NT Exampro 1

1. Define the term standard enthalpy of combustion.

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.................................................................................................................................... (3)

2. Calculate the enthalpy change for the reduction of propanoic acid to propanal:

CH3CH2COOH + H2 → CH3CH2CHO + H2O

given the following enthalpies of combustion / kJ mol–1:

propanoic acid – 1527; hydrogen – 286; propanal – 1821.

(3)

3. The thermal stability of Group 2 carbonates increases from beryllium, the carbonate of which decomposes at room temperature, to barium, the carbonate of which is stable even at quite high temperatures. Explain this trend.

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4. State Hess’s Law.

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5. (i) Write an equation, the enthalpy change for which would be the enthalpy of formation of zinc sulphide, ZnS.

.......................………................................................................................................. (1)

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NT Exampro 2

(ii) In the smelting of zinc ores, the following reaction occurs:

ZnS(s) + 1½O2(g) → ZnO(s) + SO2(g) ∆H = –441 kJ mol–1

Use this, together with the data below, to calculate a value for the enthalpy of formation of ZnS.

Data: Zn(s) + 1½O2(g) → ZnO(s) ∆H = –348 kJ mol–1

S(s) + O2(g) → SO2 (g) ∆H = –297 kJ mol–1

(5)

(Total 6 marks)

6. (a) State Hess’s Law.

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.................................................................................................................................. (2)

(b) Define the term standard enthalpy change of combustion.

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(c) The equation for the combustion of ethanol in air is

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

and the structural representation of this is:

H C C O H + 3O O 2O C O + 3H O H

H

H

H

H

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NT Exampro 3

(i) Calculate the enthalpy change for this reaction using the average bond enthalpy

values given below.

Bond Average bond enthalpy/kJ mol–1

Bond Average bond enthalpy/kJ mol–1

CH +412 CC +348

CO +360 OH +463

OO +496 CO +743 (3)

(ii) Draw and label an enthalpy level diagram to represent this reaction.

enthalpy

(2)

(Total 10 marks)

7. The enthalpy change of combustion of two fuels is listed below.

fuel enthalpy of combustion/kJ mol–1

hydrogen, H2 –280

octane, C8H18 –5510

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NT Exampro 4

(a) Calculate the enthalpy change per unit mass for each of the fuels.

Hydrogen:

Octane:

(3)

(b) Suggest, giving two reasons, which substance is the more useful as a fuel for motor cars and give your reasoning.

Reason 1.................................................................................................................

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Reason 1.................................................................................................................

............................................................................................................................... (2)

(c) Suggest one disadvantage of using the fuel chosen in (b)

............................................................................................................................... (1)

(Total 6 marks)

8. You are required to carry out a quantitative exercise to find the enthalpy change when a Group 1 compound, D, dissolves in water. (13 marks]

Quantitative exercise to find the enthalpy change when a Group 1 compound dissolves in water

You are provided with:

• a sample of a Group 1 compound, D, in a specimen tube.

You are required to find the molar enthalpy change when D dissolves in water.

D(s) + aq→D(aq)

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NT Exampro 5

PROCEDURE

(1) Rinse out a 50 cm3 measuring cylinder with distilled water then use the measuring cylinder to measure 50 cm3 of distilled water into a dry plastic cup held firmly in a beaker. Place the thermometer in the distilled water in the cup.

(2) Weigh the specimen tube containing D. Record the mass of the tube and D in the first table below.

(3) Measure the temperature of the distilled water in the cup to the nearest 0.2 °C. Record the temperature in the second table below.

(4) Empty the sample of D from the specimen tube into the plastic cup, stirring gently with the thermometer. (The accuracy of your results will not be affected if a small amount of D is left in the tube.) Continue stirring until all of the solid has dissolved then record the steady temperature of the solution to the nearest 0.2°C. Record the temperature in the second table below.

(5) Weigh the emptied specimen tube and record the mass in the first table below.

Mass of specimen tube + D g

Mass of emptied specimen tube g

Mass of D used g

(2) - Temperature of distilled water before adding D, T1, °C

Steady temperature of solution, T2 °C

Calculate the change in temperature ∆T by subtracting T1, from T2

∆T = T2 – T1 = = ....................°C (6)

Calculations and questions

(a) Calculate the molar enthalpy change when D dissolves in water using the formula

∆H = – usedofMass1.21

DT∆×

kJ mol–1

Include a sign and units with your value of ∆H which should be expressed to an appropriate number of significant figures.

∆H = (3)

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NT Exampro 6

(b) Suggest a change to the procedure, using the same apparatus, that may lead to a more

accurate value of the molar enthalpy change. Explain the reason for your suggestion.

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(Total 13 marks)

9. In an experiment to find the enthalpy change when copper is displaced from a solution of copper ions excess zinc was added to 50.0 cm3 of 1.00 mol dm–3 aqueous copper(ii) sulphate in a plastic cup.

Zn(s) + Cu2+(aq) → Zn2+(aq) + Cu(s)

The temperature of the solution in the cup was measured every minute for 10 minutes with the zinc being added after 3.5 minutes.

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NT Exampro 7

The temperature readings are shown on the graph below.

70

60

50

40

30

20

100 1 2 3 4 5 6 7 8 9 10

Time/minutes

Tem

pera

ture

/°C

× × ×

××

××

××

×

(a) Suggest two reasons why a series of temperature readings is taken rather than simply initial and final readings.

First reason..............................................................................................................

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Second reason..........................................................................................................

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(b) Use the graph to calculate the maximum temperature change, ∆T, as the reaction takes place.

∆T = ...........................................°C (2)

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NT Exampro 8

(c) Calculate the enthalpy change for the reaction using the formula below, giving your

answer to an appropriate number of significant figures.

∆H = –4.18 × ∆T kJ mol–1

(2)

(Total 6 marks)

10. Alkanes are often used as fuels in a variety of situations. Methane, CH4 is used as a gas to heat houses and offices whereas octane, C8H18 is used in petrol.

Methane burns to form carbon dioxide and water as follows:

CH4 + 2O2 → CO2 + 2H2O

The enthalpy of combustion of methane is –890 kJ mol–1. The enthalpy of combustion of octane is –5512 kJ mol–1. Octane might seem a much better fuel than methane in all circumstances but this is not necessarily so.

(a) (i) Define the term standard enthalpy of combustion.

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(ii) Why does methane not react with air unless a flame or spark is applied to the mixture?

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….……………………………………………………………………………. (2)

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NT Exampro 9

(ii) Draw a reaction profile diagram for the combustion of methane.

(3)

(b) (i) Write a balanced equation for the complete combustion of octane in oxygen.

….……………………………………………………………………………. (2)

(ii) What volume of air, which contains 20% oxygen, would be needed to burn one mole of octane? (Under the conditions of the experiment, 1 mole of gas occupies 24 dm³).

(2)

(iii) Burning 1.00 g of octane produces 48.4 kJ of energy. Calculate the heat energy produced by burning 1.00 g of methane.

(2)

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NT Exampro 10

(iv) Burning 1.00 dm³ of liquid octane releases 33800 kJ of energy. Calculate the heat

energy released on burning 1.00 dm³ of gaseous methane. (Under the conditions of the experiment, 1 mole of gas occupies 24 dm³).

(2)

(v) Suggest why octane is preferred to methane as a fuel for motor cars.

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(c) What role does methane play in the industrial production of ammonia?

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…...…………………………………………………………………………………. (2)

(Total 21 marks)

11. (a) List all the measurements you would make, when using the apparatus shown in the diagram below, in an experiment to find the enthalpy of combustion of ethanol.

water

ethanol

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NT Exampro 11

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(b) Using the results obtained with the apparatus shown in the diagram a student calculates and records the following value for the enthalpy of combustion of ethanol.

∆Hcθ (C2H5OH) = 853.54966 kJ mol–1

Make three criticisms of the student’s recorded value giving reasons for each.

Criticism 1 …………………………………………………………………………..

Reason ………………………………………………………………………………

Criticism 2 ………………….……………………………………………………….

Reason ……………………..………………………………………………………..

Criticism 3 …………………………………………………………….…………….

Reason ………...……………………………………………………………………. (6) ww

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NT Exampro 12

(c) Suggest two reasons why the student ’s value is much lower than that given in a data

book.

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(Total 13 marks)

12. (a) (i) Define the term standard enthalpy of formation, ∆Hf .

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(ii) The following table shows some values of standard enthalpy of formation.

Name Formula ∆Hf /kJ mol–1

ethene C2H4(g) +52.3

hydrogen bromide HBr(g) –36.2

bromoethane C2H5Br(g) –60.4

Use the data in the table above to calculate the standard enthalpy change for the following reaction.

C2H4(g) + HBr(g) → C2H5Br(g)

(2)

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NT Exampro 13

(iii) State the significance of the sign of the value obtained in part (a)(ii) above.

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(b) Enthalpy changes can also be calculated using average bond enthalpy data.

Bond Average bond enthalpy/kJ mol–1

C == C +612

C C +348

C H +412

C Br +276

H Br +366

Use the data in the table above to recalculate the enthalpy change for the reaction in part (a)(ii).

C2H4(g) + HBr(g) → C2H5Br(g)

(3)

(c) Suggest why the value obtained in part (b) above is likely to be less accurate than that obtained in part (a)(ii).

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.................................................................................................................................... (2)

(Total 11 marks)

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NT Exampro 14

13. Part 3: you re required to carry out a quantitative exercise to determine an enthalpy change.

Quantitative exercise to determine an enthalpy chance involving compound A.

You are provided with:

A sample tube containing solid A (this is labelled solid A for part 3).

You are required to find the molar enthalpy change when A dissolves in water.

Procedure

(1) Use a measuring cylinder to measure 25 cm3 of water into a polystyrene cup held securely in a beaker. Place the thermometer in the water.

(2) Weigh the tube containing solid A. Record the mass of the tube plus A in Table 2.

(3) Measure the steady temperature of the water in the cup to the nearest 0.2 °C. Record the temperature in Table 3.

(4) Empty the sample of A from the tube into the water and stir with the thermometer. Continue stirring until all the solid has dissolved and record the temperature to the nearest 0.2 °C. Record this temperature in Table 3.

(5) Weight the empty tube and record the mass in Table 2.

Table 2

Mass of tube + solid A g

Mass of empty tube g

Mass of solid A used g

(2)

Table 3

Temperature of water before adding solid A, T1 °C

Steady temperature of solution, T2 °C

Calculate the temperature chance ∆T by subtracting T1 from T2.

∆T = T2 – T1 = ......................... o C (6)

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NT Exampro 15

Calculation and question

(a) Calculate the molar enthalpy change of solution when A dissolves in water using the formula:

∆H = 1–molkJ

(g)usedofmassΔT10.2–

A×

(3)

(b) Describe two ways in which the accuracy of this experiment could be improved. Give the reason for each of the ways suggested.

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(Total 15 marks)

14. The reaction of an acid with a base to give a salt is an exothermic reaction. In an experiment to determine the enthalpy of neutralisation of hydrochloric acid with sodium hydroxide, 50.0cm3 of 1.00 mol dm–3 HCl was mixed with 50.0 cm3 of 1.10 mol dm–3 NaOH. The temperature rise obtained was 6.90 °C.

(a) Define the term enthalpy of neutralisation.

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.................................................................................................................................... (1)

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NT Exampro 16

(b) Assuming that the density of the final solution is 1.00 g cm–3 and that its heat capacity is

4.18 J K–1 g–1, calculate the heat evolved during the reaction.

(3)

(c) 0.0500 mol of acid was neutralised in this reaction; calculate ∆Hneutralisation in kJ mol–1.

(2)

(d) Suggest why sodium hydroxide is used in slight excess in the experiment.

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(Total 7 marks)

15. (a) (i) Define the term standard enthalpy of combustion.

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(ii) The values for the standard enthalpy of combustion of graphite and carbon monoxide are given below:

∆Hc /kJ mol–1

C (graphite) –394

CO(g) –283 Use these data to find the standard enthalpy change of formation of carbon

monoxide using a Hess’s law cycle.

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NT Exampro 17

C(graphite) + 21 O2(g) → CO(g)

(3)

(iii) Suggest why it is not possible to find the enthalpy of formation of carbon monoxide directly.

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............................................................................................................................ (1)

(iv) Draw an enthalpy level diagram below for the formation of carbon monoxide from graphite.

(1)

(b) Natural gas consists of methane, CH4. When methane burns completely in oxygen the reaction occurs as shown in the equation

CH4(g) + 2O2(g) → CO2(g) + 2H2O(l) ∆Hc = –890 kJ mol–1

Methane does not burn unless lit.

Use this information to explain the difference between thermodynamic and kinetic stability.

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NT Exampro 18

(c) Suggest why methane is not used as a fuel for motor vehicles whereas octane is.

The following data may be useful: CH4(g) ∆Hc = –37 kJ m–3; C8H18(l) ∆Hc = –3530 kJ m–3.

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(Total 15 marks)

16. A student was required to determine the enthalpy change for the reaction between iron and copper sulphate solution.

The student produced the following account of their experiment.

A piece of iron, mass about 3 g, was placed in a glass beaker. 50 cm of0.5 mol dm aqueous copper sulphate solution was measured using ameasuring cylinder and added to the beaker. The temperature of themixture was measured immediately before the addition and every minuteafterwards until no further change took place.

Fe + CuSO FeSO + Cu4 4

–3

3

Timing beforeaddition

1 min 2 mins 3 mins 4 mins 5 mins

Temperature/°C 22 27 29 26 24 22

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NT Exampro 19

(a) Suggest two improvements you would make to this experiment. Give a reason for each of

the improvements suggested.

Improvement 1 ...........................................................................................................

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Reason 1 ....................................................................................................................

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Improvement 2 ...........................................................................................................

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Reason 2 ....................................................................................................................

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(b) In an improved version of the same experiment a maximum temperature rise of 15.2 °C occurred when reacting excess iron with 50.0 cm3 of 0.500 mol dm–3 aqueous copper sulphate solution.

(i) Using this data and taking the specific heat capacity of all aqueous solutions as 4.18 Jg–1 deg–1 calculate the heat change.

(1)

(ii) Calculate the number of moles of copper sulphate used.

(1) ww

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NT Exampro 20

(iii) Calculate the enthalpy change of this reaction in kJ mol–1.

(2)

(Total 8 marks)

17. (a) Define the term standard enthalpy of combustion, making clear the meaning of standard in this context.

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(b) Use the enthalpies of combustion given below to find the enthalpy change for the reaction:

2C(graphite) + 2H2(g) + O2(g) → CH3COOH(l)

∆Hcombustion/kJ mol–1

C(graphite) –394

H2(g) –286

CH3COOH(l) –874

(3)

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NT Exampro 21

(c) With reference to ethanoic acid, CH3COOH, what is the enthalpy change obtained

in (b) called?

………….………………………………………………………………………….. (1)

(d) Draw an enthalpy level diagram to represent the enthalpy change for the combustion of graphite. Show both the enthalpy levels of the reactants and products and an energy profile which represents the activation energy for the reaction.

(3)

(e) Use your diagram given in (d) to explain the terms thermodynamic and kinetic stability with reference to the combustion of graphite.

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(Total 14 marks)

18. Part 3: you are required to carry out a quantitative exercise to find the enthalpy change when aqueous hydrochloric acid is neutralised by aqueous sodium hydroxide.

Quantitative exercise to find the enthalpy change when aqueous hydrochloric acid is neutralised by aqueous sodium hydroxide.

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NT Exampro 22

You are provided with:

• Solution E, aqueous sodium hydroxide; • Solution F, aqueous hydrochloric acid which is exactly ten times more concentrated than

solution D used in Part 2.

PROCEDURE

1. Rinse out the burette used in Part 2 with solution E then fill the burette with solution E.

2. Rinse out the pipette used in Part 2 with solution F then use the pipette to measure 25.0 cm3 of solution F into a dry polystyrene cup held firmly in a beaker. Place the thermometer in solution F in the cup.

3. Measure the temperature of the solution in the polystyrene cup to the nearest 0.2°C. Record the temperature in Table 2.

4. Add 5.00 cm3 of solution E from the burette to the polystyrene cup. Stir the solution with the thermometer then record, in Table 2, the steady temperature of the solution to the nearest 0.2°C.

5 Add further 5.00 cm3 volumes of solution E to the polystyrene cup. Stir after each addition recording the temperature of the solution to the nearest 0.2°C in Table 2.

6. Continue to add 5.00 cm3 portions of solution E until a total of 50.0 cm3 have been added.

Table 2

Volume E added / cm3 0.00 5.00 10.00 15.00 20.00 25.00 30.00 35.00 40.00 45.00 50.00

Temperature /°C

(2)

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NT Exampro 23

(a) On the grid below plot the temperatures (vertical axis) against volumes of E (horizontal

axis) recorded in Table 2. The scale on the vertical axis should allow for plotting a maximum temperature slightly above the highest reading in Table 2. Plan your scales to use the grid as fully as possible; there is no need to begin the temperature scale at 0 °C.

(2)

(b) Draw a smooth line through the points as the temperature rises and a second line as the temperature falls. You may have to extend the lines until they meet.

(1)

(c) From your graph read off the highest temperature reached and the volume of E added when the two lines meet.

Temperature = …………………….. °C

Volume of E = …………………….. cm3 (2)

(d) (i) Calculate the difference between the temperature when no E has been added and the temperature recorded in (c).

∆T = = °C (5)

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NT Exampro 24

(ii) Calculate the total volume of solution in the polystyrene cup when the volume of

E recorded in (c) has been added.

Total volume, VT = = cm3 (1)

(e) (i) Calculate the concentration of solution F, using your answer to Part 2(a) and the information given at the start of this question concerning solution F.

(1)

(ii) Calculate the enthalpy of neutralisation of hydrochloric acid and sodium hydroxide using the formula

∆H = –VT × HClC25

2.4T××∆ kJ mol–1

where CHCl is the concentration of solution F. Give your answer to an appropriate number of significant figures.

(1)

(f) What would be the effect on the accuracy of your ∆H value if the experiment were repeated with the same volumes of E and F but with both solutions at higher concentrations? Explain your answer.

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(Total 17 marks)

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NT Exampro 25

19. (a) This question is about finding the formula of copper hydroxide. The method is as follows:

20.0 cm3 of an aqueous solution of a copper salt of concentration 1.00 mol dm–3 was placed in a polystyrene cup and its temperature measured using a thermometer graduated in 0.1 °C intervals.

A burette was filled with aqueous sodium hydroxide, of concentration 2.00 mol dm–3.

2.00 cm3 of sodium hydroxide solution was run into the solution of the copper salt and the temperature was measured immediately.

As soon as possible a further 2.00 cm3 of sodium hydroxide solution was run in and the temperature measured again.

This process of adding 2.00 cm3 portions of sodium hydroxide solution and measuring the temperature was continued until a total of 36.0 cm3 of the sodium hydroxide solution had been added.

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NT Exampro 26

The temperature readings are shown in the graph below.

30

29

28

27

26

25

24

23

22

21

200 4 8 12 16 20 24 28 32 36 40

Volume of NaOH(aq) / cm

Temperature / ºC

–3

(i) Explain why the temperature reaches a maximum and then falls slightly on addition of further sodium hydroxide solution.

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(ii) From the graph, what volume of the aqueous sodium hydroxide was required for complete reaction?

………….……………….…………………………………………………….. (1)

(iii) Calculate the amount (number of moles) of sodium hydroxide in this volume of solution.

(1)

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NT Exampro 27

(iv) Calculate the amount (number of moles) of copper ions that have reacted.

(1)

(v) Write the ratio of moles of copper ions to hydroxide ions reacting.

(1)

(vi) Write the formula of the copper hydroxide that is produced.

(1)

(b) The data call be used to find the enthalpy change for the reaction between sodium hydroxide and the copper salt.

(i) Use the graph to find the temperature rise that occurs for complete reaction.

………….……………….…………………………………………………….. (1)

(ii) Find the heat change, q, that occurs in the polystyrene cup for complete reaction. Use the formula

q = 168 × ∆T joules

(1)

(iii) Use your results from (a)(iv) and (b)(ii) above, to find the molar enthalpy change, ∆H, for the reaction. Give the correct sign and units to the answer.

(3)

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NT Exampro 28

(c) Identify one potential source of error in this experiment, and say what you would do

to reduce its effect.

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(Total 14 marks)

20. Urea, which is used as a fertillser in much of mainland Europe, Asia and Africa, is manufactured by the reaction of ammonia and carbon dioxide.

2NH3(g) + CO2(g) → NH2CONH2(s) + H2O(l)

(a) Define the term standard enthalpy of formation, ∆Hf , of urea.

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NT Exampro 29

(b) Calculate the enthalpy change, ∆H for the reaction above, given the following standard

enthalpies of formation.

Substance ∆Hf / kJ mol–1

NH3(g) –46.2

CO2(g) –393.5

NH2CONH2(s) –632.2

H2O(l) –285.8

(3)

(Total 6 marks)

21. (a) Halogenoalkanes react with many nucleophiles.

Define the term nucleophile.

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...............................................................................................................................

............................................................................................................................... (2)

(b) (i) Identify the reagent and conditions necessary for the conversion of iodoethane to ethylamine, C2H5NH2.

Reagent:.........................................................................................................

Conditions:.....................................................................................................

...................................................................................................................... (3)

(ii) State why the rate of reaction would be slower if bromobutane were used in place of iodoethane, with all other conditions remaining the same.

......................................................................................................................

...................................................................................................................... (1)

(c) Iodoethane reacts with water to form ethanol and hydrogen iodide.

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NT Exampro 30

C2H5I + H2O → C2H5OH + HI ∆Hf = +36 kJ mol–1

Use some or all of the data below to calculate the CI bond enthalpy.

Bond Bond enthalpy / kJ mol–1

Bond Bond enthalpy / kJ mol–1

CH 413 HI 298

CC 347 CO 358

HO 464

(3)

(d) Ethanol was heated under reflux with an excess of a mixture of potassium dichromate(VI) and dilute sulphuric acid. Draw the full structural formnula of the organic product.

(1)

(Total 10 marks)

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NT Exampro 31

22. (a) Ammonia is manufactured by the Haber process

N2(g) + 3H2(g) 2NH3(g) ∆H = –92.4 kJ mol–1

The usual conditions for this process are a catalyst of iron, a temperature of 400 °C and a pressure of 200 atmospheres.

Draw, on the axes below, an energy profile diagram for the uncatalysed reaction. Mark on your diagram the activation energy and the enthalpy change.

Enthalpy

Extent of reaction (reaction co-ordinate) (4)

(b) (i) Draw, on the axis below, the Maxwell-Boltzmann distribution that could apply at 400°C and mark on your diagram the activation energies for the catalysed and the uncatalysed reaction.

Fraction ofmolecules ofenergy E

Energy (3)

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NT Exampro 32

(ii) Use your diagram to explain why the reaction is faster in the presence of the iron

catalyst.

......................................................................................................................

......................................................................................................................

......................................................................................................................

......................................................................................................................

...................................................................................................................... (3)

(c) A mixture of nitrogen and hvdro2en is kinetically stable at 25°C but kinetically unstable at 400°C. Explain why this is so.

...............................................................................................................................

...............................................................................................................................

...............................................................................................................................

...............................................................................................................................

............................................................................................................................... (3)

(Total 13 marks)

23. You are required to carry out a quantitative exercise to determine the molar enthalpy change when the compound A reacts with a solution of the acid C.

Quantitative exercise to find the enthalpy change when solid A reacts with a solution of C.

You are provided with:

• A sample tube containing solid A, labelled “Solid A”; • Solution C.

Procedure

1. Use a measuring cylinder to measure 60 cm3 of solution C into a polystyrene cup securely held in a beaker. Place the thermometer into the solution.

2. Weigh accurately the sample tube containing solid A. Record this mass in Table 1.

3. Measure the temperature of the solution in the cup to the nearest 0.2 °C. Record this in Table 2.

4. Empty the solid from the sample tube into the solution and stir with the thermometer until the solid has dissolved and record the final temperature to the nearest 0.2 °C. Record this in Table 2.

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NT Exampro 33

5. Reweigh the empty sample tube and record the mass in Table 1; calculate the actual mass

of solid A transferred, and record this in Table 1.

Table 1

Mass of tube + solid A g

Mass of tube after transfer g

Mass of solid A used g

(3)

Table 2

Temperature of solution before adding solid A, T1 °C

Temperature of solution after adding solid A, T2 °C

Calculate the temperature change ∆T by subtracting T1 from T2.

∆T = T2 – Tl = ................................ °C (6)

Calculation and question

(a) Calculate the molar enthalpy change of the reaction, ∆H, using the formula:

∆H = (g)usedofmassΔT4.185.04–

A××

kJ mol–1

(3)

(b) Consider the values given in the formula in part (a). Would the result for ∆H be more accurate if the mass of A were known to 4 decimal places? Give a reason.

...............................................................................................................................

...............................................................................................................................

...............................................................................................................................

...............................................................................................................................

............................................................................................................................... (2)

(Total 14 marks)

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NT Exampro 34

24. You are required to carry out a quantitative exercise to determine the molar enthalpy change

when the acid G reacts with a solution of E.

Quantitative exercise to find the enthalpy change when solid G reacts with a solution of E.

You are provided with:

• A sample tube containing solid G, labelled “Solid G” • A solution of E labelled “Solution of E”.

Procedure

1. Use a measuring cylinder to measure 60cm3 of the solution of E into a polystyrene cup securely held in a beaker. Place the thermometer into the solution.

2. Weigh accurately the sample tube containing solid G. Record this mass in Table 1.

3. Measure the temperature of the solution in the cup to the nearest 0.2°C. Record this in Table 2.

4. Empty the solid from the sample tube into the solution and stir with the thermometer until the solid has dissolved and record the final temperature to the nearest 0.2°C. Record this in Table 2.

5. Reweigh the empty sample tube and record the mass in Table 1; calculate the actual mass of solid G transferred, and record this in Table 1.

Table 1

Mass of tube + solid G g

Mass of tube after transfer g

Mass of solid G used g

(3)

Table 2

Temperature of solution before adding solid G, T1 °C

Temperature of solution after adding solid G, T2 °C

Calculate the temperature change ∆T by subtracting T1 from T2.

∆T = T2 – Tl = ................................ °C (6)

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NT Exampro 35

Calculation and question

(a) Calculate the molar enthalpy change, ∆H, of the reaction using the formula:

∆H = (g)usedofmassΔT4.185.31–

G××

kJ mol–1

(3)

(b) Consider the values given in the formula in part (a). Would the result for ∆H be more accurate if the temperature of the solution were known to 3 decimal places? Give a reason.

...............................................................................................................................

...............................................................................................................................

...............................................................................................................................

...............................................................................................................................

...............................................................................................................................

............................................................................................................................... (2)

(Total 14 marks)

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