a2 17 qu - frankly chemistryfranklychemistry.co.uk/20to9/active_learning/chemistry/curriculum... ·...

ENERGETICS IIA2 TOPIC 17

ENTHALPY OF ATOMISATION

LATTICE ENTHALPY

BORN-HABER CYCLES I

BORN-HABER CYCLES II

ENTROPY AND FREE ENERGY CHANGE

ENTROPY AND STATE

TEST QUESTION I

TEST QUESTION II

TEST QUESTION III

PRINCIPLES OF ELECTROCHEMICAL CELLS

ELECTRODE POTENTIALS AND CELL EMF

MEASUREMENT OF ELECTRODE POTENTIALS

FACTORS INFLUENCING ELECTRODE POTENTIALS ANDCELL EMF

PREDICTION OF REDOX CHANGES FROM STANDARDELECTRODE POTENTIALS

FUEL CELLS

TEST QUESTION IV

TEST QUESTION V

Questionsheet 1

Questionsheet 2

Questionsheet 3

Questionsheet 4

Questionsheet 5

Questionsheet 6

Questionsheet 7

Questionsheet 8

Questionsheet 9

Questionsheet 10

Questionsheet 11

Questionsheet 12

Questionsheet 13

Questionsheet 14

Questionsheet 15

Questionsheet 16

Questionsheet 17

QUESTIONSHEETS A2 LevelCHEMISTRY

13 marks

16 marks

16 marks

16 marks

17 marks

21 marks

15 marks

15 marks

15 marks

20 marks

17 marks

13 marks

18 marks

18 marks

16 marks

16 marks

17 marks

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Curriculum Press • Bank House • 105 King Street • Wellington • Shropshire • TF1 1NU

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AuthorsTrevor Birt John BrockingtonDonald E Caddy Kevin FrobisherAndrew Jones Andy ShepherdAdrian Bond Stuart Barker

EditorsJohn BrockingtonStuart Barker

Do not writein margin

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ENTHALPY OF ATOMISATION

13

TOPIC 17 Questionsheet 1

a) Distinguish between the standard enthalpy of atomisation of an element and the standard enthalpy ofatomisation of a covalent compound.

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

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

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

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

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

....................................................................................................................................................................... [4]b) What is the relationship between the enthalpy of atomisation of hydrogen and the bond dissociation enthalpy

of the H—H bond?

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

c) (i) Given that the standard enthalpy of atomisation of water is 920 kJ mol-1, calculate the mean bonddissociation enthalpy of the O—H bond in water.

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

(ii) According to the data books, the mean bond dissociation enthalpy of the O—H bond is 463 kJ mol-1.How do you account for the difference between this value and the one you have calculated in c) (i)?

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

d) (i) Write an equation for the formation of methane from its elements.

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

(ii) Given that the standard enthalpy of formation of methane is –74.9 kJ mol-1, and the standard enthalpiesof atomisation of carbon and hydrogen are 715 and 218 kJ mol-1 respectively, calculate the mean bonddissociation enthalpy of the C—H bond in methane.

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LATTICE ENTHALPYTOPIC 17 Questionsheet 2

a) Define the lattice enthalpy of a simple ionic compound.

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

....................................................................................................................................................................... [3]b) (i) For the chlorides of the metals of Group 1, how does the magnitude of lattice enthalpy change on

progressing from LiCl to CsCl? Explain your answer.

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

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

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

(ii) From lattice enthalpy considerations only, would you expect the following physical properties of theGroup1 chlorides to increase, decrease or remain approximately constant on progressing from LiCl toCsCl?

Melting point .......................................................................................................................................... [1]

Solubility in water .................................................................................................................................. [1]

c) Lattice enthalpies and enthalpies of hydration are given in the table for a selection of compounds and ions.

Salt Lattice enthalpy Ion Hydration enthalpy/kJ mol-1 /kJ mol-1

BaF2 (s) 2352 Ba2+ -1275

BaCl2(s) 2056 F- -530

Cl- -394

(i) Use the appropriate data to construct an enthalpy diagram for the dissolving of barium fluoride in water,and calculate the enthalpy of solution of BaF

2(s).

[6](ii) Use your value from c)(i), and the information that the enthalpy of solution of BaCl

2(s) is –7 kJ mol-1, to

explain why BaF2(s) is less soluble than BaCl

2(s).

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

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

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


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BORN-HABER CYCLES ITOPIC 17 Questionsheet 3

a) Given the thermodynamic data below, construct a Born-Haber cycle for the formation of lithium fluoride andthen calculate a value for its lattice enthalpy.

kJ mol-1

Enthalpy of atomisation of Li(s) 159.5Enthalpy of atomisation of F

2(g) 79.0

First ionisation energy of Li(g) 520.0First electron affinity of F(g) -334.0Enthalpy of formation of Li+F-(s) -616.0

Marks are awarded for correct labelling of both enthalpy levels and arrows.

Born-Haber cycle

[12]Lattice enthalpy of lithium fluoride

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

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

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

b) The lattice enthalpy of a compound, determined from a Born-Haber cycle as in a), generally differs from thevalue that can be calculated from the charge numbers and radii of its ions. Suggest a reason for this.

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


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BORN-HABER CYCLES IITOPIC 17 Questionsheet 4

a) Magnesium chloride has the formula MgCl2, i.e. Mg2+ (Cl-)

2, and a standard enthalpy of formation of -642 kJ mol-1.

Use the data below to calculate the standard enthalpy of formation of the hypothetical chloride MgCl, i.e.Mg+ Cl-, and hence explain why, when magnesium and chlorine react together, the product is MgCl

2 rather

than MgCl. Begin your answer by drawing a Born-Haber cycle.

Atomisation enthalpy of magnesium = 150 kJ mol-1

Atomisation enthalpy of chlorine = 121 kJ mol-1

First ionisation energy of magnesium = 736 kJ mol-1

First electron affinity of chlorine = -364 kJ mol-1

Lattice enthalpy of MgCl = 756 kJ mol-1

(i) Born-Haber cycle for the formation of MgCl

[6]

(Continued...)


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BORN-HABER CYCLES IITOPIC 17 Questionsheet 4 Continued

16

(ii) Standard enthalpy of formation of MgCl

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

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

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

(iii) Why MgCl2 is formed, rather than MgCl

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

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

b) Which one of the data listed in a) would have been obtained by calculation rather than experiment?

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

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

c) If you were constructing a Born-Haber cycle for MgCl2 rather than MgCl, what addtional data would you

need?

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

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

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

d) Would you expect the lattice enthalpy of MgCl to be greater or less than that of MgCl2? Explain you answer.

Greater or less than ∆Hêlattice

(MgCl2) ........................................................................................................ [1]

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

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


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17

ENTROPY AND FREE ENERGY CHANGETOPIC 17 Questionsheet 5

a) Give the name and units of each physical quantity in the equation: ∆Gê = ∆Hê - T.∆Sê

∆Gê .............................................................................................................................................................. [1]

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

∆Hê .............................................................................................................................................................. [1]

∆Sê .............................................................................................................................................................. [1]

b) State, with reasons, whether you would expect the standard entropy change in each of the following reactionsto increase, decrease or remain the approximately the same.

(i) 2Mg(s) + O2(g) → 2MgO(s)

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

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

(ii) Mg(s) + 2HCl(aq) → MgCl2(aq) + H

2(g)

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

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

(iii) H2(g) + Cl

2(g) → 2HCl(g)

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

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

(iv) 2NH3(g) → N

2(g) + 3H

2(g)

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

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

c) The combustion of ethanol is represented by the equation: C2H

5OH(l) + 3O

2(g) → 2CO

2(g) + 3H

2O(l)

(i) Given the following data: Sê/J K-1 mol-1: C2H

5OH 160.7; O

2(g) 0; CO

2(g) 213.6; H

2O(l) 69.9;

calculate the standard entropy change for the reaction.

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

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

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

(ii) Given the additional information that the standard enthalpy of combustion of ethanol is -1367.3 kJ mol-1,calculate the standard free energy change for the reaction.

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

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

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


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ENTROPY AND STATETOPIC 17 Questionsheet 6

a) Using these standard entropies (J K-1 mol-1) :

H2(g) 130; Cl

2(g) 223; HCl(g) 187; Na(s) 51; NaCl(s) 72

(i) Explain why two values are considerably lower than the others

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

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

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

(ii) Calculate the standard entropy change (per mole chloride formed) for the reaction of chlorine with:

Sodium

[3]Hydrogen

[3]

(iii) Comment on the sign of the value calculated in (ii) for NaCl in the light of the reaction being exothermic.

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

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

(Continued...)


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TOPIC 17 Questionsheet 6 Continued

HF

[2]

HCl

[2]

(ii) Explain why the values in (i) can be calculated using the function given.

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

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

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

(iii) Suggest why HF has a lower vaporisation entropy than the other hydrogen halides which are similar invalue.

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

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

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

b) Consider the standard enthalpy of vaporisation and boiling point for these hydrogen halides:

Hydrogen halide Boiling point (K) ∆Hêvaporisation

(kJ mol-1)

HF 293 7.5

HCl 188 16.2

HBr 206 17.6

HI 238 19.8

(i) ∆Sê

vap =

- ∆Hêvap (T = boiling point in K) T

Use these values to calculate the standard entropies of vaporisation for

ENTROPY AND STATE


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A2 Level TOPIC 17 Questionsheet 7

TEST QUESTION I

(Continued...)

The reaction between sodium and water can be represented by the following equation:

Na(s) + H2O(l) → Na+(aq) + OH-(aq) + ½H

2(g); ∆Hê = -183.7 kJ mol-1

a) The reaction can be broken down into the following seven steps.Name the enthalpy change for each step.

Step 1 Na(s) → Na(g); ∆Hê = + 109 kJ mol-1

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

Step 2 Na(g) → Na+(g) + e-; ∆Hê = + 500 kJ mol-1

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

Step 3 Na+(g) + aq → Na+(aq); ∆Hê = -406 kJ mol-1

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

Step 4 H2O(l) + aq → H+(aq) + OH-(aq); ∆Hê

x

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

Step 5 H+(aq) → H+(g) + aq ; ∆Hê = +1090 kJ mol-1

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

Step 6 H+(g) + e- → H(g); ∆Hê = -1316 kJ mol-1

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

Step 7 H(g) → ½H2(g); ∆Hê = -218 kJ mol-1

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

b) (i) Sketch an enthalpy diagram for this reaction.

[4]


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TEST QUESTION I

b) (ii) Apply Hess’s law to your diagram and then calculate the standard enthalpy change (∆Hêx) for Step 4.

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

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

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

c) From your answer to b) (ii), predict a value for the enthalpy of neutralisation of a strong acid by a strong baseand explain your answer.

Predicted value............................................................................................................................................. [1]

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

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


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TEST QUESTION II

(Continued...)

Magnesium oxide is an ionic compound that can easily by prepared by burning magnesium in excess oxygen. Ithas a melting point of 2827 °C and so is commonly used for manufacturing linings for open-hearth steel furnaces.Use the information in the table below to answer the following questions.

∆Hê / kJ mol-1

O2(g) → 2O(g) +498

O(g) + e- → O-(g) -142O-(g) + e- → O2-(g) +844Mg(s) → Mg(g) +147Mg(g) → Mg+(g) + e- +738Mg+(g) → Mg2+(g) + e- +1451Mg2+(g) + O2-(g) → MgO(s) -3884

a) Write a chemical equation which represents the standard enthalpy of formation of magnesium oxide.

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

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

b) (i) Construct a Born-Haber cycle which could be used to calculate the standard enthalpy of formation ofmagnesium oxide. Write, by each arrow, the value of the enthalpy change it represents.

[4]


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TEST QUESTION II

(ii) Calculate the standard enthalpy of formation of magnesium oxide.

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

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

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

c) The standard molar entropy, Sê, for each species involved in the standard enthalpy of formation of magnesiumoxide is shown below.

Sê / J K-1 mol-1

Mg(s) 32.7

O2(g) 205.0

MgO(s) 27.0

Use the data to calculate the standard entropy change for the formation of magnesium oxide.

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

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

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

d) (i) Calculate the standard free energy change at 298 K for the formation of magnesium oxide. Show theequation on which your calculation is based.

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

(ii) Why does this reaction not happen spontaneously at 298 K?

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TEST QUESTION III

(Continued...)

Magnesium hydroxide, Mg(OH)2, and calcium hydroxide, Ca(OH)

2, are both hydroxides of Group 2 metals but

have different solubilities in water.

a) Explain, with a suitable example, the meaning of the following terms.(i) Enthalpy of hydration (of an ion)

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

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

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

(ii) Enthalpy of solution (of an ionic compound)

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

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

b) Use the following data to answer parts (i) and (ii) of this question.

Enthalpy change ∆Hê /kJ mol-1

Lattice enthalpy of MgCl2(s) +2493

Enthalpy of solution of MgCl2(s) -155

Enthalpy of hydration of Cl-(g) -364

(i) Construct an enthalpy diagram for the dissolving of MgCl2(s) in water. Your diagram should include theenthalpy of hydration of Mg2+(g), as well as the enthalpy changes shown in the table.

[3]

(ii) Apply Hess’s law to your enthalpy diagram to write an equation which relates the enthalpy of solutionof a compound to its lattice enthalpy and the hydration enthalpies of its ions.

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

(iii) Calculate a value for the enthalpy of hydration of Mg2+(g)

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

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


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TEST QUESTION III

c) (i) Describe how the value of ∆Hêsolution

changes as solubility increases.

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

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

(ii) Explain why, on descending Group 2, the lattice enthalpy of the metal hydroxides decreases.

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

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

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

(iii) Explain why calcium hydroxide is more soluble in water than magnesium hydroxide under standardconditions, even though the hydration enthalpy of cations decreases down Group 2.

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a) A redox reaction can occur between metallic zinc and aqueous copper(II) sulfate.

(i) Write down the two ionic half-equations, one for the oxidation half-reaction and the other for thereduction half-reaction.

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

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

(ii) Combine these so as to give an ionic equation for the overall reaction.

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

(iii) The reaction can be carried out in a beaker, by adding powdered zinc to an aqueous solution ofcopper(II) sulfate. Describe briefly what you would observe when this reaction occurs.

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

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

b) The reaction can also be conducted in an electrochemical cell, so that the two half-reactions occur inseparate half-cells as shown below.


(Continued...)

(i) What would you observe while the cell was in use?

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

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

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

(ii) What is the essential difference between the energy released here and when the experiment is carriedout in a beaker?

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

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

SALT BRIDGE CuZn

ZnSO4(aq)

CuSO4(aq)


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c) (i) Why must every electrochemical cell comprise two half-cells?

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

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

(ii) What is the purpose of the salt bridge and what might it contain?

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

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

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

(iii) What particles flow through the external circuit, and in which direction do they flow?

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

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

(iv) What are the principal particles which flow through the salt bridge, and in which direction do theyflow?

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

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20




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ELECTRODE POTENTIALS AND CELL EMFTOPIC 17 Questionsheet 11

a) (i) Explain how an electrode potential can develop when a metal rod is partially immersed in an aqueoussolution of one of its salts.

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

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

(ii) What three energy changes contribute to the electrode potential of a metal?

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

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

(iii) For comparison purposes, electrode potentials must be measured under standard conditions oftemperature and concentration. What are these standard conditions?

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

(iv) Under the same conditions of temperature and concentration, different metals give different electrodepotentials. Why is this?

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

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

b) In some redox electrodes, the reductant is not a metal rod: instead, the reductant and oxidant are twochemically related species in solution, e.g. Fe2+(aq) and Fe3+(aq). When setting up a standard electrode ofthis sort:

(i) how is electrical contact made with the rest of the circuit?

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

(ii) what should be the concentration of each species in solution?

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

c) (i) What is meant by the electromotive force (e.m.f.) of an electrochemical cell?

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

(ii) How could it be measured in the laboratory?

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

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

(iii) Write down a mathematical equation which relates the e.m.f. of a cell to the potentials of its electrodes.

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


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MEASUREMENT OF ELECTRODE POTENTIALS


a) (i) Why must all electrode potentials be related to that of a reference electrode?

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

(ii) By international convention, the standard hydrogen electrode (SHE) is used as a primary referenceelectrode and assigned a potential of 0.00 V. Label the diagram of this electrode, making clear thenecessary conditions.

[4]

b) The following cell was used to determine the standard electrode potential of the Fe3+(aq)/Fe2+(aq) couple.

Temperature =

13

(i) Write ionic half-equations for the two half-reactions.

Reduction half-equation ..................................................................................................................... [1]

Oxidation half-equation...................................................................................................................... [1]

(ii) Hence write an ionic equation for the complete cell reaction.

....................................................................................................................................................................... [1](iii) Calculate the standard electrode potential of the Fe3+(aq)/Fe2+(aq) couple. Given that Eê (Cu2+(aq)/

Cu(s)) = +0.34 V and the e.m.f. of the cell was found to be +0.43V.

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

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

....................................................................................................................................................................... [2](iv) If in the diagram Fe3+(aq)/Fe2+(aq) was replaced by Ti3+(aq)/Ti2+(aq) (Eê= -0.37V), calculate the e.m.f

of the cell.

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

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

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

SALT BRIDGE CuPt

1M FeCl3(aq)

1M CuSO4(aq)

& 1M FeCl2(aq)

V+

_


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FACTORS INFLUENCING ELECTRODE POTENTIALS AND CELL EMF


An electrochemical cell was set up comprising a standard hydrogen electrode and a standard silver electrode

Ag+(aq)/Ag(s) Eê = +0.80 V H+(aq)/ ½ H2(g) Eê = 0.00 V

a) (i) Give the conventional cell representation.

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

(ii) Calculate the cell e.m.f.

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

(iii) In which direction do electrons flow through the external circuit?

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

(iv) Write an ionic equation for the cell reaction.

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

(v) If the experiment is left for a long time, the pH of the hydrogen electrode changes. State whether itincreases or decreases, and why.

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

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

b) What would be the effect on the potential of the electrode concerned, and hence on the cell e.m.f., of eachof the following modifications?

(i) Increasing the surface area of the platinum foil of the hydrogen electrode.

Effect on electrode potential .............................................................................................................. [1]

Effect on e.m.f. ..................................................................................................................................... [1]

(ii) Increasing the hydrogen ion concentration of the hydrogen electrode.


Effect on e.m.f. ..................................................................................................................................... [1]

(iii) Raising the temperature of the hydrogen electrode.


Effect on e.m.f. ..................................................................................................................................... [1]

(iv) Increasing the gas pressure of the hydrogen electrode..


Effect on e.m.f. ..................................................................................................................................... [1]

(v) Increasing the silver ion concentration of the silver electrode..


Effect on e.m.f. ..................................................................................................................................... [1]


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PREDICTION OF REDOX CHANGES FROM STANDARD ELECTRODE POTENTIALS


For all parts of this Questionsheet you should refer to the following data.

½ S2O

8 2- (aq) + e- ¾ SO

42-(aq) Eê = +2.01 V

½ Cl2(g) + e- ¾ Cl-(aq) Eê = +1.36 V

½Cr2O

72-(aq) + 7H+(aq) + 3e- ¾ Cr3+(aq) + 7/

2H

2O(l) Eê = +1.33 V

½ O2(g) + 2H+(aq) + 2e- ¾ H

2O(l) Eê = +1.23 V

ClO-(aq) + H2O(l) + e- ¾ ½ Cl

2(g)

+ 2OH-(aq) Eê = +1.20 V

Fe3+(aq) + e- ¾ Fe2+(aq) Eê = +0.77 V

2SO42-(aq)

+ 10H+(aq)

+ 5e- ¾ S

2O

32-(aq) + 5H

2O(l) Eê = +0.74 V

½I2(aq) + e- ¾ I-(aq) Eê = +0.54 V

½ S4O

62-(aq) + e- ¾ S

2O

32-(aq) Eê = +0.09 V

a) Give the formulae of all the species produced when the following pairs of substances are mixed together inaqueous solution under standard conditions. (If there are no products, write “none”.)

(i) Sodium thiosulfate and iodine.

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

(ii) Sodium thiosulfate and chlorine.

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

(iii) Chromium(III) sulfate and potassium chloride.

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

(iv) Iron(II) iodide and potassium dichromate(VI) in acidic solution.

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

(Continued...)


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PREDICTION OF REDOX CHANGES FROM STANDARD ELECTRODE POTENTIALS


18

b) Imagine that each of the following reactions were to be carried out in an electrochemical cell, with oxidationin one half-cell and reduction in the other. Calculate the e.m.f. of the cell and then decide the outcome if thereaction were to be attempted in a beaker: would it go to completion, exist in dynamic eqilibrium, or notoccur at all under standard conditions?

(i) 4Fe3+(aq) + 2H2O(l) → 4Fe2+(aq) + O

2(g) + 4H+(aq)

Eêcell

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

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

(ii) Cr2O

72-(aq) + 6I-(aq) + 14H+(aq) → 2Cr3+(aq) + 3I

2(aq) + 7H

2O(l)

E êcell

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

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

(iii) 2Cl2(g) + 2H

2O(l) → O

2(g) + 4Cl

-(aq) + 4H

+(aq)

E êcell

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

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

(iv) Cl2(g) + 2OH-(aq) → ClO-(aq) + Cl-(aq) + H

2O(l)

E êcell

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

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

c) Comment on each of the following observations.

(i) On mixing together solutions of sodium peroxodisulfate, Na2S

2O

8, and potassium iodide at room

temperature, there is no immediate colour change.

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

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

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

(ii) When concentrated hydrochloric acid is added to potassium dichromate(VI) solution, chlorine gas isevolved.

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

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

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


TOTAL /

A2 Level

16


FUEL CELLS

a) What are the essential differences between a fuel cell and an ordinary electrochemical cell?

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

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

b) The following diagram depicts an oxygen-hydrogen fuel cell. Several of these cells can be used togetherto provide power, for example, for public transport vehicles.

(i) Write an ionic half-equation for the reaction at the electrode involving hydrogen.

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

(ii) Write an ionic half-equation for the reaction at the electrode involving oxygen.

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

(iii) Write an overall equation for the cell reaction.

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

(iv) Suggest why the graphite electrodes need to be porous.

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

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

(Continued...)

_+

H2(g)O

2(g)

Unreacted O2(g) H

2O(g) + some

unreacted H2(g)

Porous graphite electrodes

Hot concentrated KOH(aq)


TOTAL /

A2 Level

16


FUEL CELLS

c) (i) Calculate the volume of hydrogen, at 1 atm and 298 K, needed to generate 1.00 kWh of electricity.1 kilowatt hour = 4.3 × 106 C1 Faraday = 96 500 C mol-1

1 mole of a gas at 1 atmosphere pressure and 298 K occupies 24 dm3.

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

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

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

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

(ii) If the cell’s efficiency is rated at 72%, what is the mass of hydrogen required?

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

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

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

d) (i) Suggest two advantages of using fuel cells for powering city buses.

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

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

(ii) Suggest two ways in which the problem of storing hydrogen gas on a bus could be reduced.

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

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


TOTAL /


TEST QUESTION IV

a) Explain the term standard electrode potential, Eê, as applied to the reduction of aqueous halogens.

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

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

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

....................................................................................................................................................................... [5]

b)

By referring to the electrode potential data, state how the oxidising power of the elemental halogens variesdown Group 7.

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

c) Write full ionic equations and calculate the e.m.f. of possible reactions between the following pairs ofspecies. Comment on whether or not each reaction is feasible.

(i) Fe2+(aq) and Cl2(aq)

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

e.m.f. ..................................................................................................................................................... [1]

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

(ii) Fe2+(aq) and I2(aq)

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

e.m.f. ..................................................................................................................................................... [1]

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

(iii) Fe3+(aq) and I-(aq)

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

e.m.f. ..................................................................................................................................................... [1]

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

d) In the light of your answer to c) (iii), comment on the stability of iron(III) iodide.

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

I Fe3+(aq) + e- ¾ Fe2+(aq) +0.77

II Cl2(aq) + 2e- ¾ 2Cl-(aq) +1.36

III Br2(aq) + 2e-¾ 2Br-(aq) +1.07

IV I2(aq) + 2e- ¾ 2I-(aq) +0.54

Eê / volts

16


TOTAL /


TEST QUESTION V

17

a) Hydrogen is a common fuel for fuel cells.Suggest the missing words in this statement from these possibilities:

chemical positive catalyst electrolyte redox

electrochemical oxidant biomass carrier negative

catalysis combustion reduction electrolysis renewable

“ A fuel cell is an (A) energy conversion device. Fuel goes in on the (B) side and the (C) side takes the electronacceptor (the (D)). These two materials react in the presence of an (E). A fuel works by using a (F), such as aplatinum alloy. Hydrogen acts as an energy (G) not an energy source.”

A= ............................................. B= ................................................... C= ....................................................

D= ............................................ E= ................................................... F= ...................................................

G= ............................................. [7]

b) Hydrogen can be used as a fuel gas. Suggest two advantages and disadvantages if the gas is used for thispurpose.

Advantages .......................................................................................................................................................

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

Disadvantages ..................................................................................................................................................

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

c) The standard hydrogen electrode forms a cell with a half-cell of copper(II) sulfate and copper under standardconditions. The standard electrode potential for Cu2+/Cu = +0.34V.

(i) Give an ionic half-equation to show the reaction at the reducing electrode

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

(ii) The copper(II) sulfate concentration is increased. State and explain the effect on the cell e.m.f.

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

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

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

(iii) What is the standard cell e.m.f?

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

a2 17 qu - frankly chemistryfranklychemistry.co.uk/20to9/active_learning/chemistry/curriculum... ·...

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