3.2.1 Energetics. Bond Enthalpy. 98 minutes. 96 marks. Page 1 of 16

Transcription

1 3..1 Energetics Bond Enthalpy 98 minutes 96 marks Page 1 of 16

2 Q1. (a) State what is meant by the term mean bond enthalpy. () (b) Ethanal has the structure Gaseous ethanal burns as shown by the equation CH 3 CHO(g) + ½O (g) H O(g) + CO (g) Use the mean bond enthalpy data given below to answer the following questions. Bond Mean bond enthalpy/kj mol 1 C H +413 C C +347 C==O +736 O==O +498 O H +464 Calculate the enthalpy change which occurs when all the bonds in the reactants shown in the above equation are broken. Calculate the enthalpy change which occurs when all the bonds in the products shown in the above equation are formed. Page of 16

3 (iii) Hence, calculate the enthalpy change for the complete combustion of ethanal as shown in the equation above. (5) (Total 7 marks) Q. The table below contains some mean bond enthalpy data. Bond H H C C C=C N N N H Mean bond enthalpy / kj mol (a) Explain the term mean bond enthalpy. () (b) Write an equation for the formation of one mole of ammonia, NH 3, from its elements. Use data from the table above to calculate a value for the enthalpy of formation of ammonia. (4) Page 3 of 16

4 (c) Use the following equation and data from the table above to calculate a value for the C H bond enthalpy in ethane. (Total 9 marks) Q3. (a) The table below contains some mean bond enthalpy data. Bond H O O O O=O Mean bond enthalpy/kj mol The bonding in hydrogen peroxide, H O, can be represented by H O O H. Use these data to calculate the enthalpy change for the following reaction. H O (g) H O (g) + O (g) (b) The standard enthalpy of formation, ΔH f for methane, is 74.9 kj mol 1. Write an equation, including state symbols, for the reaction to which this enthalpy change applies. () Page 4 of 16

5 (c) The enthalpy changes for the formation of atomic hydrogen and atomic carbon from their respective elements in their standard states are as follows. H (g) H(g) ΔH = +18 kj mol 1 C(s) C(g) ΔH = +715 kj mol 1 By reference to its structure, suggest why a large amount of heat energy is required to produce free carbon atoms from solid carbon. Parts (b) and (c) give enthalpy data for the formation of CH 4 (g), H(g) and C(g). Use these data and Hess s Law to calculate the value of the enthalpy change for the following reaction. CH 4 (g) C(g) + 4H(g) (iii) Use your answer from part (c) to calculate a value for the mean bond enthalpy of a C H bond in methane. (5) (Total 10 marks) Q4. (a) Explain the meaning of the terms mean bond enthalpy and standard enthalpy of formation. Mean bond enthalpy... Standard enthalpy of formation... (5) Page 5 of 16

6 (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 H 4, and hydrogen peroxide, H O + H O O H N N + 4 H O H (c) Some standard enthalpies of formation are given below. N H 4 (g) H O (g) H O(g) H f ο /kj mol These data can be used to calculate the enthalpy change for the reaction in part (b). N H 4 (g) + H O (g) N (g) + 4H O(g) State the value of H f ο for N (g). Use the H f ο values from the table to calculate the enthalpy change for this reaction. (4) Page 6 of 16

7 (d) Explain why the value obtained in part (b) is different from that obtained in part (c). (Total 13 marks) Q5. (a) Define the term standard enthalpy of combustion, H c ο (b) Use the mean bond enthalpy data from the table and the equation given below to calculate a value for the standard enthalpy of combustion of propene. All substances are in the gaseous state. Bond C == C C C C H O == O O == C O H Mean bond enthalpy/ kj mol (c) State why the standard enthalpy of formation, H f ο, of oxygen is zero. Page 7 of 16

8 (d) Use the data from the table below to calculate a more accurate value for the standard enthalpy of combustion of propene. Compound C 3 H 6 (g) CO (g) H O(g) Standard enthalpy of formation, ο H f / kj mol (e) Explain why your answer to part (b) is a less accurate value than your answer to part (d). () (Total 1 marks) Q6. Hydrogen gas is used in the chemical industry. (a) Tungsten is extracted by passing hydrogen over heated tungsten oxide (WO 3 ). State the role of the hydrogen in this reaction. Write an equation for this reaction. (iii) State one risk of using hydrogen gas in metal extractions. Page 8 of 16

9 (b) Hydrogen is used to convert oleic acid into stearic acid as shown by the following equation. oleic acid + H CH 3 (CH ) 16 COOH stearic acid Use your knowledge of the chemistry of alkenes to deduce the type of reaction that has occurred in this conversion. State the type of stereoisomerism shown by oleic acid. (c) Hydrogen reacts with nitrogen in the Haber Process. The equation for the equilibrium that is established is shown below. N (g) + 3H (g) NH 3 (g) State Le Chatelier s principle. Use Le Chatelier s principle to explain why an increase in the total pressure of this equilibrium results in an increase in the equilibrium yield of ammonia. () Page 9 of 16

10 (d) Hydrogen reacts with oxygen in an exothermic reaction as shown by the following equation. H (g) + O (g) H O(g) H = 4 kj mol 1 Use the information in the equation and the data in the following table to calculate a value for the bond enthalpy of the H H bond. O H O=O Mean bond enthalpy / kj mol (Total 11 marks) Q7. A student carried out an experiment to study the rates of hydrolysis of some haloalkanes. (a) In the experiment, two different haloalkanes were placed in separate test tubes containing silver nitrate solution. The haloalkanes reacted with the water in the silver nitrate solution. The student timed how long it took for the first appearance of the silver halide precipitate in each tube at a constant temperature. This time was used to provide a measure of the initial rate of reaction. The student obtained the following results. 1-bromobutane 1-iodobutane Time to form a precipitate / s State the meaning of the term hydrolysis. State the colour of the precipitate formed when iodide ions react with silver nitrate and write the simplest ionic equation for this reaction. Colour of precipitate... Simplest ionic equation () Page 10 of 16

11 (iii) Use your knowledge of the reactions of halide ions with silver nitrate to suggest why the student did not include 1-fluorobutane in this experiment. () (b) The student used the following enthalpy data to try to account for the different initial rates of hydrolysis of the haloalkanes used in part (a). The student deduced that the rate of hydrolysis of a haloalkane is influenced by the strength of the carbon halogen bond in the haloalkane. C Br C I Bond enthalpy / kj mol State how the experimental evidence enabled the student to make this deduction. (c) The student had read that the reaction of water with haloalkanes was similar to the reaction of aqueous sodium hydroxide with haloalkanes and was an example of a nucleophilic substitution reaction. State the meaning of the term nucleophile. When a hydroxide ion collides with a molecule of 1-bromobutane, the following reaction occurs. CH 3 CH CH CH Br + OH CH 3 CH CH CH OH + Br Outline the nucleophilic substitution mechanism for this reaction. () Page 11 of 16

12 (d) The reaction of hydroxide ions with -bromo--methylpropane may occur by a different mechanism from the one in part (c). This different mechanism involves the formation of a carbocation. Complete the following equation by drawing the structure of the carbocation formed when the C Br bond in -bromo--methylpropane is broken. Suggest one reason why this reaction occurs by a mechanism involving a carbocation, but the reaction in part (c) does not. (Total 11 marks) Q8. Hydrazine (N H 4 ) decomposes in an exothermic reaction. Hydrazine also reacts exothermically with hydrogen peroxide when used as a rocket fuel. (a) Write an equation for the decomposition of hydrazine into ammonia and nitrogen only... (b) State the meaning of the term mean bond enthalpy () Page 1 of 16

13 (c) Some mean bond enthalpies are given in the table. N H N N N N O H O O Mean bond enthalpy / kj mol Use these data to calculate the enthalpy change for the gas-phase reaction between hydrazine and hydrogen peroxide (Total 6 marks) Q9. Ethanol is an important industrial compound. (a) Ethanol can be produced by the hydration of ethene. The equation for the equilibrium that is established is H C=CH (g) + H O(g) CH 3 CH OH(g) ΔH = 4 kj mol 1 The operating conditions for the process are a temperature of 300 o C and a pressure of 7 MPa. Under these conditions, the conversion of ethene into ethanol is 5%. Identify the catalyst used in this process. Deduce how an overall yield of 95% is achieved in this process without changing the operating conditions () Page 13 of 16

14 Use your knowledge of equilibrium reactions to explain why a manufacturer might consider using an excess of steam in this process, under the same operating conditions (iii) At pressures higher than 7 MPa, some of the ethene reacts to form a solid with a relative molecular mass greater than Deduce the identity of this solid. Give one other reason for not operating this process at pressures higher than 7 MPa. Do not include safety reasons () (b) Write an equation for the reaction that has an enthalpy change that is the standard enthalpy of formation of ethanol... () (c) When ethanol is used as a fuel, it undergoes combustion. Define the term standard enthalpy of combustion Page 14 of 16

15 Consider these bond enthalpy data. C H C C C O O=O C=O O H Bond enthalpy / kj mol Use these data and the equation to calculate a value for the enthalpy of combustion of gaseous ethanol. CH 3 CH OH(g) + 3O (g) CO (g) + 3H O(g) (d) Gaseous ethanol can be used to convert hot copper(ii) oxide into copper. Deduce the role of ethanol in this reaction... Draw the structure of the organic compound with M r = 60 that is produced in this reaction. (Total 17 marks) Page 15 of 16

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