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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
lead(II) nitrate solution
sodium chloride solution
molten lead(II) chloride
dilute ethanoic acid
sodium hydroxide solution
aqueous ammonia
Non-electrolyte
Solid lead(II) chloride
solid sodium chloride
magnesium
glucose 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)
Sodium chloride
Lead(II) oxide
Potassium bromide
Cation
Name
Sodium ion
Lead(II) ion
Potassium ion
Anion
Formula
Na+
Pb2+
K+
Name
Chloride ion
Oxide ion
Bromide ion
Formula
Cl–
O2–
Br–
2.
Electrolyte
(molten)
(i) Aluminium oxide
Substance discharged at the electrodes
and the half equation
Anode
Cathode
Oxygen gas
Aluminium
Half equation:2O2–  O2 + 4e
Half equation: Al3+ + 3e  Al
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WAJA F4 Chemistry 2009
Electrolyte
(molten)
(ii) Potassium iodide
Chapter 6: Electrochemistry
Substance discharged at the electrodes
and the half equation
Anode
Cathode
Iodine
Potassium
Half equation : 2I–  I2 + 2e
Half equation: K+ + e  K
(iii) Sodium chloride
Chlorine gas
Half equation: 2Cl–  Cl2 + 2e
Sodium
Half equation: Na+ + e  Na
(iv) Zinc bromide
Bromine gas
Half equation: 2Br–  Br2 + 2e
Zinc
Half equation: Zn2+ + 2e  Zn
Activity 5
1. Electrolysis of molten lead(II) bromide
(a) Lead(II) ions and bromide ions (or Pb2+ and Br– ions), (b) Bromide ions (or Br– ions), (c)
Lead(II) ions or (Pb2+ ions), (d) 2Br–  Br2 + 2e, (e) Pb2+ + 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 ZnCl2, (a) Zn2+ and Cl– ions, (b) Cl– ions, (c) Zn2+ ions,
(d) 2Cl–  Cl2 + 2e, (e) Zn2+ + 2e  Zn, (f) chlorine gas, (g) zinc metal.
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+, Ca2+, Mg2+, Al3+, Zn2+, Fe2+, Sn2+, Pb2+, H+, Cu2+, Ag+
Anion: F- , SO42- , NO3- , Cl- , Br - , I- , OH4. (a) lower, selectively (b) high, selectively discharged (c) solution, corrode, dissolved
Activity 7
(a) Cations: copper(II) ion, Cu2+ and hydrogen ion, H+
Anions: sulphate ion, SO42– and hydroxide ion, OH–
(b) Anode: electrode K
Cathode: electrode J
–
(c) (i) hydroxide ion, OH
(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-  H2O + O2 + 4e
(d) (i) copper(II) ion, Cu2+
(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)Cu2+ + 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–
Na+, Cl–, H+ , OH–
Cu2+, SO42–, H+, OH–
4
Na+, H+
Na+, H+
Cu2+, H+
5
2H+ + 2e  H2
2H+ + 2e  H2
Cu+ + 2e  Cu
6
Gas bubbles are formed
Gas bubbles are formed
Brown solid deposited on the
cathode
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Cl–, OH– ions
Cl–, OH– ions
SO42–, OH–
8
2Cl–  Cl2 + 2e
4OH–  2H2O + O2 + 4e
Cu  Cu2+ + 2e
9
Gas bubbles are formed
Gas bubbles are formed
Copper plate becomes thinner
Activity 9
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
Purification of copper
from impure mined
copper
Cathode: Pure copper
Anode: impure copper
Aluminium oxide and
cryolite
Cathode:
Al3+ + 3e  Al
Anode:
2O2–  O2 + 4e
Copper(II) sulphate
solution
Cathode:
Cu2+ + 2e  Cu
Anode:
Cu  Cu2+ + 2e
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Electroplating of
iron spoon with
silver
Cathode: iron
spoon
Anode: silver
Silver nitrate
solution
Cathode:
Ag+ + e  Ag
Anode:
Ag  Ag+ + e
WAJA F4 Chemistry 2009
Chapter 6: Electrochemistry
Activity 10
1. different, electrolyte, wires
2. chemical, electrical
3.
Cation formed and number of electrons
released during the process
Electrochemical series of metals
K
K+ + e
Na
Na+ + e
Ca
Ca2+ + 2e
Mg
Al
Zn
Fe
Sn
Tendency
of metal
atoms to
donate
electrons
to form ions
increases
Mg2+ + 2e
Al3+ + 3e
Zn2+ + 2e
Fe2+ + 2e
Sn2+ + 2e
Pb
Pb2+ + 2e
H
H+ + e
Cu
Cu2+ + 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 Mg2+
formed moved into the solution.
Mg(s)  Mg2+ (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  H2 (g)
(g) The overall equation for the reaction is given as follows.
Mg(s) + 2H+(aq)  Mg2+(aq) + H2(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  Zn2+ + 2e
(c) Cu2+ +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
Electrolytic cell
It requires a source of electric current
The electrical energy causes
chemical reactions to occur at the
electrodes.
Electrical energy  chemical energy
DIFFERENCES
Aspect
Source of
electric
current
Conversion
of energy
5
Chemical cell
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
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).
Ions donate electrons at the positive
electrode.
(Oxidation)
Ions receive electrons at the negative
electrode.
(Reduction)
Activity 13
(a) Experiment I
(b)
= 1.6 V
DIFFERENCES
Aspect
Chemical cell
Type of
The electrodes must be of two different
electrodes metals
Electrons flow from the more
Direction of
electropositive metal (negative terminal)
flow of
to the less electropositive metal (positive
electrons
terminal).
Type of
Ions receive electrons at the positive
reaction at electrode.
positive
(Reduction)
terminal
Type of
Atoms donate electrons at the negative
reaction at electrode.
negative
(Oxidation)
terminal
Experiment II = 0.8 V
Experiment III = 3.2V
Experiment
Negative
Positive
Voltmeter
terminal
terminal
reading (V)
I
P
Q
1.6
II
S
Q
0.8
III
S
T
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 cm3 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  P2+ + 2e
(h) P, S, Q, T
(i) 2.4 V
(j) No, because metal X is less electropositive than S.
(k) Cu + 2TNO3  Cu(NO3)2 + 2T
(l) Please refer to page 109 of the Form Four Chemistry Text Book.
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