INTRODUCTION
 In
chemistry, you might have learnt that some reactions proceed easily
(spontaneously) either naturally or on application of heat. Other
reactions can only occur (need to be forced to proceed) by supplying energy with an externally applied electric current, for example, a battery. This process is called electrolysis.
Electrolysis
is important in that it is used in the extraction of reactive metals,
such as potassium, sodium, calcium, magnesium and aluminium;
electroplating metals to improve their appearance and reduce
corrosion/prevent rusting and in refining metals such as copper. For
example, copper is mined in Kilembe and is transported to Jinja for
purifying electrolytically. Electroplating can be done at home once you
have the necessary substances to use and intend to start a small
business, for example, coating iron or aluminium bangles with copper.
ELECTROLYSIS
This unit deals with
(i) Characterisation of substances as electrolytes, non – electrolytes, conductors and non-conductors.
(ii) Selective discharge of an ion at an electrode.
(iii) Reactions at the electrodes during electrolysis.
(iv) Laws of electrolysis.
(v) Electrochemical cells.
In dealing with this unit the teacher should have knowledge on the following:
(i) Definitions of strong and weak electrolytes, conductors, non- conductors and electrolysis.
(ii) Examples of strong and weak electrolytes, non-electrolytes, conductors and non-conductors.
(iii) The conditions required for electrolysis to take place such as:
Figure 1- Diagram showing the requirements for electrolysis
(iv) The factors which affect discharge of an ion at an electrode. These include:
Position of the ion in the activity series: Illustrate this factor using copper(II) sulphate with carbon electrodes or dilute sulphuric acid using platinum electrodes.
Concentration of electrolytes: Illustrate this factor using dilute and concentrated hydrochloric acid, dilute and concentrated sodium chloride solutions.
Caution: if
you are performing the experiment take note that chlorine is poisonous.
Therefore, you should avoid inhaling too much of it. In order to do so
keep the windows open to ensure proper ventilation.
Nature of electrodes: This could be illustrated by using copper(II) sulphate solution with copper electrodes.
Figure 2- Electrolysis of copper(II) sulphate using graphite electrodes
|
Cu2+(aq) + SO42-(aq)
| at anode (positive) | at cathode (negative) | |
| a | anode dissolves | pink solid forms |
| b | anode dissolves | pink solid forms |
| c | color gas forms | color gas forms |
| d | color gas forms | pink solid forms |
17. Why is cryolite, Na3AlF6, used in the extraction of aluminium from aluminium oxide?
a. to dissolve aluminium oxide
b. to prevent the anodes from burning away
c. to prevent the oxidation of the aluminium
d. to remove impurities from the aluminium oxide
18.
When sodium chloride was electrolysed, sodium was produced at the
negative electrode. In which form was the sodium chloride during the
electrolysis?
a. concentrated aqueous solution
b. dilute aqueous solution
c. molten
d. solid
19. In which instance is there no change in the concentration of the solution during electrolysis?
a. concentrated sodium chloride solution between carbon electrodes
b. copper(II) sulfate solution between copper electrodes
c. copper(II) sulfate solution between platinum electrodes
d. dilute sodium chloride solution between platinum electrodes
20. An example of a weak electrolyte is
a. alcohol
b. salt solution
c. sugar solution
d. ammonia solution
21. Electroplating iron with zinc is called galvanising. The reaction at the cathode is shown by the equation
a. Fe (s) ---> Fe2+ (aq) + 2e-
b. Fe2+ (aq) + 2e- ---> Fe (s)
c. Zn (s) ---> Zn2+ (aq) + 2e-
d. Zn2+ (aq) + 2e- ---> Zn (s)
22. The circuit shown below was set up, with brass as the anode.
Which electrode reactions will occur on closing the switch?
Anode reaction Cathode reaction
a. Copper dissolves preferentially. Copper is deposited.
b. Copper dissolves preferentially. Hydrogen is evolved.
c. Zinc dissolves preferentially. Hydrogen is evolved.
d. Zinc and copper both dissolve. Copper is deposited.
23.
During the electrolysis of concentrated sodium chloride in a cell,
chlorine, hydrogen, and sodium hydroxide are produced. What is the molar
ratio of these products?
Chlorine Hydrogen Sodium hydroxide
a. 1 1 1
b. 2 1 2
c. 2 1 1
d. 2 2 1
Answers
1. b
2. c
3. a
4. a
5. c
6. c
7. a
8. b
9. a
10. c
11. a (H+ and K+ ions in the electrolyte migrate to the cathode. H+ are preferentially discharged to form hydrogen gas because it is lower down in the electrochemical series than K+ ions)
12. b (the negative chloride ions will migrate to the anode and become oxidised at the anode to form chlorine gas)
13. b (the ions attracted to the cathode are H+ and Na+ ions. H+ is preferentially discharged to form hydrogen gas)
14. b
15. c
16. b
17. a
18. c
19. b
20. d
21. d
22. c
23. b
Structured Question Worked Solutions
1. Dilute sulphuric acid will conduct an electric current.
a. Give the formulae of all of the ions present in dilute sulphuric acid
b. Name the gaseous products which you would expect to be formed during the electrolysis of aqueous potassium sulphate using inert electrodes
at the anode:_____
at the cathode:______
c. Name a metal which is used to electroplate
i. bicycle handlebars
ii. teaspoon
d. Explain why a metal such as aluminium can conduct an electric current but a non-metal such as sulphur cannot conduct a current
a. Give the formulae of all of the ions present in dilute sulphuric acid
b. Name the gaseous products which you would expect to be formed during the electrolysis of aqueous potassium sulphate using inert electrodes
at the anode:_____
at the cathode:______
c. Name a metal which is used to electroplate
i. bicycle handlebars
ii. teaspoon
d. Explain why a metal such as aluminium can conduct an electric current but a non-metal such as sulphur cannot conduct a current
Solution
a. H+, OH-, SO42-
b. cathode: Hydrogen
anode: Oxygen
ci. chromium
cii. silver
d.
Aluminium consists of positively charged particles in a sea of
electrons. The electrons are able to move freely and thus electricity
can flow. In sulphur, the atomic arrangement is fixed, so there is no
movement of electrons. When electricity is passed through sulphur,
electricity will not be able to flow.
2a.
When concentrated aqueous sodium chloride is electrolysed using graphite
electrodes, hydrogen is collected at the cathode and chlorine at the
anode.
When concentrated aqueous sodium chloride is electrolysed using iron electrodes, hydrogen is again collected at the cathode but much less chlorine is collected at the anode.
i. Give the equations for the electrode reactions by which hydrogen and chlorine are formed
ii. Explain why much less chlorine is collected when iron electrodes are used.
iii. Name the product, other than hydrogen and chlorine, which is manufactured by the electrolysis of concentrated aqueous sodium chloride. Give a major use of this product
b. Why is the electrolysis of concentrated hydrochloric acid not used for the manufacture of chlorine?
When concentrated aqueous sodium chloride is electrolysed using iron electrodes, hydrogen is again collected at the cathode but much less chlorine is collected at the anode.
i. Give the equations for the electrode reactions by which hydrogen and chlorine are formed
ii. Explain why much less chlorine is collected when iron electrodes are used.
iii. Name the product, other than hydrogen and chlorine, which is manufactured by the electrolysis of concentrated aqueous sodium chloride. Give a major use of this product
b. Why is the electrolysis of concentrated hydrochloric acid not used for the manufacture of chlorine?
Answers
ai. 2H+ (aq) + 2e- -----> H2 (g)
2Cl- (aq) -----> Cl2 (g) + 2e-
ii.
When iron anode is used, some oxygen gas is produced at the same time.
Some of the electrical energy is used to liberate oxygen. So less Cl2 is produced.
iii. Sodium hydroxide. It is used to manufacture soap
b.
because concentrated HCl is not a cheaply and readily available raw
material. It is also a volatile acid. A lot of HCl gas will be emitted.
3.
The diagram shows the electrolytic cell used to produce aluminium. The electrolyte contains aluminium oxide and cryolite (sodium aluminium fluoride) and is molten at about 800oC. The electrodes are made from graphite.a. Why is a mixture of cryolite and aluminium oxide, and not pure aluminium oxide, used as the electrolyte?
b. Write the equations for the reactions occurring at
i. the positive electrode
ii. the negative electrode
c. Explain why the graphite anodes need to be replaced at regular intervals
d. Calculate the maxiumum mass of aluminium that can be made from 408 tonnes of aluminium oxide.
ei. Aluminium foil is used to make food containers because it does not corrode easily. Explain why aluminium does not corrode easily.
eii. Give a use of aluminium, other than for food containers, together with the physical property that makes aluminium suitable for that use.
eiii. Give a further use of aluminium, that makes aluminium suitable for that use. (excluding that from ei and eii)
Solution
a.
Aluminium oxide has a very high melting point. With the addition of
cryolite, the melting point is greatly reduced, making it more
economical.
bi. 2O2- (l) ---> O2 (g) + 4e-
bii. Al3+ (l) + 3e- ---> Al (s)
c. The oxygen produced at the graphite anode oxidises the graphite to CO2
d. From the equation 2Al2O3 ---> 4Al + 3O2
1 mole of aluminium oxide gives 2 moles of aluminium. So 102g of Al2O3 gives 54g of aluminium.
Thus 408 tonnes of Al2O3 will give (54/102) x 408 = 216 tonnes of aluminium
ei.
Aluminium forms aluminium oxide in the presence of air. This oxide is
insoluble and resistant to corrosion so it forms a protective coating
for aluminium.
eii. It is used in making cooking
utensils since it has very good conductivity, in addition to its good
appearance and resistance to corrosion.
eiii. it is
a component in several alloys used in aircraft construction. Its
favorable use is due to its low density and high tensile strength.
4. Complete the table by naming the products formed when the following liquids are electrolysed using inert electrodes
| liquid | product formed at cathode | product formed at anode |
| dilute sulphuric acid | oxygen | |
| molten calcium bromide | ||
| concentrated aqueous sodium chloride |
Solution
| liquid | product formed at cathode | product formed at anode |
| dilute sulphuric acid | hydrogen | oxygen |
| molten calcium bromide | calcium | bromine |
| concentrated aqueous sodium chloride | hydrogen | chlorine |
5. Aqueous copper(II) sulphate was electrolysed in two cells using different electrodes as shown below.
a. Write ionic equations, with state symbols, for the reactions which take place at the anode in each cell.b. Describe one change that you would see happen in both cells.
c. Describe one change that you would see happen in Cell 1 but not in Cell 2.
Solution
a. Anode reaction cell 1: 4OH- (aq) --> O2 (g) + 2H2O (l) + 4e-
Anode reaction cell 2: Cu (s) --> Cu2+ (aq) + 2e-
b. The size of the cathode increases as copper metal is plated onto the cathode in both cells.
Cu2+ (aq) + 2e- --> Cu (s)
c.
The blue colour of the electrolyte in cell 1 fades when more and more
Cu2+ ions are reduced to copper metal and plated onto the cathode as a
pink deposit.
Cu2+ + 2e- --> Cu
6a. Write an ionic equation for the reaction between zinc and aqueous copper(II) sulphate.
This reaction can be used to generate electricity in a cell.
b. Draw an arrow on the diagram to show the direction of the flow of electrons in the wire.c. The voltage of the cell was measured when the following metals were used as electrode 2.
copper iron lead zinc
Complete the table by entering the metals in the correct order.
| meter reading/V | Metal |
| 1.10 | |
| 0.78 | |
| 0.21 | |
| 0.00 |
d. When metal M was used as electrode 2, it produced a higher voltage than zinc. Suggest a name for metal M.
Solution
a. Zn (s) + Cu2+ (aq) --> Zn2+ (aq) + Cu (s)
Note: zinc, being a more reactive metal, displaces copper ions out of solution as copper metals
b. arrow direction from electrode 2 to electrode 1
Note:
Zinc, being a more reactive metal, loses electrons more easily when
connected to a metal of lower reactivity, in this case copper.
c.
| meter reading/V | Metal |
| 1.10 | zinc |
| 0.78 | iron |
| 0.21 | lead |
| 0.00 | copper |
d. Magnesium
7. Electroplating can be used to coat nickel with a thin coating of silver.
a. Draw a labelled diagram of an apparatus that can be used to electroplate silver onto nickel.
b. Write equations, with state symbols, for the reactions at the anode and cathode.
c. Solutions of two salts, A and B, were electrolysed using carbon electrodes. The following products were collected.
a. Draw a labelled diagram of an apparatus that can be used to electroplate silver onto nickel.
b. Write equations, with state symbols, for the reactions at the anode and cathode.
c. Solutions of two salts, A and B, were electrolysed using carbon electrodes. The following products were collected.
| Salt | Products |
| A | oxygen and hydrogen |
| B | chlorine and hydrogen |
i. Suggest the names of the two salts, A and B.
ii. Describe tests to confirm the identities of the three gases collected.
Solution
a.
b. reaction at anode: Ag (s) --> Ag+ (aq) + e-
ci. Salt A: sodium sulphate
Salt B: sodium chloride
cii.
Collect samples of each gas using test tubes. To test for oxygen,
insert a glowing splint into the test tube of gas. The gas that relights
the glowing splint is oxygen. To the remaining samples, place a lighted
splint at the mouth of each test tube. The gas that extinguishes the
lighted splint with a "pop" sound is hydrogen. To identity chlorine,
place a piece of moist blue litmus at the mouth of test tube of gas. The
litmus turns red and bleaches.
8. One important
use of a gas Y is to sterilize swimming pool water. The gas is prepared
in the laboratory by the electrolysis of a solution. A student tried
to prepare gas Y by the electrolysis of a very dilute sodium chloride
solution as shown below. Contrary to the student's expectation, a
colorless gas, instead of gas X, was liberated at the anode. 
a. What is the colorless gas liberated?
b. Suggest a chemical test for the colorless gas
c. The experiment was then modified to prepare gas Y.
i. Suggest how the experiment could be modified. Explain your answer.
ii. Suggest the solution left after the electrolysis
iii. Suggest one common use of the solution left.
a. What is the colorless gas liberated?b. Suggest a chemical test for the colorless gas
c. The experiment was then modified to prepare gas Y.
i. Suggest how the experiment could be modified. Explain your answer.
ii. Suggest the solution left after the electrolysis
iii. Suggest one common use of the solution left.
Solution
8a. oxygen
8b. The gas relights a glowing splint.
8ci.
Use concentrated sodium chloride solution instead of a very dilute
sodium chloride solution. Concentration of chloride ions in the solution
would be much greater than that of hydroxide ions. Therefore, chloride
ions would be preferentially discharged to form chlorine gas.
8ciii. manufacture of bleach
9. A dilute copper(II) sulphate solution is electrolysed using carbon electrodes.
a. Describe and explain what would happen at the two carbon electrodes
b. Write half-equations for the reactions at carbon exlectrodes X and Y.
c. What will be the charge in the electrolyte as electrolysis proceeds for some time? Explain your answer.
d. Explain what could happen to the copper(II) sulphate solution if copper electrodes are used in the above experiment.
a. Describe and explain what would happen at the two carbon electrodesb. Write half-equations for the reactions at carbon exlectrodes X and Y.
c. What will be the charge in the electrolyte as electrolysis proceeds for some time? Explain your answer.
d. Explain what could happen to the copper(II) sulphate solution if copper electrodes are used in the above experiment.
Solution
9a.
At carbon electrode X:
The
sulphate ions and hydroxide ions migrate to electrode X. A hydroxide
ion is a stronger reducing agent than a sulphate ion. So hydroxide ions
are preferentially discharged.
At carbon electrode Y:
The
copper(II) ions and hydrogen ions migrate to electrode Y. A copper(II)
ion is a stronger oxidizing agent than a hydrogen ion. So copper(II)
ions are preferentially discharged to form a deposit of copper on
electrode Y.
9b.
electrode X:
4OH- (aq) --> O2 (g) + 2H2O (l) + 4e-
electrode Y:
Cu2+ (aq) + 2e- --> Cu (s)
9c.
The solution becomes sulphuric acid because copper(II) ions and
hydroxide ions are consumed in the electrolysis. Hydrogen ions and
sulphate ions remain in the solution.
9d. The net
effect is the transfer of copper from electrode X to electrode Y. The
dilute copper(II) sulphate solution remains the same.
10.
Tuning knobs on radios are often made of plastics plated with metal
coatings. The plastic knobs are first coated with copper and then
electroplated with nickel. The electroplating can be conducted using the
following setup.
a. Explain the term 'electroplating'
b. Why is the plastic knob first coated with copper before electroplating?
c. Explain why nickel(II) sulphate solution can conduct electricity
d. Which is the anode, the nickel electrode or the copper-coated knob?
e. Write an ionic half-equation for the reaction at the copper-coated knob
f. Explain why it is better to use a nickel electrode than a carbon electrode in the above process
g. In a nickel-plating factory, the waste water is treated with sodium hydroxide solution to remove nickel(II) ions before discharge. Suggest 2 reasons why it is necessary to remove nickel(II) ions from the waste water before discharge
a. Explain the term 'electroplating'
b. Why is the plastic knob first coated with copper before electroplating?
c. Explain why nickel(II) sulphate solution can conduct electricity
d. Which is the anode, the nickel electrode or the copper-coated knob?
e. Write an ionic half-equation for the reaction at the copper-coated knob
f. Explain why it is better to use a nickel electrode than a carbon electrode in the above process
g. In a nickel-plating factory, the waste water is treated with sodium hydroxide solution to remove nickel(II) ions before discharge. Suggest 2 reasons why it is necessary to remove nickel(II) ions from the waste water before discharge
Solution
10a. It is the coating of an object with a thin layer of a metal by electrolysis
10b. to make the knob conduct electricity
10c. the solution contains mobile ions
10d. the nickel electrode
10e. Ni2+ (aq) + 2e- --> Ni (s)
10f. the concentration of nickel(II) ions in the electrolyte can be maintained
10g.
- to recover the nickel metal
- nickel(II) ions are harmful to marine lives. Humans may get poisoned by eating contaminated seafood
11. The following circuit is set up. Electrodes A and B are made of carbon while electrode C and D are made of copper.
a. What are the functions of the ammeter and rheostat respectively?
b. Explain why no current flows when potassium iodide is in solid state, but a current flows when water is added.
ci. What would be observed at electrode A and B respectively?
cii. Write ionic half-equations for the reactions at electrodes A and B
di. What would be observed at electrodes C and D respectively?
dii. Write ionic half-equations for the reactions at electrodes C and D
diii. Would you expect any color change in the dilute copper(II) sulphate solution during the process? Explain your answer.
a. What are the functions of the ammeter and rheostat respectively?b. Explain why no current flows when potassium iodide is in solid state, but a current flows when water is added.
ci. What would be observed at electrode A and B respectively?
cii. Write ionic half-equations for the reactions at electrodes A and B
di. What would be observed at electrodes C and D respectively?
dii. Write ionic half-equations for the reactions at electrodes C and D
diii. Would you expect any color change in the dilute copper(II) sulphate solution during the process? Explain your answer.
Solution
11a.
The ammeter is an instrument used to measure the electric current
passing through the circuit. The rheostat is used to vary the resistance
in the circuit and regulate the current.
11b. In
solid state, the ions in potassium iodide are held together by strong
attraction. They are not free to move. So solid potassium iodide does
not conduct electricity. When water is added to the compound, the
compound dissolves in the water and the ions become mobile and a current
can then flow through the solution.
11ci. A brown color develops around electrode A. A colorless gas is given off from electrode B.
11cii.
electrode A:
2I- (aq) --> I2 (aq) + 2e-
electrode B:
2H+ (aq) + 2e- --> H2 (g)
11di. Electrode C dissolves. A reddish brown deposit forms on electrode D.
11dii.
electrode C:
Cu (s) --> Cu2+ (aq) + 2e-
electrode D:
Cu2+ (aq) +2e- --> Cu (s)
11diii.
The blue color of the dilute copper(II) sulphate solution does not
change because the concentration of copper(II) ions in the solution
remains the same.
12. A student used the following set-up for passing electricity through some solutions.
The results are shown below:
The results are shown below:| Experiment | Solution | Observations |
| 1 | sugar solution | zero ammeter reading |
| 2 | dilute sulphuric acid | gas bubbles given off from both electrodes |
| 3 | dilute sodium iodide solution | ? |
| 4 | dilute silver nitrate solution | gas bubbles given off from one electrode and silvery solid deposited on the other electrode |
a. Why is zero ammeter reading recorded in Experiment 1?
b. For Experiment 2,
i. name the gas liberated at electrodes X and Y respectively
ii. write ionic half-equations for the reactions at the electrodes
c. For Experiment 3,
i. what substance would you expect to form at electrode X. Explain briefly.
ii. what substance would you expect to form at electrode Y. Explain briefly.
d. For Experiment 4,
i. write ionic half-equations for the reactions at the electrodes
ii. state the change in the solution after electricity has been passed through for some time.
Solution
12a. Sugar solution is not a conductor of electricity
12bi. X: oxygen; Y: hydrogen
12bii.
at electrode X:
4OH- (aq) --> O2 (g) + 2H2O (l) + 4e-
at electrode Y:
2H+ (aq) + 2e- --> H2 (g)
12ci.
The concentration of iodide ions in the solution is much greater than
that of hydroxide ions. Iodide ions are preferentially discharged to
form iodine.
12cii. A hydrogen ion is a stronger
oxidizing agent than a sodium ion. So hydrogen ions are preferentially
discharged to form hydrogen gas.
12ciii. Hydrogen
ions and iodide ions are consumed in the electrolysis. Sodium ions and
hydroxide ions remain in the solution. Eventually, the solution becomes
sodium hydroxide solution.
12di.
electrode X:
4OH- (aq) --> O2 (g) + 2H2O (l) + 4e-
electrode Y:
Ag+ (aq) + e- --> Ag (s)
12dii.
Silver ions and hydroxide ions are consumed in the electrolysis.
Hydrogen ions and nitrate ions remain in the solution. The solution
eventually becomes nitric acid solution.
13. When the circuit in the set-up shown below is closed, the acidified potassium permanganate solution loses its color gradually.
a.
Write a half equation for the reaction that occurs in the acidified
potassium permanganate solution. Explain whether the permanganate ion is
oxidized or reduced.b. What would be observed in the iron(II) sulphate solution after some time. Write a half equation for the reaction that would occur.
c. Identify the direction of electron flow in the external circuit.
d. Write an ionic equation for the reaction that occurs when acidified potassium permanganate solution and iron(II) sulphate solution are mixed together.
ei. Give the function of the salt bridge set up
eii. Explain whether a sodium sulphite solution can be used instead of a potassium nitrate solution in the salt bridge.
As an alternative to iron(II) sulphate and acidified potassium permanganate solution, potassium iodide solution and iron(III) sulphate solution may be used on the left hand side and the right hand side respectively.
fi. What would be observed in the potassium iodide solution after some time? Write a half equation for the reaction that would occur.
fii. What would be observed in the iron(III) sulphate solution after some time? Write a half equation for the reaction that would occur.
Solution
13a. MnO4- (aq) + 8H+ (aq) + 5e- --> Mn2+ (aq) + 4H2O (l)
MnO4- is reduced because it receives electrons and the oxidation number of Mn changes from +7 to +2.
13b. The solution changes from green to yellow gradually because iron(II) ions are oxidised to iron(III) ions
Fe2+ (aq) --> Fe3+ (aq) + e-
13c. From iron(II) sulphate solution to potassium permanganate solution
13d. 5Fe2+ (aq) + 8H+ (aq) + MnO4- (aq) --> Mn2+ (aq) + 5Fe3+ (aq) + 4H2O (l)
13ei. to allow migration of ions between the two beakers or to complete the circuit
13eii. No. sodium sulphite reacts with potassium permanganate or the sulphite ions can be oxidised by permanganate ions
13fi. the solution turns brown or yellow
2I- (aq) --> I2 (aq) + 2e-
13fii. the solution changes from yellow to green
Fe3+ (aq) + e- --> Fe2+ (aq)
13fiii. 2I- (aq) + 2Fe3+ (aq) --> I2 (aq) + 2Fe2+ (aq)
14. The diagram shows a dry cell
ai. which substance is the positive electrode?
aii. write an ionic half equation for the reaction at the positive electrode
bi. which substance is the negative electrode?
bii. write an ionic half equation for the reaction at the negative electrode
c. explain the function of manganese(IV) oxide
d. the voltage of the cell drops if current is drawn from the cell rapidly. Explain briefly.
e. explain why the zinc case of a used cell is thinner than that of a new cell
f. explain why disposal of 'flat' cells present a pollution problem
g. explain why zinc-carbon cells should be removed from electric appliances when not in use for a long period
ai. which substance is the positive electrode?
aii. write an ionic half equation for the reaction at the positive electrode
bi. which substance is the negative electrode?
bii. write an ionic half equation for the reaction at the negative electrode
c. explain the function of manganese(IV) oxide
d. the voltage of the cell drops if current is drawn from the cell rapidly. Explain briefly.
e. explain why the zinc case of a used cell is thinner than that of a new cell
f. explain why disposal of 'flat' cells present a pollution problem
g. explain why zinc-carbon cells should be removed from electric appliances when not in use for a long period
Solution
14ai. carbon rod
14aii. 2NH4+ (aq) + 2e- --> 2NH3 (aq) + H2 (g)
14bi. zinc case
14bii. Zn (s) --> Zn2+ (aq) + 2e-
14c. Hydrogen is produced and collected on the surface of the positive electrode. Since hydrogen is a poor conductor of electricity, the accumulation of hydrogen at the positive electrode may hinder further reactions and decrease the current of the cell. Manganese(IV) oxide, an oxidizing agent, is used to remove the hydrogen.
14c. Hydrogen is produced and collected on the surface of the positive electrode. Since hydrogen is a poor conductor of electricity, the accumulation of hydrogen at the positive electrode may hinder further reactions and decrease the current of the cell. Manganese(IV) oxide, an oxidizing agent, is used to remove the hydrogen.
14d.
If a current is drawn from the cell rapidly, the gaseous product cannot
be removed fast enough. The voltage drops as a result.
14e. The zinc case undergoes oxidation to give zinc ions in the cell reaction.
14f.
The materials inside the cells do not decompose even after a long time.
These materials may combine with other compounds and form harmful
substances which pollute the environment.
14g.
There is a slow direct reaction between the zinc electrons and ammonium
ions. After some time, the zinc case becomes too thin and the paste
leaks out. This may cause damage to electric appliances.
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