to completion, how much energy will be produced? 4

16 CHEM J-5 June 2004 Water solutions of NaOH (100 ml, 2.0 M) and HCl (100 ml, 2.0 M), both at 24.6 C, were mixed together in a coffee cup calorimeter. The temperature of the solution rose to 38.0 C during the reaction process. Write a balanced chemical equation to describe the reaction in the calorimeter. 5 Is the process an endothermic or exothermic reaction? Assuming a perfect calorimeter, determine the standard enthalpy change for the neutralisation reaction. Assume the density of water is 1.00 g ml 1. The heat capacity of water is 4.18 J K 1 g 1.

17 CHEM J-7 June 2004 Aluminium metal is a very effective agent for reducing oxides to their elements. For example, it is used as a component of the solid fuel in the space shuttle, and in the thermite reaction shown in lectures: Fe 2 O 3 (s) + 2Al(s) Al 2 O 3 (s) + 2Fe(s) Write a balanced equation for the reduction of CuO(s) to the base metal by Al(s). 5 Given the following thermochemical data, evaluate the enthalpy change per gram of reactant for the CuO and Fe 2 O 3 reactions above. Compound H f (kj mol 1 ) Fe 2 O Al 2 O CuO -157 Which would make the best rocket fuel on the basis of most energy provided per mass of fuel (i.e. biggest bounce per ounce )?

18 CHEM1001/CHEM1101 combined 2003-N-9 November 2003 Diborane (B 2 H 6 ) is a highly reactive compound, which was once considered as a possible rocket fuel for the US space program. Calculate the heat of formation of diborane at 298 K from the following reactions. 2 Reaction H r (kj mol 1 ) 2B(s) + 3 /2O 2 (g) B 2 O 3 (s) 1273 B 2 H 6 (g) + 3O 2 (g) B 2 O 3 (s) + 3H 2 O(g) 2035 H 2 (g) + 1 /2O 2 (g) H 2 O(l) 286 H 2 O(l) H 2 O(g) +44