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CHigher (National 6) Unit 3: Chemistry in Society 3c Chemical Energy

1. What are the meanings of the terms exothermic reaction and endothermic reaction?

2. Opposite is a potential energy diagram for the combustion of a fuel.

(a) What is the energy of activation for this reaction and what is the enthalpy change?

(b) What is the activation energy for the reverse reaction?

(c) What is formed at X?

3. Calculate the amount of heat energy (in kJ) required to raise the temperature of 250cm3 of water by 15˚C.

4.

(a) Write the equation to show the enthalpy change of combustion of propan-1-ol.

(b) In an experiment to discover the enthalpy of combustion of propan-1-ol a pupil heated 100g of water using a spirit burner containing the fuel. 0.5g of propan-1-ol burned. This resulted in a temperature change from 10.5˚C to 42.5˚C.

Calculate the enthalpy of combustion of propan-1-ol from these results.

(c) Draw a labelled diagram of the assembled apparatus.

(d) Provide reasons as to why the experimental value is often lower than the actual value.

(e) Which piece of equipment could be used to improve the reliability and accuracy of the results.

5. Using the value quoted on page 10 of the data booklet calculate the heat released when 4g of methane burns in excess oxygen.

6. The booster of the Saturn V rocket burned methyl hydrazine CH3NHNH2:

4CH3NHNH2(I) + 5N2O4(g) 4CO2(g) + 12H2O(I) + 9N2 (g) ∆H= -5116 kJ

(a) What was the purpose of the dinitrogen tetroxide N2O4?

(b) Draw the full structural formula for methyl hydrazine CH3NHNH2.

(c) Calculate the enthalpy of combustion for methyl hydrazine.

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C7. A pupil uses a cylinder of camping gas to measure the enthalpy of combustion of butane. The experiment was set up as shown opposite.

The pupil found that 2.g of butane burned to give out 72.4kJ of energy.

(a) Write a balanced chemical equation to show the reaction which corresponds to the enthalpy of combustion of butane.

(b) Apart from the mass of the butane cylinder at the start and end of the experiment state 3 additional measurements which the pupil would have to make.

(c) Calculate the experimental combustion of methane.

8. At room temperature a lump of white phosphorus reacts with air forming phosphorus (V) oxide.

4P(s) + 5O2(g) P4O10(s) ∆H= -3000 kJ

(a) Use this information to calculate the enthalpy of combustion of white phosphorus.

(b) State the effect of using powdered phosphorus in place of a lump on:

(i) the rate of reaction and

(ii) the enthalpy change for the reaction.

9. A page of a pupil’s notebook shows instructions on how to measure the enthalpy of combustion of an alcohol.

(a) Draw a neat labelled diagram of the apparatus which the pupil could use to carry out this experiment.

(b) Write a balanced chemical equation to show the reaction which corresponds to the enthalpy of combustion of methanol.

(c) The pupil found that when 0.23g of methanol burned the heat produced raised the temperature of 100g of water by 9.2˚C.

Using information from page 10 of the data booklet calculate the enthalpy of combustion of methanol.

(d) The pupil’s result is well below the value in the data booklet. Even with insulation much heat is lost to the surroundings, including the apparatus.

Suggest one other reason why the experimental result is low.

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10.

(a) State Hess’s Law.

(b) Describe three experiments you would carry out the chemical: solid sodium hydroxide, hydrochloric acid and water, to prove Hess’s law.

11. Using Hess’s Law, calculate the enthalpy of formation of propane using the enthalpies of combustion for carbon, hydrogen and propane in the Data book.

12. Given the enthalpies of combustion of enthalpies of ethane and carbon monoxide are – 1543 kJ mol -1 and -284 kJ mol-1 respectively, calculate, using Hess’s Law, the enthalpy change of the reaction:

C2H6 + 2.5O2 2CO + 3 H2O

13.

KOH(s) + (aq) KOH(aq) ∆H = a

KOH(s) + HCI(s) KC(aq) + H2O(I) ∆H = b

KOH(aq) + HCI(aq) KCI(aq) + H2O(I) ∆H = c

From the above data write an equation to show the relationship between a, b and c.

14.

(a) Given that the enthalpy of formation magnesium oxide is -1162 kJ and that of silver(l)oxide is -5 kJ, calculate the enthalpy change for the reaction:

Mg(s) + Ag2O(s) 2Ag(s) + MgO (s)

(b) Given the chemical name for the above reaction.

(c) What information does the enthalpy of formations given about the relative:

(i) reactivity of the two elements, and

(ii) the stability of their compounds?

15. Using the information below

CH3NHNH2(I) + 2.5O2(g) CO2(g) + 3H2O(I) + N2 (g) ∆H= 1305 kJ

And the enthalpies of combustion of carbon and hydrogen in the Data book, calculate the enthalpy change for the reaction:

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CC(s) + 3H2(g) + N2(g)--- CH3NHNH2(I)

16. Using bond enthalpies, calculate the enthalpy change for the combustion of hydrogen to produce water shown by the equation below.

2H2 (g) + O2 (g) 2H2O (g)

17. Use bond enthalpy values from the data book to calculate the enthalpy change for the following reaction.

CH4 (g) + Br2 (g) CH3Br (g) + HBr (g)

18. The data book gives the enthalpy of combustion of methane as –891kJ mol–1. Use bond enthalpies to calculate the enthalpy change for this reaction.

CH4 (g) + 2O2 (g) CO2 (g) + 2H2O (g)

19. Using bond enthalpy values, calculate the enthalpy change for the following addition reaction.

C2H4 (g) + HBr (g) C2H5Br (g)

20. Use the bond enthalpy values quoted in the data book to calculate the enthalpy change for the hydrogenation of but-1-ene.

C4H8 (g) + H2 (g) C4H10 (g)

21. Using bond enthalpy values, calculate the enthalpy change for the addition reaction between iodine and propene.

C3H6 (g) + I2 (g) C3H6I2 (g)

22. Hydrogen chloride can react with ethyne in a two-stage addition process to give a saturated product. Calculate the enthalpy change for this reaction using bond enthalpy values from the data book.

C2H2 (g) + 2HCl (g) C2H4Cl2 (g)

23. The data book gives the enthalpy of combustion of ethanol as –1367 kJ mol–1. Use bond enthalpies to calculate the enthalpy change for this reaction.

C2H5OH (g) + 3O2 (g) 2CO2 (g) + 3H2O (g)

24. Calculate the enthalpy of formation for ethene using the enthalpy of sublimation and bond enthalpy values from the data book.

2C (s) + 2H2 (g) C2H4 (g)

25. The data book quotes the mean bond enthalpy for a carbon-to-carbon double bond (C=C) as 602 kJ mol–1. Use the enthalpy of formation given and bond enthalpies from the data book to calculate the enthalpy of the C=C bond in ethene.

2C (s) + 2H2 (g) C2H4 (g) ∆Hformation = 52 kJ mol–1

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Exam type questions (2011 – 2014)

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37. The enthalpies of combustion of some alcohols are shown in the table.

Name of alcohol Enthalpy of combustion/ kJmol-1

Methanol -727Ethanol -1367Propan-1-ol -2020

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C(a) Using this data predict the enthalpy of combustion of butan-1-ol, in kJ mol-1.

(b) The value for the enthalpy of combustion of butan-2-ol, C4H9OH, can be determined experimentally using the apparatus shown.

Use these results to calculate the enthalpy of combustion of butan-2-ol, in kJmol -1.

(c) Enthalpy changes can also be calculated using Hess’s Law.

The enthalpy of formation for pentan-1-ol is shown below.

Using this value, and the enthalpies of combustion of carbon and hydrogen from the data booklet, calculate the enthalpy of combustion of pentan-1-ol, in kJmol-1.

38. Ethanol C2H5OH, can be used as a fuel in some camping stoves.

(a) The enthalpy of combustion of ethanol is -1367 kJmol-1.

Using this value calculate the number of moles of ethanol required to raise the temperature of 500g of water from 18˚C to 100˚C.

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CShow your working clearly.

(b) Suggest two reasons why less energy is obtained from burning ethanol in the camping stove than is predicted from its enthalpy of combustion.

39. The compound diborane (B2H6) is used as a rocket fuel.

(a) It can be prepared as shown.

Balance the above equation.

(b) The equation for the combustion of diborane is shown below.

Calculate the enthalpy of combustion of diborane (B2H6) using the following data.

(c) Diborane can be used to manufacture pentaborane (B5H9).

Pentaborane was also considered for use as a rocket fuel because its enthalpy of combustion is -9037 kJmol -1.

Calculate the energy released, in kJ, when 1 kilogram of pentaborane is completely burned.

40. Mobile phones are being developed that can be powered by methanol.

Methanol can be made by a two-stage process. The first stage is shown below.

Use the data below to calculate the enthalpy change, in kJmol-1, for the forward reaction in the first stage.

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Unit 3c: Chemical Energy Ink Exercise

1. The table below gives the heats of combustion (enthalpies of combustion) of three alcohols:

Alcohol ∆H (kJmol-1)Methanol -726Ethanol -1370Propanol -2020

(a) Estimate the heat of combustion of butanol. (1)

(b) Ethanol and methoxy methane are isomers. Their structures are shown below:

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Why do they have different heats of combustion? (1)

2.

(a) Write a balanced chemical equation for the complete combustion of propane. (1)

(b) Use the value quoted in your data booklet to calculate the heat released when 2.2g of propane burns completely in oxygen. (2)

(c) Suggest the name of a device that could be used it to improve the reliability and accuracy of the result.

(1)

3. Methanol can be prepared industrially by reacting a mixture of carbon monoxide and water in the presence of a chromium (III) oxide catalyst.

CO(g) + 2H2(g) CH3OH(l)

Using bond enthalpies calculate the enthalpy change for the reaction shown above. (2)

4. Use the enthalpies of combustion given in your data booklet to calculate the enthalpy change for the reaction:

2C(s) + 3H2(g) C2H6(g) (2)

5. Using Hess’s Law and the information in the table opposite, calculate the enthalpy change for the reaction:

3C(g) + 4H2(g) C3H8(g) (2)

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Enthalpy change ∆H (kJ)C(s) C(g) 7153C(s) + 4H2(g) C3H8(g) -104

(Bond dissociation of 1075 kJmol-1)