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TOPIC 5 Questionsheet

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AS Level

ENTHALPY OF DISPLACEMENT

16

1

An experiment was carried out to find the enthalpy change for the reaction:

Zn(s) + Cu2+(aq) → Zn2+(aq) + Cu(s)

The temperature of 20.0 cm3 of copper sulfate solution (1.0 mol dm-3) in a plastic cup was measured everyminute for 3 minutes. Exactly on the third minute 1.95g of zinc powder was added rapidly and the mixture stirred.The temperature every half-minute was noted for a further 3 minutes.

a) Calculate:(i) the mol copper ion used

....................................................................................................................................................................... [1](ii) the mol zinc used

....................................................................................................................................................................... [1]

b) Calculate and explain the mol used in the calculation

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

....................................................................................................................................................................... [2]

c) The following graph was drawn :

(i) Why was the zinc added after 3 minutes?

....................................................................................................................................................................... [1](ii) Explain how the value of the temperature change ?T was derived from the graph

....................................................................................................................................................................... [1]

(iii) Explain why the method in (ii) was necessary

....................................................................................................................................................................... [1]

Tem

pera

ture

(o C

)

3 Time (min)

∆T

(Continued..)



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AS Level TOPIC 5 Questionsheet 1 ContinuedENTHALPY OF DISPLACEMENT

d) The value of ∆T was derived as 39.5 0C.

(i) Calculate the joules of energy exchange with the surroundings (assuming that 1 cm3 solution requires4.2 J to increase the temperature by 1 0C).

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

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....................................................................................................................................................................... [2]

(ii) Calculate a value for the enthalpy of reaction

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

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....................................................................................................................................................................... [4]

e) If the experiment was repeated using magnesium instead of zinc, how and why would you expect the enthalpyof reaction to compare with your value in d)(ii) ?

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....................................................................................................................................................................... [3]

16



TOTAL /

AS Level

15

THERMOMETRIC TITRATIONa) The following results were obtained from an experiment to determine the heat of neutralisation of an acid

with an alkali:Hydrochloric acid added to 50 cm3 of 2.0 M sodium hydroxide

Volume of acid added / cm3

0.05.0

10.015.020.025.030.035.040.045.050.0

Temperature of mixture / 0C

17.622.026.030.033.435.434.033.031.630.829.6

(i) Plot a graph of temperature against volume of acid added and use the graph to determine the volume ofacid required to neutralise the 50 cm3 of 2.0 M sodium hydroxide.

2

Volume of acid required = ......................................................................................................................... [5] (Continued..)



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AS Level

15

(ii) Calculate the molar concentration of the acid.

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

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

....................................................................................................................................................................... [3]

(iii) Calculate the enthalpy of neutralisation of the acid by the alkali. (Take the specific heat capacity ofdilute solutions to be 4.18 J g-1 K-1)

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

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

....................................................................................................................................................................... [3]

b) To confirm this result, 40 cm3 of 2 M hydrochloric acid were put into a Thermos flask and its temperaturerecorded.Then 60 cm3

of 2 M sodium hydroxide were added and the temperature rose by 10.4 0C. The flask

was rinsed out and 100 cm3 of water put in together with a small electrical heating coil. The coil supplied 30Joules every second and after 145 seconds the same temperature rise was achieved. Use the electrical data tocalculate the enthalpy of neutralisation of the acid by the alkali.

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

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....................................................................................................................................................................... [4]

THERMOMETRIC TITRATIONTOPIC 5 Questionsheet 2 Continued



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AS Level

12

3

ENTHALPY OF FORMATION

a) Define standard enthalpy of formation of a compound.

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

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

....................................................................................................................................................................... [3]

b) Calculate the standard enthalpy change for the oxidation of ammonia according to the equation:4NH

3(g) + 5O

2(g) → 4NO(g) + 6H

2O(g)

Standard enthalpies of formation are as follows: NH

3(g) = -46 kJ mol-1 NO(g) = +90 kJ mol-1 H

2O(g) = -242 kJ mol-1

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

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

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....................................................................................................................................................................... [3]

c) Ethyne burns in oxygen according to the equation:2C

2H

2(g) + 5O

2(g) → 4CO

2(g) + 2H

2O(l)

Calculate the standard enthalpy of combustion of this gas given the following standard enthalpies of formation:C

2H

2(g) = + 228 kJ mol-1 CO

2(g) = -394 kJ mol-1 H

2O(l) = -286 kJ mol-1

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

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....................................................................................................................................................................... [3]

d) Calculate the standard enthalpy of formation of magnesium carbonate, using the following information.The standard enthalpy of combustion of magnesium is – 602 kJ mol-1 and that of carbon is – 394 kJ mol-1.The standard enthalpy change for the decomposition of magnesium carbonate in the reaction:

MgCO3(s) → MgO(s) + CO

2(g)

is +100 kJ mol-1.

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

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....................................................................................................................................................................... [3]



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AS Level

16

8

HESS’S LAW WITH CALORIMETRY

A student poured exactly 50 cm3 of 2 M hydrochloric acid into a polystyrene cup and recorded its temperatureeach minute for three minutes. At 3½ minutes, 0.24 g of magnesium turnings was added. The mixture was stirredand further temperature readings taken until a total time of nine minutes had elapsed. The recorded temperatureswere as follows.

Time / minutes

Temperature / 0C

0 1 2 3 4 5 6 7 8 9

20 19.7 19.5 19.2 43.8 43.0 42.3 41.5 40.8 40.0

a) Plot a graph of temperature against time using these results and hence determine the maximum temperaturechange accompanying the reaction.

(Continued...)Maximum temperature change ..............................................................................................................[5]



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AS Level

18

HESS’S LAW WITH CALORIMETRYb) Use the value from a) and the data given in the question to calculate the enthalpy change for the reaction.

(Assume the density of all solutions to be 1 g cm-3 and their specific heat capacity to be 4.2 J g-1 K-1)

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....................................................................................................................................................................... [3]

c) In a similar experiment, 0.01 mol of magnesium carbonate was added to exactly 50 cm3 of 2 M hydrochloricacid in a polystyrene cup. The maximum temperature rise was 5.2 0C. Calculate the enthalpy change for thereaction.

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

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....................................................................................................................................................................... [2]

d) Write a balanced chemical equation to represent the standard enthalpy of formation for magnesium carbonate.

....................................................................................................................................................................... [2]

e) Given the additional data for the standard enthalpies of formation of H2O(l) and CO

2(g) as –286 and –394 kJ

mol-1 respectively, together with the reactions above, construct an enthalpy cycle or diagram to show how thestandard enthalpy of formation for magnesium carbonate can be calculated.

[4]

f) Apply Hess’s law to the cycle or diagram you have devised and the results from b) and c) to calculate thestandard enthalpy of formation for magnesium carbonate.

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

....................................................................................................................................................................... [2]

TOPIC 5 Questionsheet 8 Continued



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AS Level 9

15

HESS’S LAW WITH BOND DISSOCIATION ENTHALPY

Ethanol production in the UK is mainly from the hydration of ethene. In this process, ethene and steam arereacted together at 300 0C and 70 atm pressure in the presence of phosphoric acid, which acts as a catalyst:

CH2=CH

2(g) + H

2O(g) → C

2H

5OH(l)

a) Apply Hess’s law to an enthalpy cycle or diagram to calculate the enthalpy change for this reaction, given thefollowing data:

Standard enthalpy of formation of ethanol = -278 kJ mol-1

C(s) → C(g); ∆H = + 715 kJ mol-1Mean bond dissociation enthalpies / kJ mol-1:

H—H = 436 O O = 496 C C = 612 O—H = 464 C—H = 413

[8]

b) Calculate the enthalpy change for the same reaction, using the following standard enthalpies of combustion:

CH2=CH

2 = –1411 kJ mol-1 CH

3CH

2OH = –1368 kJ mol-1

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....................................................................................................................................................................... [3]

c) Why is the value calculated from bond dissociation enthalpies in a) different from that obtained in b)?

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

....................................................................................................................................................................... [1]

d) For the formation of gaseous ethanol, CH2=CH

2(g) + H

2O(g) → C

2H

5OH(g)

would you expect the enthalpy of reaction to be more negative or less negative than the value for the formationof liquid ethanol? Give your reason.

More or less negative .................................................................................................................................... [1]

Reason ...............................................................................................................................................................

....................................................................................................................................................................... [2]

————



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AS Level

17

10

TEST QUESTION I

NOTE In this Questionsheet, assume that dilute aqueous solutions have a specific heat capacity of 4.18 J g-1 K-1

and a density of 1.00 g cm-3.

a) (i) Define the term molar enthalpy of solution.

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(ii) When 5.35 g of ammonium chloride was dissolved in 100 g of water, the temperature fell from 20.55 0Cto 17.10 0C. Calculate the molar enthalpy of solution of ammonium chloride.

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(iii) What is the significance of the sign of the value calculated in (ii)?

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....................................................................................................................................................................... [1]

b) 50.0 cm3 of 0.5 mol dm-3 NH3(aq) at 20.00 oC. was mixed with 50.0 cm3 0.5 mol dm-3 HCl(aq), also at 20.00 oC.

The temperature of the mixture rose to 23.12 oC. Calculate the molar enthalpy of neutralisation of HCl(aq) byNH

3(aq).

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(Continued..)



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AS Level 10 Continued

c) (i) Write an equation to represent the formation of 1 mole of solid ammonium chloride from its elements intheir standard states.

....................................................................................................................................................................... [1]

(ii) Use the following data, together with your answers from a) and b), to calculate the molar enthalpy ofsolution of ammonia. Your answer should include an enthalpy cycle or diagram to which Hess’s law isclearly applied.

∆Hθf / kJ mol-1 : NH

3(g) = -43.2 HCl(g) = -92.3 NH

4Cl(s) = -315

Molar enthalpy of solution of HCl(g) = -187 kJ mol-1

Enthalpy cycle or diagram

[5]

Application of Hess’s law

............................................................................................................................................................... [1]

Molar enthalpy of solution of ammonia

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TEST QUESTION I

19



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AS Level

17

11

TEST QUESTION II

(Continued...)

A spirit burner containing ethanol was weighed. 80 cm3 of water was measured into a metal calorimeter and thetemperature measured. The burner was ignited under the calorimeter and extinguished after 5 minutes.

The temperature of the water and the mass of the burner were then immediately measured. The following resultswere obtained:

Mass of spirit burner at start (g) 32.44

Mass of spirit burner after combustion (g) 31.96

Temperature of water at start (oC) 16.4

Temperature of water after combustion (oC) 33.1

a) Calculate(i) the mass of ethanol burned

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....................................................................................................................................................................... [1]

(ii) the temperature rise of the water

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AS Level 11 Continued

19

TEST QUESTION II

b) Assuming 1cm3 of water rises 1oC for 4.2 J energy , calculate(i) the energy exchange with the surroundings (in joules)

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

....................................................................................................................................................................... [2]

(ii) the energy yield for ethanol in joules per g

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....................................................................................................................................................................... [2]

(iii) the enthalpy of combustion for ethanol

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....................................................................................................................................................................... [4]

(iv) Explain why it is not possible to state that the answer to (iii) is the‘standard enthalpy of combustion’.

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....................................................................................................................................................................... [2]

(v) Give an assumption used when calculating an answer to (i)

....................................................................................................................................................................... [1]

c) If the standard enthalpy of combustion is -1400 kJ mol-1, suggest and explain two reasons for the difference.

Reason 1 ....................................................................................................................................................... [1]

Explanation ......................................................................................................................................................

....................................................................................................................................................................... [2]

Reason 2 ....................................................................................................................................................... [1]

Explanation ......................................................................................................................................................

....................................................................................................................................................................... [2]

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