Page 1

Topic VII – Redox, Chemical cell & Electrolysis (Part C) a set-up for electrolysis

by Ricky Tsui

is a setup to convert electrical energy to chemical energy.

= decomposition by electrical energy

Steps for handling an electrolysis question

d.c. supply (直流電) a.c. supply is NOT appropriate (polarity of electrode will be changed throughout electrolysis)

1

Electrode (電極棒)

2

If…Inert electrode: Platinum, Pt or graphite, C OR made up of metals (i.e. Zn, Cu and Mg etc.)

external circuit (外電路) 正長負短 負極出電子

3

Direction of electron flow in external circuit is determined by the polarity of d.c. supply

Polarity of d.c. supply = Polarity of electrode

4 Electrolyte (電解質) Contains mobile ions

溶) state or molten (熱熔) state

Either in aqueous (水

Factors affecting the formation of

Industrial application (工業應用)

products of electrolysis

of electrolysis

1. Position of ions in the

1. Metal extraction for reactive metals

electrochemical series (E.C.S.) p.6-7 2. Concentration of ions p.13 3. Nature of electrode p. 20

p.3

2. Aluminum anodization p.9 3. Electrolysis of sea water p.14-15 4. Copper purification p.20-21 5. Electroplating p.26 Ch 33 Electrolysis

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Page 1


Topic VII – Redox, Chemical cell & Electrolysis (Part C)

by Ricky Tsui

Case 1 of 4 – inert electrode + molten electrolyte

Cations and anions are held together by STRONG ionic bond

PbBr2(s)

ions are NOT mobile

(s) (l)

In molten state, ions become mobile and move to opposite charged electrode.

X(anode) :

2Br-  Br2 + 2e(brown fume produced)

Y(cathode) : Pb2+ + 2e-  Pb

m.p of PbBr (s) = 373 C 2

o

Overall: PbBr2(l)  Pb(l) + Br2(g) PbBr2(l) is broken down into Pb(l) and Br2(g) by electrolysis.

1. At the anode:

At the cathode:

Overall reaction:

m.p of NaCl(s) = 801 C

Observations:

o

_____________________ is evolved Provided information:

at the anode ; _________________ is formed around the cathode.

Ch 33 Electrolysis

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Page 2


Topic VII – Redox, Chemical cell & Electrolysis (Part C)

by Ricky Tsui

[03CE/7(a)/6 marks] 2. The set-up shown below is used to investigate the electrical conductivity of lead(II) bromide.

When the lead(II) bromide becomes molten, the bulb lights up. (i)

What would be observed at electrode X? Write the half equation for the reaction involved.

(ii)

State ONE potential hazard when carrying out the experiment.

(iii)

State what will happen to the bulb when heating is stopped and the molten lead(II) bromide is allowed to cool down gradually to room temperature. Explain your answer.

Industrial application (工業應用) of electrolysis – extraction of reactive metals 

Reactive metals are extracted by electrolysis of their molten ores.

The ores used are usually chlorides of K, Na, Ca and Mg and oxide of Al. For example, in the extraction of aluminium from its

molten aluminium oxide.

MRS

Methods of extraction

K At the cathode: Al3+(l)

+ 3e  Al(l) -

At the anode: 2-

2O

(l)

Overall reaction: 2Al2O3(l)  4Al(l) + 3O2(g)

 O2(g) + 4e

-

Na Ca

Electrolysis of its

Mg

molten

Al

ores

extraction of aluminum - electrolysis of its molten ores cryolite (冰晶石) Ch 33 Electrolysis

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Page 3


Topic VII – Redox, Chemical cell & Electrolysis (Part C)

by Ricky Tsui

3. Electrolysis means decomposition by A. heat. B. light. C. electricity. D. electrons.

4. Which of the following statements about electrolysis are INCORRECT? (1) An alternating current (a.c.) power source could be used for electrolysis. (2) It refers to the decomposition of compounds by chemical energy. (3) Redox reactions take place in an electrolytic cell spontaneously. A. (1) and (2) only B. (1) and (3) only C. (2) and (3) only D. (1), (2) and (3)

5. Which of the following substances CANNOT be decomposed by electricity? (1) Molten aluminium (2) Solid lead(II) bromide (3) Solid sodium chloride A. (1) and (2) only B. (1) and (3) only C. (2) and (3) only D. (1), (2) and (3)

6. Which of the following statements concerning the electrolysis of molten lead(II) bromide is correct? A.Bromide ions lose electrons at the anode to form bromine molecules. B. Lead(II) ions gain electrons at the anode to form lead metal. C. Bromide ions undergo oxidation at the cathode to form bromine molecules. D.Lead(II) ions undergo reduction at the anode to form lead metal.

7. Magnesium can be obtained from its ores by electrolysis of molten magnesium chloride. What are the electrodes made of? A. Copper B. Platinum C. Magnesium D. Graphite

8. Which of the following metals is usually NOT extracted by electrolysis? A. Sodium B. Calcium C. Aluminium D. Iron

*9. The following set-up shows an electrolysis using inert electrodes.

lead

lead(II) bromide

In the apparatus, the bulb glows when (1) lead is in molten state and lead(II) bromide is in solid state. (2) lead is in solid state and lead(II) bromide is in molten state. (3) both lead and lead(II) bromide are in molten states. A. (1) and (2) only B. (1) and (3) only C. (2) and (3) only D. (1), (2) and (3) Ch 33 Electrolysis

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Page 4


Topic VII – Redox, Chemical cell & Electrolysis (Part C)

by Ricky Tsui

[04CE/MC#16] 10. Magnesium can be obtained from magnesium oxide by A. B. C. D.

electrolysis of the molten oxide. heating the oxide strongly. heating the oxide with carbon. heating the oxide with zinc powder.

[12DSE/MC#9] 11.

[07CE/6/6 marks] 12. Read the paragraph below and answer the questions that follow. Magnesium is a useful metal. Scientists adopt different methods to extract magnesium from magnesium oxide. In 1828, a scientist obtained magnesium in two steps. In the first step, magnesium oxide reacts with chlorine and carbon to form magnesium chloride. In the second step, the magnesium chloride formed reacts with potassium to give magnesium. In 1951, some scientists adopted another chemical process to obtain magnesium from magnesium chloride. Potassium is not used in this process, and there is even no need to use any other chemicals. (a)

(b)

(c)

Write a chemical equation for the reaction that occurred in the first step of the method used by the scientist in 1828. (1 mark) Name the type of reaction between potassium and magnesium chloride. Why can potassium react with magnesium chloride to give magnesium? (2 marks) (i) What would be the chemical process that can obtain magnesium from magnesium chloride, without using potassium or other chemicals, in 1951?

(ii)

What property does magnesium chloride possess so as to make the chemical process possible?

(2 marks) (d)

Suggest one use of magnesium in daily life. (1 mark) Ch 33 Electrolysis

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Page 5


Topic VII – Redox, Chemical cell & Electrolysis (Part C)

by Ricky Tsui

Case 2 of 4 – inert electrode + aqueous electrolyte Solution 1 – NaCl(aq) From NaCl(s)

B

C

Mobile ions

From self ionization of water

Cations Anions Na+ ClH+ OH-

H3O+(aq) + OH-(aq)

2H2O(l)

H3O+(aq) = H+(aq)

A

H+(aq)

Pure water ionizes very slightly.

actually does not exist in the water.

We can write: H2O H+ + OH-

A

B

C

Solution 2 – H2SO4(aq) From H2SO4(aq)

H2SO4(aq)  2H+(aq) + SO42-(aq) From H2O(l) H2O H+ + OH-

Cations Anions H+ SO42H+ OH-

Ch 33 Electrolysis

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Page 6


Topic VII – Redox, Chemical cell & Electrolysis (Part C)

by Ricky Tsui

How about these? Write down the MOBILE cations and anions in the space provided.

2. CH3COOH(aq) Cations Anions

1. HCl(aq) Cations Anions

4. PbBr2(l) Cations Anions

3. Sodium nitrate solution Cations Anions

4. AgCl(s) 5. Copper(II) sulphate solution 6. NiSO4(aq) Cations Anions Cations Anions Cations Anions

Preferentially discharge (優先 放電) refer to the electrochemical series (E.C.S)

Anions ClOH-

E.C.S (anion)

Cations Na+ H+

E.C.S (cation)

ClOH-

2Cl4OH-

 Cl2 + 2e 2H2O + O2 +

Very dilute NaCl(aq)

4e-

Na+ + e-  Na OH-(aq) is preferentially discharged to form O2(g) and H2O(l)

2H+ + 2e-  H2

H+(aq) is

preferentially discharged to form H2(g)

Explain the formation of product on the anode:

13. Explain the formation of product on cathode:

In very dilute NaCl(aq), it contains mobile ions Na+(aq), Cl-(aq), H+(aq) and OH-(aq).

__________________________________________ __________________________________________

Both Cl-(aq) and OH-(aq) will move to anode, __________________________________________ as OH-(aq) is at a higher position than Cl-(aq) in the electrochemical series, OH-(aq) will be preferentially discharged to form O2(g).

__________________________________________ __________________________________________ __________________________________________ Ch 33 Electrolysis

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Page 7


Topic VII – Redox, Chemical cell & Electrolysis (Part C)

by Ricky Tsui

One more example: Hofmann voltameter

Cations Anions H+ SO42H+ OH-

At the cathode:

At the anode:

2H+(aq) + 2e-  H2(g)

4OH-(aq)  O2(g) + 2H2O(l) + 4e-

H+(aq) ions are attracted towards the cathode and discharged here.

Both OH-(aq) ions and SO42-(aq) ions are attracted towards the anode. As OH-(aq) is higher than SO42-(aq) in the E.C.S., OH-(aq) ions are preferentially discharged.

Overall reaction: 2H2O(l)

 2H2(g) + O2(g)

The volume ratio of H2 : O2 = 2 : 1 It is because when two water molecules are decomposed, two hydrogen molecules and one oxygen molecule are formed. When H+(aq) ions and OH-(aq) ions are discharged, more water molecules ionize. The net effect is decomposition of water. Ch 33 Electrolysis

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Page 8


Topic VII – Redox, Chemical cell & Electrolysis (Part C)

by Ricky Tsui

Industrial application of electrolysis – aluminium anodization (鋁陽極氧化)

 http://www.engineeringhobbyist.com/projects/completed/anodizing/ Al object to be anodized 駁 正極 (anode) Al(s) sheet 駁 負極 (cathode) See what’s happening below for a better understanding!

1 Anions SO OH 24 -

Cations H H

At the anode: Both SO42-(aq) ions and OH-(aq) ions are

+

attracted towards the anode. OH-(aq) ions

+

are preferentially discharged at the anode to form oxygen:

4OH-(aq)  O2(g) + 2H2O(l) + 4e-

3

thickness of the aluminium oxide layer

2 The oxygen produced reacts with the aluminium object (anode) to form aluminium oxide.

has been increased 4

4Al(s) + 3O2(g)  2Al2O3(s)

1. more corrosion resistant (for making window frame)

2. can be dyed to give attractive colours (for making dyed aluminium objects)

[09CE/MC#41] [09CE/MC$47] 14. Anodised aluminium is more commonly used than 15 Which of the following statements concerning the iron for making window frames. This is because anodisation of an aluminium object are correct? (1) the cost for extracting aluminium is lower than the cost for extracting iron. (2) anodised aluminium is more corrosion resistant than iron. (3) anodised aluminium is harder than iron. A. B. C. D.

(1) only (2) only (1) and (3) only (2) and (3) only

(1) The electrolyte used can be dilute sulphuric acid. (2) A layer of aluminium oxide is formed on the surface of the object. (3) The aluminium object should be connected to the negative terminal of the power supply. A. B. C. D.

(1) and (2) only (1) and (3) only (2) and (3) only (1), (2) and (3)

Ch 33 Electrolysis

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Page 9


Topic VII – Redox, Chemical cell & Electrolysis (Part C) [06CE/MC#34] 16. Which of the following changes occurs after an aluminium article has been anodized? A. B. C. D.

Its electrical conductivity increases. Its tensile strength increases. It becomes more easily dyed. It becomes more easily oxidized.

18. Which of the following materials is/are usually used as inert electrode(s) in electrolysis? (1) Copper (2) Platinum (3) Graphite A. B. C. D.

(1) only (2) only (1) and (3) only (2) and (3) only

by Ricky Tsui

[02CE/MC#8] 17. Which of the following statements concerning aluminium is correct? A. Aluminium is used to make stainless steel. B. The strength of aluminium can be enhanced by anodization. C. Aluminium is the most abundant element in the earth’s crust. D. When aluminium is exposed to air, a layer of aluminium oxide is formed on its surface. 19. Which of the following combinations concerning the electrolysis of dilute magnesium chloride solution using graphite electrodes is correct?

A. B. C. D.

Ions oxidized at anode OH(aq) H+(aq) Cl(aq) Cl(aq)

Ions reduced at cathode H+(aq) OH(aq) Mg2+(aq) H+(aq)

20. Which of the following statements concerning the electrolysis of sodium nitrate solution using graphite electrodes are INCORRECT? (1) Na+(aq) ions are preferentially discharged at the cathode to form Na(s). (2) OH(aq) ions are preferentially discharged at the anode to form O2(g). (3) The concentration of the sodium nitrate solution remains unchanged after electrolysis. A. (1) and (2) only B. (1) and (3) only C. (2) and (3) only D. (1), (2) and (3)

21. Which of the following statements concerning the electrolysis of dilute copper(II) chloride solution using graphite electrodes are correct? (1) The main product at the cathode is Cu(s). (2) OH(aq) ions are preferentially discharged at the anode to form O2(g). (3) The copper(II) chloride solution becomes hydrochloric acid after electrolysis. A. (1) and (2) only B. (1) and (3) only C. (2) and (3) only D. (1), (2) and (3)

22. Dilute nitric acid is electrolysed using graphite electrodes. Which of the following equations correctly represents the reaction taking place at the cathode?

23. The electrolysis of dilute nitric acid is carried out by using inert electrodes. What is the expected volume ratio of hydrogen and oxygen collected over the electrodes after electrolysis?

A. B. C. D.

4H+(aq) + NO3(aq) + 3e  NO(g) + 2H2O(l) 4OH(aq)  O2(g) + 2H2O(l) + 4e 2H+(aq) + 2e  H2(g) 2H2O(l)  2H2(g) + O2(g)

A. B. C. D.

H2 : O2 = 1 : 1 H2 : O2 = 1 : 2 H2 : O2 = 2 : 1 H2 : O2 = 4 : 1

Ch 33 Electrolysis

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Page 10


Topic VII – Redox, Chemical cell & Electrolysis (Part C) 24. The following diagram shows the electrolysis of dilute sodium chloride solution using a Hofmann voltameter. What are gases X and Y collected in the apparatus?

by Ricky Tsui

*25. The diagram below shows the electrolysis of dilute sulphuric acid with a few drops of litmus solution added, using graphite electrodes.

dilute sodium chloride solution

graphite electrode X

graphite electrode Y

gas Y

gas X

dilute sulphuric acid with litmus solution

Which of the following statements concerning the above electrolytic cell are correct? platinum foil

platinum foil

A. B. C. D.

Gas X Oxygen Hydrogen Hydrogen Chlorine

Gas Y Hydrogen Chlorine Oxygen Hydrogen

(1) The gas collected at electrode Y can relight the glowing splint. (2) The colour of solution around electrode X becomes red. (3) The volume of gas collected at electrode X is less than that at electrode Y. A. B. C. D.

(1) and (2) only (1) and (3) only (2) and (3) only (1), (2) and (3)

[04CE/6(a)/6 marks] 26. Water (H2O) is an oxide of hydrogen. Electrolysis of water in the presence of sulphuric acid gives hydrogen and oxygen in a volume ratio of 2:1. (i)

Suggest suitable electrodes to be used in the electrolysis.

(ii)

Write the half equation for the reaction at the cathode and that at the anode during the electrolysis.

(iii)

What is the function of sulphuric acid in the electrolysis?

(iv)

Is it possible to deduce the formula of water from the results of the electrolysis? Explain your answer.

Ch 33 Electrolysis

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Page 11


Topic VII – Redox, Chemical cell & Electrolysis (Part C)

by Ricky Tsui

**[02CE/9(c)/8 marks] 27. A student used the set-up shown below to conduct a microscale experiment on electrolysis.

(i) (1)

The initial colour of the drop shown above was green. State the colour change of the liquid around carbon rod A after a current was passed through the circuit for some time. Explain your answer with the help of a half equation.

(2) A gas was liberated at carbon rod B. What was the gas? Explain its formation.

(ii) Some objects readily available in daily life contain carbon rods which can be used in this experiment. Suggest ONE such object.

(iii) The use of microscale experiments in studying chemistry is becoming more popular nowadays. Suggest TWO advantages of carrying out experiments in microscale.

Ch 33 Electrolysis

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Page 12


Topic VII – Redox, Chemical cell & Electrolysis (Part C)

by Ricky Tsui

Case 3 of 4 – inert electrode + aqueous electrolytes with different concentrations In general, for ions that occupy similar positions in the E.C.S., the higher the concentration of an ion, the easier the discharge of this ion. The concentration effect on anions (OH-, I-, Br- and CI-) is significant whereas the concentration effect on cation is seldom considered.

VERY DILUTE NaCl(aq) as electrolyte

B

CONCENTRATED NaCl(aq) as electrolyte

A

[01CE/9/8 marks]

Cations Anions Na+ ClH+ OH-

Electrode A:

Cations Anions Na+ ClH+ OH-

2H+ + 2e-  H2

[05/CE/9(a)(b)/5 marks]

Electrode A: 2H+ + 2e-  H2

Electrode B:

Electrode B:

4OH  2H2O + O2 + 4e

2Cl-  Cl2 + 2e-

Both Cl-(aq) and OH-(aq) will move to

Both Cl-(aq) and OH-(aq)

-

-

‘蟻多摟死象’

OH-

anode (positive electrode),

vs Clwill move to anode (positive electrode),

as OH-(aq) is at a higher position than

as concentration of Cl-(aq) is higher than

Cl-(aq) in electrochemical series,

that of OH-(aq),

OH-(aq) will be preferentially

Cl-(aq) will be preferentially discharged to

discharged to form O2(g) and H2O(l).

form Cl2(g).

After electrolysis: becomes more concentrated NaCl(aq) Theoretical volume ratio H2 : O2 = 2 : 1 (理論上)

After electrolysis: becomes NaOH(aq) Some properties of Cl2(g): - a yellowish-green gas - slightly soluble in water Cl2 + H2O

How about the case of conc. NaCl(aq)? H2 : Cl2 = ? : ?

Pt(s) CANNOT be used as electrode, because Cl2(g) will attack Pt(s)

HCl + HOCl

Ch 33 Electrolysis

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Page 13


Topic VII – Redox, Chemical cell & Electrolysis (Part C)

Graphite for BOTH electrodes

Cations Anions Na+ ClH+ OHElectrode B: 2Cl-  Cl2 + 2eElectrode A: 2H+ + 2e-  H2

Mercury(Hg) as cathode

by Ricky Tsui

Na+ + e-  Na This reaction is very unfavorable, because Na itself is a very reactive metal.

e-

Cations Anions Na+ ClH+ OH-

B

Electrode B: 2Cl-  Cl2 + 2eElectrode A: Na+ + e-  Na

Na(s) + Hg(l)  Na/Hg(l)

Sodium amalgam (汞合金)

e- e e e A

Na/Hg is an alloy much more stable than Na itself.

alternative experimental set-up

Industrial electrolysis of brine (conc. NaCl)  will be discussed!

Ch 33 Electrolysis

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Page 14


Topic VII – Redox, Chemical cell & Electrolysis (Part C)

by Ricky Tsui

Industrial application of electrolysis – electrolysis of sea water / brine

Flowing mercury cell

@ upper chamber:

At the anode (graphite):

2Cl-(aq)  Cl2(g) + 2e-

At the cathode (mercury):

Na+(aq) + e-  Na(s)

Na(s) + Hg(l)  Na/Hg(l)

At the cathode (mercury):

(sodium amalgam) Chlorine gas is collected.

Sodium amalgam reacts with water to give sodium

@ lower chamber:

hydroxide solution and hydrogen:

2Na/Hg(l) + 2H2O(l)  2NaOH(aq) + H2(g) + 2Hg(l) Recovered mercury is recirculated to the upper chamber.

Uses of products from electrolysis of brine: [07CE/MC#41] 28. Which of the following items does NOT require the use of the products obtained from electrolysis of brine? A. manufacture of soaps B. manufacture of polyethene C. manufacture of bleaches D. manufacture of hydrochloric acid

[00CE/MC#5] 29. Which of the following gases are the major products liberated in the electrolysis of concentrated sodium chloride solution using carbon electrodes?

A. B. C. D.

Cathode hydrogen chlorine hydrogen oxygen

Anode chlorine hydrogen oxygen hydrogen Ch 33 Electrolysis

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Page 15


Topic VII – Redox, Chemical cell & Electrolysis (Part C)

by Ricky Tsui

30. Consider the following electrolytic cell.

graphite electrode Y

graphite electrode X

concentrated sodium chloride solution

Which of the following statements concerning the above electrolytic cell are correct? (1) Chlorine gas and hydrogen gas can be collected at electrodes X and Y respectively. (2) The actual volume of chlorine gas collected is less than the actual volume of hydrogen gas collected. (3) The solution gradually becomes alkaline. A. B. C. D.

(1) and (2) only (1) and (3) only (2) and (3) only (1), (2) and (3)

31. The following set-up shows the electrolysis of concentrated sodium chloride solution with a mercury cathode. plastic-covered wire water

32. Consider the following electrolytic cell.

graphite concentrated sodium chloride solution

bare platinum wire mercury

Which of the following statements concerning the above electrolysis are correct? (1) Chlorine gas is the main product at the anode. (2) Sodium amalgam is formed at the cathode. (3) The sodium chloride solution is diluted after electrolysis. A. (1) and (2) only B. (1) and (3) only C. (2) and (3) only D. (1), (2) and (3)

electrode M

electrode N

solution X

Which of the following combinations can produce oxygen gas and hydrogen gas at electrodes M and N during electrolysis respectively? Electrode Electrode Solution X M N A. Platinum Platinum Dilute sulphuric acid B. Copper Copper Dilute copper(II) sulphate solution C. Graphite Graphite Concentrated sodium chloride solution D. Graphite Graphite Sugar solution Ch 33 Electrolysis

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Page 16


Topic VII – Redox, Chemical cell & Electrolysis (Part C)

by Ricky Tsui

33. Which of the following statements concerning the 34. Caesium, Cs, is in the same group of the electrolysis of concentrated sodium bromide solution Periodic Table as sodium. Which of the following using graphite electrodes are correct? combinations concerning the electrolysis of concentrated caesium chloride solution using (1) The main product at the anode is O2(g). graphite electrodes is correct? + (2) H (aq) ions are preferentially discharged at the Product at the Solution becomes cathode to form H2(g). cathode (3) The solution becomes sodium hydroxide A. Hydrogen gas Acidic solution after electrolysis. B. Oxygen gas Acidic C. Hydrogen gas Alkaline A. (1) and (2) only D. Caesium Alkaline B. (1) and (3) only C. (2) and (3) only D. (1), (2) and (3) 36. The following diagram shows the electrolysis of concentrated sodium sulphate solution using graphite electrodes.

35.

graphite electrode X

graphite electrode Y graphite electrode

graphite electrode

concentrated potassium bromide solution

Which of the following combinations concerning the above electrolytic cell is correct? Product formed at Product formed at electrode X electrode Y A. Oxygen Potassium B. Bromine Potassium C. Bromine Hydrogen D. Oxygen Hydrogen

[07CE/MC#45] *37. Using carbon as electrodes, which of the following solutions would give hydrogen upon electrolysis? (1) (2) (3)

1 M silver nitrate solution 2 M sodium hydroxide solution 3 M calcium chloride solution

A. B. C. D.

(1) only (2) only (1) and (3) only (2) and (3) only

concentrated sodium sulphate solution

Which of the following combinations concerning the above electrolysis is correct? Reaction at the Reaction at the anode cathode A. Na+(aq) + e  SO42(aq) + 4H+(aq) + Na(s) 2e  SO2 + 2H2O(l) B. 2H+(aq) + 2e SO42(aq) + 4H+(aq) +  H2(g) 2e  SO2 + 2H2O(l) + C. Na (aq) + e  4OH(aq)  O2(g) + Na(s) 2H2O(l) + 4e D. 2H+(aq) + 2e 4OH(aq)  O2(g) +  H2(g) 2H2O(l) + 4e

Ch 33 Electrolysis

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Page 17


Topic VII – Redox, Chemical cell & Electrolysis (Part C)

by Ricky Tsui

[05CE/9(a)(b)/5 marks] 38. An experiment was carried out to study the electrolysis of a concentrated sodium chloride solution using several zinc-carbon cells as a source of electricity. The following diagram shows the set-up used: gas Y

gas X

carbon electrode B

carbon electrode A

concentrated sodium chloride solution zinc-carbon cells (a)

(b)

(i)

What is gas X ?

(ii)

Give ONE use of X in industry.

(i)

What is gas Y ?

(ii)

If the electrolysis is replaced using a very dilute sodium chloride solution, another gas will be liberated at carbon electrode B. Suggest an explanation for this phenomenon.

[10CE/11(f)/4 marks] 39.

Ch 33 Electrolysis

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Page 18


Topic VII – Redox, Chemical cell & Electrolysis (Part C)

by Ricky Tsui

[01CE/9/8 marks] 40. (a) A student used the set-up shown below to prepare hydrogen and chlorine by electrolysis of a very dilute sodium chloride solution. Contrary to the student's expectation, a colourless gas X instead of chlorine was liberated at the anode.

(b)

(i)

What is X?

(ii)

Suggest a chemical test for X.

(2 marks) The experiment in (a) was then modified so that hydrogen and chlorine were produced at the cathode and anode respectively. (i)

Suggest how the experiment could be modified.

(ii)

Deduce the ratio of the theoretical volumes of hydrogen and chlorine produced.

(iii)

With the help of a chemical equation, explain why the volume of chlorine collected is significantly smaller than the theoretical volume.

Ch 33 Electrolysis

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Page 19


Topic VII – Redox, Chemical cell & Electrolysis (Part C)

by Ricky Tsui

Case 4 of 4 – metal as electrode(s) + aqueous electrolytes

e

-

e-

Some metals lose e- more readily than OH(e.g. Cu, Zn and Mg, etc.)

Cations Cu H 2+

+

Set-ups

Reaction on electrodes

Anions SO OH 24 -

Results

Electrode X (graphite):

4OH-  2H2O + O2 + 4ecolorless gas bubbles

Electrode Y (graphite):

0

Cu2+ + 2e-  Cu brown solid deposits

affects the reaction on

anode.

Anode (graphite)

OH-(aq) loses e-

Electrode X (copper):

Cu  Cu2+ + 2eCu electrode becomes thinner

Electrode Y (graphite):

1

Nature of electrode

or Anode (metal)

Metal loses e-

Cu2+ + 2e-  Cu brown solid deposits

Electrode X (graphite):

4OH-  2H2O + O2 + 4ecolorless gas bubbles

Electrode Y (copper):

2

Cu2+ + 2e-  Cu brown solid deposits

Electrode X (copper):

Cu  Cu2+ + 2eCu electrode becomes thinner

Nature of electrode does NOT affect the reaction

Electrode Y (copper):

3

Cu2+ + 2e-  Cu

on

cathode.

brown solid deposits

Ch 33 Electrolysis

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Page 20


Topic VII – Redox, Chemical cell & Electrolysis (Part C)

by Ricky Tsui

More on set-up 3

At the anode: Cu(s)  Cu2+(aq) + 2eBoth OH-(aq) ions and SO42-(aq) ions are attracted towards the anode. However, copper (from the electrode) is higher than OH-(aq) and SO42-(aq) in the E.C.S., copper loses electrons more readily.

At the cathode: Cu2+(aq) + 2e-  Cu(s) Both H+(aq) ions and Cu2+(aq) ions are attracted towards the cathode. As Cu2+(aq) is lower than H+(aq) in the E.C.S., Cu2+(aq) ions are preferentially discharged.

Overall reaction: Cu(s) (anode)  Cu(s) (cathode)

Brown deposit (copper) is formed on the cathode whereas the anode becomes thinner.

As the amount of copper dissolves at the anode is equal to the amount of copper deposits on the cathode, the concentration of Cu2+(aq) ions and the colour of the solution remain unchanged.

[03CE/MC#15] 41. In the electrolysis of a copper(II) sulphate solution, copper is used as the anode and carbon as the cathode. Which of the following statements concerning this electrolysis is correct? A. B. C. D.

2+

The concentration of Cu (aq) ions in the solution remains unchanged. The concentration of H+(aq) ions in the solution increases. O2(g) is liberated at the anode. H2(g) is liberated at the cathode.

[09CE/MC#44] 42. In the electrolysis of a copper (II) sulphate solution using copper cathode and graphite anode, which of the following would change? (1) (2) (3)

pH of the solution colour of the solution mass of the graphite anode

A. B. C. D.

(1) and (2) only (1) and (3) only (2) and (3) only (1), (2) and (3)

Ch 33 Electrolysis

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Page 21


Topic VII – Redox, Chemical cell & Electrolysis (Part C)

by Ricky Tsui

Industrial application of electrolysis – copper purification (銅的純化) Anions SO42OH-

impure copper 駁 正極 (anode) pure copper 駁 負極 (cathode) See what’s happening below for a better understanding!

Cations Cu2+ H+ Zn2+

At the anode: Zn(s)  Zn2+(aq) + 2eCu(s)  Cu2+(aq) + 2e-

At the cathode: Cu2+(aq) + 2e-  Cu(s)

Copper and metal impurities (e.g. Zn)

As Cu2+(aq) is lower than Zn2+(aq) in the

higher than copper in the E.C.S. will

E.C.S., Cu2+(aq) ions are preferentially

dissolve into the electrolyte.

discharge to form Cu(s).

Metal impuries (e.g. Ag) lower than

Note: The electrolyte used in purification of copper should be an aqueous solution containing Cu2+(aq) ions

copper in the E.C.S. and sand will sink to the bottom of the container. Overall reaction: Cu(s) (anode)  Cu(s) (cathode)

[99CE/MC#12] 43. Consider the electrolysis experiment shown below:

Which of the following statements concerning this experiment is correct?

[Cu2+(aq)] in electrolyte drops gradually. Anode: Cu and Zn dissolve to Cu2+ and Zn2+ Cathode: Only Cu2+ is reduced to Cu

A. The mass of the impure copper rod decreases. B. The blue colour of the copper(II) sulphate solution gradually fades off. C. Oxidation takes place at the pure copper rod. D. The electrolysis process can enhance the corrosion resistance of copper.

Ch 33 Electrolysis

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Page 22


Topic VII – Redox, Chemical cell & Electrolysis (Part C) 44. The following diagram shows the electrolysis of copper(II) sulphate solution using copper electrodes.

copper anode

copper cathode

*45. The following diagram shows the purification of impure sample of silver by electrolysis.

impure silver anode

copper(II) sulphate solution

Which of the following statements concerning the above electrolysis are correct? (1) The copper anode dissolves to form Cu2+(aq) ions. (2) Copper deposits at the copper cathode. (3) The blue colour of copper(II) sulphate solution becomes paler after electrolysis. A. B. C. D.

by Ricky Tsui

pure silver cathode

silver nitrate solution

The loss in mass of the anode was 60.0 g and the gain in mass of the cathode was 45.0 g. What was the percentage of purity of this sample of silver? A. 65.0% B. 70.0% C. 75.0% D. 80.0%

(1) and (2) only (1) and (3) only (2) and (3) only (1), (2) and (3)

*46. The electrolysis of copper(II) sulphate solution was carried out using copper electrodes. Which of the following graphs represents the mass of the copper cathode against time if a constant current was used? A. B. Mass of copper cathode Mass of copper cathode

Time

C.

Time

D.

Mass of copper cathode

Time

Mass of copper cathode

Time

Ch 33 Electrolysis

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Page 23


Topic VII – Redox, Chemical cell & Electrolysis (Part C)

by Ricky Tsui

[10CE/MC#34] 47.

*[06CE/MC#40] 48. The set-up below shows a chemical cell connected to a voltmeter:

In the set-up, electrons flow in such a direction that the concentration of Cu2+(aq) ions in each half cell becomes the same eventually. Which of the following statements concerning the set-up is correct? A. B. C. D.

The salt bridge allows electrons to flow from one half cell to the other. Oxidation occur at Y. Electrons flow from Y to X in the external circuit. The mass of X will decrease but the mass of Y will increase.

Ch 33 Electrolysis

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Page 24


Topic VII – Redox, Chemical cell & Electrolysis (Part C)

by Ricky Tsui

[07CE/11/10 marks] 49. In a chemical plant, extracting copper from its ores involves roasting copper(I) sulphide with air inside a high temperature furnance. Copper(I) sulphide reacts with oxygen in air according to the following equation:

Cu2S + O2

2Cu + SO2

The copper so extracted contains impurities including metals such as silver, iron, zinc and gold. The impure copper is then purified by electrolysis as illustrated in the diagram below:

(a)

With reference to the reaction between copper(I) sulphide and oxygen, identify the species undergoing oxidation and the species undergoing reduction. Explain your answers in terms of changes in oxidation numbers. (2 marks)

(b)

Explain briefly how impure copper can be purified by electrolysis as illustrated in the diagram above. (2 marks)

(c)

Insoluble impurities deposit under the impure copper anode as ‘anode sludge’. According to the information given, suggest what substances the anode sludge would contain. Explain your answer. (2 marks)

(d)

‘The concentration of copper(II) ions in copper(II) sulphate solution remains UNCHANGED in the above electrolysis.’ Is this statement correct? Explain your answer. (2 marks)

(e)

State TWO advantages of building a factory in which contact process is carried out near the chemical plant mentioned above. (2 marks)

Ch 33 Electrolysis

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Page 25


Topic VII – Redox, Chemical cell & Electrolysis (Part C)

by Ricky Tsui

Industrial application of electrolysis – electroplating (電鍍) 

Electroplating is the process of coating a thin layer of metal on an object by electrolysis.

Electroplating is used to prevent corrosion and improve appearance. Set-up for electroplating (e.g. nickel

plating): Ni(s) 駁 正極 (anode) Object to be plated 駁 負極 (cathode) See what’s happening below for a better understanding!

At the anode:

Ni(s)  Ni2+(aq) + 2e-

At the cathode:

Overall reaction:

Ni2+(aq) + 2e-  Ni(s) 

Ni(s) (anode)  Ni(s) (anode)

The effluent (排放物) from electroplating industries contains harmful substances: Harmful substances

acids and alkalis

Harmful effect kill aquatic lives

Treatment methods Neutralize acidic effluent with calcium hydroxide. Neutralize alkaline effluent with sulphuric acid. Remove heavy metal ions by addition

heavy metal ions

highly

(e.g. Ni2+)

toxic

of sodium hydroxide solution followed by filtration. e.g. Ni2+(aq) + 2OH-(aq)  Ni(OH)2(s)

cyanide ions

highly

(CN-)

toxic

Treat cyanides chemically to make them harmless.

Ch 33 Electrolysis

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Page 26


Topic VII – Redox, Chemical cell & Electrolysis (Part C)

by Ricky Tsui

50. The electroplating of copper on a metal fork is carried out by using copper(II) sulphate solution. Which of the following substances should be used as the negative electrode? A. Graphite B. Platinum C. Copper D. The metal fork

51. Which of the following are applications of electrolysis? (1) Electroplating silver metal on a plastic spoon (2) Purifying lead by electrolysis (3) Anodizing aluminium A. (1) and (2) only B. (1) and (3) only C. (2) and (3) only D. (1), (2) and (3)

52. Heavy metal ions are common pollutants produced by electroplating factories. Which of the following actions is NOT a proper way to remove these heavy metal ions? A. Precipitating the metal ions as hydroxides by adding sodium hydroxide solution. B. Electrolysing very dilute solutions and change metal ions to metal. C. Precipitating the metal ions as carbonates by adding sodium carbonate solution. D. Displacing metal ions by adding a more reactive metal to the solution.

[09CE/MC#38] 53. In an experiment of electroplating nickel on a copper object, which of the following combinations is correct? Anode Cathode Electrolyte A. copper object nickel CuSO4(aq) B. copper object nickel NiSO4(aq) C. nickel copper object CuSO4(aq) D. nickel copper object NiSO4(aq)

[08CE/MC#48] 54. Upon electrolysis, which of the following solutions would give hydrogen at carbon cathode and oxygen at platinum anode? (1) very dilute sodium chloride solution (2) dilute copper(II) sulphate solution (3) concentrated potassium sulphate solution A. (1) only B. (2) only C. (1) and (3) only D. (2) and (3) only [05CE/MC#31] 55. In which of the following combinations will oxygen be produced as the major product at the anode during electrolysis ?

Ch 33 Electrolysis

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Page 27


Topic VII – Redox, Chemical cell & Electrolysis (Part C)

by Ricky Tsui

[12DSE/paper1/5/8 marks] 56. In order to prepare 50 dm3 of 0.1 M CuSO4(aq), an inexperienced electroplating worker added the required exact amount of CuSO4.5H2O(s) to water in a plastic container. He then stirred the mixture with an iron rod until the CuSO4.5H2O(s) dissolved completely. Finally, he sent a sample of the solution to the Quality Control Laboratory for analysis, but found that the concentration of CuSO4(aq) was lower than 0.1M.

(a) With the aid of a chemical equation, explain why the concentration of the CuSO4(aq) prepared was lowered than 0.1M. (2 marks)

(b) The worker used the prepared CuSO4(aq) to coat a layer of copper on a metallic object by electrolysis. He used an unreasonably high voltage, and found that some bubbles were formed on the object and the copper layer easily flaked off. (3 marks) (i) Explain why copper can be coated on the metallic object by electrolysis.

(ii)

Suggest what the bubbles were, and explain why the copper layer easily flaked off.

(c) Draw a labeled diagram of the experimental set-up used in a laboratory for coating a layer of copper on a metallic object by electrolysis. (3 marks)

Ch 33 Electrolysis

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Page 28


Topic VII – Redox, Chemical cell & Electrolysis (Part C) [02CE/4/9 marks] 4.

by Ricky Tsui

Summary: THREE factors affecting the products from electrolysis

Using the electrolysis of copper(II) chloride solution as an example, briefly discuss the factors affecting the discharge of ions in electrolysis. Case 2

Case 3

Case 4

Ch 33 Electrolysis

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Page 29


Topic VII – Redox, Chemical cell & Electrolysis (Part C)

by Ricky Tsui

Challenging question 1 of 2 57. An electric circuit is set up below. Electrode B is made of copper while electrodes A, C and D are made of graphite.

a) State the functions of ammeter and rheostat.

b) Identify the polarity of each electrode.

c) State whether each electrode is a cathode or an anode.

d) Write an ionic half equation for the reaction occurs at each electrode.

e) Describe the observable change(s) at each electrode.

f) What are the changes, if any, in the solutions in beakers X and Y after the experiment?

Ch 33 Electrolysis

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Page 30


Topic VII – Redox, Chemical cell & Electrolysis (Part C)

by Ricky Tsui

Challenging question 2 of 2 58. An electric circuit is set up below. All the electrodes are made of graphite.

When the circuit is closed, a current flows in the external circuit. After some time, a reddish brown fume is observed around electrode X. a) Which pole, A or B, is the positive pole? Explain briefly.

b) What is observed at electrode W? Write the ionic half equation for the reaction involved.

c) What are the gases collected in the test tubes above electrodes Y and Z respectively? Write the ionic half equations for the reactions involved.

d) What is the ratio of the theoretical volumes of the gases collected above electrodes Y and Z?

e) What is the colour change of the universal indicator around electrodes Y and Z during the electrolysis?

f) Using the concept of preferential discharge of ions, explain the formation of the gas at electrode Y.

g) Suggest a test for the gas collected at electrode Z.

Ch 33 Electrolysis

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Page 31


Topic VII – Redox, Chemical cell & Electrolysis (Part C)

by Ricky Tsui

CHEMICAL CELL X ELECTROLYSIS Challenging question 1 of 2 59. Consider the following experimental set-up.

(a)

(b)

(i)

Write the half equations for the reactions at electrodes P and Q.

(2 marks)

(ii)

Explain why electrodes P and Q should be porous.

(1 mark)

Write down the substance liberated and explain why the substance is liberated (6 marks) (i)

at electrode R; and

(ii)

at electrode S.

Ch 33 Electrolysis

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Page 32


Topic VII – Redox, Chemical cell & Electrolysis (Part C)

by Ricky Tsui

CHEMICAL CELL X ELECTROLYSIS Challenging question 2 of 2

[07CE/11/10 marks] 60.

In the above diagram, P and Q are two different metals. When the circuit is closed, a current flows in the external circuit. After some time, 0.36 g of copper is deposited on the carbon electrode R. (i)

(1) What is the direction of electron flow in the external circuit? Explain your answer.

(2) Calculate the quantity of electricity that has passed through the circuit. (Out of Syllabus) (Hint: quantity of electricity = no of mole of electrons x 96500)

(ii)

After the circuit has been closed for some time, what would be observed (1)

at the carbon electrode S?

(2)

in the copper(II) sulphate solution?

(iii)

What is the function of set-up X in this experiment?

(iv)

Which of the metals, P or Q, occupies a higher position in the electrochemical series? Explain your answer.

(Relative atomic mass : Cu = 63.5; 1 faraday = 96500 coulombs) Ch 33 Electrolysis

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Page 33


Topic VII – Redox, Chemical cell & Electrolysis (Part C)

by Ricky Tsui

Answer: 1.

2Cl-(l)  Cl2(g) + 2eNa+(l) + e-  Na(l) 2NaCl(l)  2Na(l) + Cl2(g) Greenish gas (chlorine) / Silvery liquid (sodium)

2.

3. C 12.

13.

4. D

5. D

6. A

7. D

8. D

9. C

In very dilute NaCl(aq), it contains mobile ions Na+(aq), Cl-(aq), H+(aq) and OH-(aq). Both Na+(aq) and H+(aq) will move to cathode, as H+(aq) is at a lower position than Na+(aq) in the electrochemical series, H+(aq) will be preferentially discharged to form H2(g).

18. D

19. A

20. B

21. D

22. C

23. C

24. C

25. C

Ch 33 Electrolysis

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Page 34


Topic VII – Redox, Chemical cell & Electrolysis (Part C)

by Ricky Tsui

26.

27.

30. D

31. D

32. A

33. C

34. C

35. C

36. D

Ch 33 Electrolysis

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Page 35


Topic VII – Redox, Chemical cell & Electrolysis (Part C)

by Ricky Tsui

38.

39.

40.

44. A

45. C

46. A Ch 33 Electrolysis

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Page 36


Topic VII – Redox, Chemical cell & Electrolysis (Part C)

by Ricky Tsui

49.

50. D

51. D

52. D

56.

Ch 33 Electrolysis

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Page 37


Topic VII – Redox, Chemical cell & Electrolysis (Part C)

by Ricky Tsui

56.

57. a) Ammeter: measure the electric current Rheostat: adjust the current b) A and C: negative B and D: positive c) A and C: cathode B and D: anode d) At A: Cu2+(aq) + 2e-  Cu(s) At B: Cu(s)  Cu

2+

(aq)

-

+ 2e

e) At A: Brown deposit is formed on the electrode surface. At B: The electrode gradually dissolves/becomes thinner. At C: Colourless gas bubbles are evolved. At D: Greenish gas is evolved. f) Beaker X: The solution remains unchanged. Beaker Y: The solution becomes sodium hydroxide solution gradually.

At C: 2H+(aq) + 2e-  H2(g) At D: 2Cl-(aq)  Cl2(g) + 2e58. a) B The reddish brown fume formed at electrode X

d) Ratio of theoretical volumes of H2 to O2 =2 : 1

should be bromine vapour. Hence electrode X is the anode and is connected to the positive pole of the battery. b) Silvery deposit is formed. / Silvery liquid is formed. Pb2+(l) + 2e-  Pb(s /l)

c) At Y: from colourless to blue At Z: from colourless to red d) Both H+(aq) ions and Na+(aq) ions are attracted towards electrode Y. As H+(aq) is lower than Na+(aq) in the E.C.S., H+(aq) ions are preferentially discharged.

c) At Y: hydrogen; 2H+(aq) + 2e-(aq)  H2(g)

e) The gas relights a glowing splint.

At Z: oxygen; 4OH-(aq)  O2(g) + 2H2O(l) + 4eCh 33 Electrolysis

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Page 38


Topic VII – Redox, Chemical cell & Electrolysis (Part C) 59. (a)

(i)

(ii) (b)

(i)

(ii)

At electrode P: H2(g) + 2OH-(aq)  2H2O(l) + 2eAt electrode Q: O2(g) + 2H2O(l) + 4e-  4OH-(aq) The porous electrode allow the flow of H2 and OH- in and out the compartments. Chlorine gas is evolved. Since the concentration of Cl- ions is much higher than that of OH- ions, Cl- ions will preferentially discharge to form Cl2(g) 2Cl-  Cl2 + 2eCopper metal is formed. Since copper is in the position lower than hydrogen in the electrochemical series, Cu2+ ions will preferentially discharge to form Cu(s) Cu2+ + 2e-  Cu

by Ricky Tsui

(1) (1) (1) (1) (1) (1) (1) (1) (1)

60.

Ch 33 Electrolysis

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Page 39


Topic 7 redox reactions part c electrolysis  
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