WAJA F4 Chemistry 2009 Chapter 6: Electrochemistry
CHAPTER 6: ELECTROCHEMISTRY
ANSWER
Activity 1
(a) electricity, (b) molten, (c) aqueous, (d) solution, (e) chemical, (f) changes, (g) electricity, (h) molten (i) aqueous (j) moving ions (k) molecules, (l) moving ions, (m) moving ions.
Electrolyte molten aluminium oxide
Non-electrolyte
Solid lead(II) chloride lead(II) nitrate solution sodium chloride solution molten lead(II) chloride solid sodium chloride magnesium glucose solution dilute ethanoic acid aqueous ammonia sodium hydroxide solution glacial ethanoic acid molten naphthalene ethanol silver tetrachloromethane
Activity 2
(a) molten, (b) aqueous, (c) elements, (d) electricity, (e) positive, (f) negative, (g) battery, (h) switch, (i) carbon, (j) carbon, (k) molten lead(II) bromide
Activity 3
1. Solid sodium chloride contains sodium ions and chloride ions which are in fixed position and not freely moving.
2. In solid state, sodium ions and chloride ions are strongly attracted by electrostatic forces in a lattice.
3. Aqueous sodium chloride contains freely moving ions to conduct electricity.
4. During electrolysis cations are attracted to the cathode and anions are attracted to the anode.
5. Electric circuit is complete due to the flow of electrons along the connecting wires and movement of ions in the solution.
6. If the electrodes are placed further apart, the ammeter reading will decrease because there will be an increase in internal resistance.
Activity 4
1
Electrolyte (Molten)
2.
Name
Cation
Sodium ion Sodium chloride
Lead(II) oxide Lead(II) ion
Potassium bromide Potassium ion
Formula
Na +
Pb 2+
K +
Name
Anion
Chloride ion
Oxide ion
Bromide ion
Formula
Cl
–
O 2 –
Br
–
Electrolyte
(molten)
Substance discharged at the electrodes and the half equation
Anode Cathode
(i) Aluminium oxide Oxygen gas
Half equation:2O 2 – 
O
2
+ 4e
Aluminium
Half equation: Al 3+ + 3e

Al
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WAJA F4 Chemistry 2009 Chapter 6: Electrochemistry
Electrolyte
(molten)
Substance discharged at the electrodes and the half equation
Anode Cathode
(ii) Potassium iodide Iodine
Half equation : 2I
– 
I
2
+ 2e
Potassium
Half equation: K + + e

K
(iii) Sodium chloride
(iv) Zinc bromide
Chlorine gas
Half equation: 2Cl
–
Bromine gas
Half equation: 2Br
–

Cl
2

Br
2
+ 2e
+ 2e
Sodium
Half equation: Na + + e

Na
Zinc
Half equation: Zn 2+ + 2e

Zn
Activity 5
1. Electrolysis of molten lead(II) bromide
(a) Lead(II) ions and bromide ions (or Pb 2+ and Br
–
Lead(II) ions or (Pb 2+ ions), (d) 2Br
– 
Br
2
ions), (b) Bromide ions (or Br
+ 2e, (e) Pb 2+
–
ions), (c)
+ 2e

Pb, (f) bromine gas, (g) lead metal.
2. Electrolysis of molten zinc chloride
Note: The flow chart is similar to that shown in question 1.
Name of substance : Molten ZnCl
2
, (a) Zn
(d) 2Cl
– 
Cl
2
+ 2e, (e) Zn
2+ and Cl
–
ions, (b) Cl
–
ions, (c) Zn
2+ + 2e

Zn , (f) chlorine gas, (g) zinc metal.
2+ ions,
Activity 6
1 (a) Position of ions in the electrochemical series
(b) Concentration of ions in the electrolysis
(c) Types of electrodes used in the electrolysis
2 hydrogen H + ; hydroxide, OH
—
3. Cation: K + , Na + , Ca 2+ , Mg 2+ , Al 3+ , Zn 2+ , Fe 2+ , Sn 2+ , Pb 2+ , H + , Cu 2+ , Ag +
Anion: F , SO
4
2 , NO 3 , Cl , Br , I , OH -
4. (a) lower, selectively (b) high, selectively discharged (c) solution, corrode, dissolved
Activity 7
(a) Cations: copper(II) ion, Cu 2+
Anions: sulphate ion, SO
4
2 –
and hydrogen ion, H +
and hydroxide ion, OH
–
(b) Anode: electrode K
(c) (i) hydroxide ion, OH
–
Cathode: electrode J
(ii) hydroxide ion is lower than sulphate ion in the electrochemical series.
(iii) Gas bubbles
(iv) Place a glowing wooden splinter into the test tube containing the gas. If splinter rekindles, then the gas is confirmed to be oxygen.
(v) 4OH  H
2
O + O
2
+ 4e
(d) (i) copper(II) ion, Cu 2+
(ii) Brown solid deposited on the electrode
(iii) The position of copper(II) ion is lower than hydrogen ion in the electrochemical series (or concentration of copper(II) ion is higher than the concentration of hydrogen ion)
(iv)Cu 2+ + 2e  Cu
(e) Blue colour of copper(II) sulphate solution become paler.
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WAJA F4 Chemistry 2009 Chapter 6: Electrochemistry
Activity 8
1
2
3 Na
+ , Cl
–
, H + , OH
–
4 Na
+ , H +
5 2H
+ + 2e

H
2
6 Gas bubbles are formed
7 Cl
–
, OH
–
ions
8 2Cl – 
Cl
2
+ 2e
9 Gas bubbles are formed
Activity 9
Na
Na
2H
+
+
+
, Cl
, H
, OH
–
Gas bubbles are formed
Cl
–
, OH
4OH
–
+
–
, H +
+ 2e

H
–
ions

2H
2
2
O + O
2
+ 4e
Gas bubbles are formed
Cu
Cu
2+
2+
Cu +
Cu
, SO
, H +
4
2 –
, H +
+ 2e

Cu
, OH
Brown solid deposited on the cathode
SO
4
2 –
, OH
–

Cu 2+ + 2e
–
Copper plate becomes thinner
1. (a) extraction of metals, (b) purification of metals, (c) electroplating of metals, (d) carbon, (e) cryolite, (f) cathode, (g) anode, (h) electroplating metal, (i) object/substance, (j) electroplated, (k) attractive, (l) resistant.
2. Below are shown the three uses of electrolysis in industries. Fill in the blanks.
1. Substance used as cathode and anode
2. Electrolyte used
3. Half equation representing the process.
Extraction of aluminium from bauxites
Cathode: Carbon
Anode: carbon
Aluminium oxide and cryolite
Cathode:
Al 3+ + 3e

Al
Anode:
2O 2 – 
O
2
+ 4e
Purification of copper from impure mined copper
Cathode: Pure copper
Anode: impure copper
Copper(II) sulphate solution
Cathode:
Cu 2+ + 2e

Cu
Anode:
Cu

Cu 2+ + 2e
Electroplating of iron spoon with silver
Cathode: iron spoon
Anode: silver
Silver nitrate solution
Cathode:
Ag + + e

Ag
Anode:
Ag

Ag + + e
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WAJA F4 Chemistry 2009 Chapter 6: Electrochemistry
Activity 10
1. different, electrolyte, wires
2. chemical, electrical
3.
Electrochemical series of metals
K
Na
Ca
Mg
Al
Zn
Fe
Sn
Pb
Cation formed and number of electrons released during the process
K + + e
Na + + e
Ca 2+ + 2e
Mg 2+ + 2e
Al 3+ + 3e
Zn 2+ + 2e
Fe 2+ + 2e
Sn 2+ + 2e
Pb 2+ + 2e
H + + e H
Cu Cu 2+ + 2e
Ag Ag + + e
[ One way (mnemonic) to remember the ECS: Kalau Nak Cari Makan, Ali, Zainal, Fee-fee
Sentiasa Pergi Hutan Cari Argentum]
4. (a) An example of a simple voltaic cell is a magnesium ribbon and a copper plate immersed in dilute sodium chloride solution.
(b) Magnesium is placed higher than copper in the electrochemical series.
(c) Hence magnesium atom releases electrons more easily than a copper atom and the magnesium strip act as the negative terminal of the cell.
(d) At the negative terminal, each magnesium atom releases two electrons and the Mg 2+ formed moved into the solution.
Mg(s)  Mg 2+ (aq) + 2e
(e) The electrons then flows from the magnesium ribbon to the copper plate through the wire and this results in the flow of electrical current.
(e) At the positive terminal which is the copper plate, the electrons are accepted by the H + ions in sodium chloride solution.
2H + + 2e  H
2
(g)
(g) The overall equation for the reaction is given as follows.
Mg(s) + 2H + (aq)  Mg 2+ (aq) + H
2
(g)
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WAJA F4 Chemistry 2009 Chapter 6: Electrochemistry
Activity 11
1.(a)
(b) (i) Zinc
(ii) Tendency of zinc to donate electrons is higher than copper. (Zinc is more electropositive than copper)
(iii) Zn  Zn 2+ + 2e
(c) Cu 2+ +2e  Cu
(d) (i) Zinc strip becomes thinner
(ii) Brown solids deposited (copper strip becomes thicker)
(e) 1. To allow ions to flow into the beakers and maintain electrical neutrality of the solutions
2. To complete the circuit.
2. Please refer to page 106 of the Form Four Chemistry Text Book for answer to question 2
Activity 12
DIFFERENCES
Electrolytic cell Aspect Chemical cell
It requires a source of electric current Source of electric current
The electrical energy causes chemical reactions to occur at the electrodes.
Electrical energy  chemical energy
Conversion of energy
It does not require a source of electric current
The chemical reactions that occur at the electrodes produce an electric current.
Chemical energy  electrical energy
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WAJA F4 Chemistry 2009 Chapter 6: Electrochemistry
Electrolytic cell
The electrodes may be of the same material such as carbon
Electrons flow from the positive electrode (anode) to the negative electrode (cathode).
DIFFERENCES
Aspect
Type of electrodes
Direction of flow of electrons
Chemical cell
The electrodes must be of two different metals
Electrons flow from the more electropositive metal (negative terminal) to the less electropositive metal (positive terminal).
Ions donate electrons at the positive electrode.
(Oxidation)
Ions receive electrons at the negative electrode.
(Reduction)
Type of reaction at positive terminal
Type of reaction at negative terminal
Ions receive electrons at the positive electrode.
(Reduction)
Atoms donate electrons at the negative electrode.
(Oxidation)
Activity 13
(a) Experiment I = 1.6 V Experiment II = 0.8 V Experiment III = 3.2V
(b)
Experiment Negative terminal
Positive terminal
Voltmeter reading (V)
I
II
III
P
S
S
Q
Q
T
1.6
0.8
3.2
Note: Must have at least three headings consisting (i) Experiment, (ii) Negative terminal or positive and (iii) voltmeter reading, V
(c) Apparatus: Voltmeter, 100 cm 3 beakers, copper wires with crocodile clips
Materials: Sodium nitrate solution,1.0 mol dm -3 , sandpaper, metal P, metal Q, metal R, metal
S, metal T
(d) 1. Manipulated variable: Pair of metals (or distance between two metals in the electrochemical series)
2. Responding variable: Voltmeter reading
3. Controlled variable: Concentration of sodium nitrate solution (or 1.0 mol dm -3 sodium nitrate solution)
(e) The further the distance between two metals in the electrochemical series, the higher the voltmeter reading.
(f) Metal P is more electropositive than Q. (or metal P is placed higher than metal Q in the electrochemical series.)
(g) P  P 2+ + 2e
(h) P, S, Q, T
(i) 2.4 V
(j) No, because metal X is less electropositive than S.
(k) Cu + 2TNO
3
 Cu(NO
3
)
2
+ 2T
(l) Please refer to page 109 of the Form Four Chemistry Text Book.
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