Chemistry 12 (HL)
Unit 4 / IB Topics 9.5 and 19.2
Oxidation and Reduction Reactions 6
Electrolytic Cells – Introduction
Answers – Please do not print.
1.
Given this reaction in an electrolytic cell:
2+
+
2 Ag(s) + Cu (aq) à 2 Ag + Cu(s)
a)
Give the half reaction that occurs at the cathode.
2+
reduction @ cathode:
b)
–
Cu (aq) + 2e à Cu(s)
Give the half reaction that occurs at the anode.
+
oxidation @ anode:
Ag(s) à Ag (aq) + e
c)
d)
State the charge on the anode. positive
Which electrode gains mass? cathode
e)
What is the minimum voltage required?
f)
In which direction do the electrons travel?
from Ag(s) (anode) to Cu(s) (cathode)
g)
Draw the cell.
e
–
–
0.46 V
–
e
↘
↗
negative
positive
anode = Ag(s) ––––––––––––
––––––––– cathode = Cu(s)
electrolyte
oxidation half rx:
+
Ag(s) à Ag (aq) + e
Cu
–
↘
Ag+
NOTE: The
cathode in this
cell could be
copper, or an
inert electrode. reduction half rx:
2+
2+
–
Cu (aq) + 2e à Cu(s)
-
NO3
2+
NOTE: This is a somewhat unusual electrolytic cell. At the cathode, Cu ions will initially
+
be reduced to form solid copper. As the cell runs and the concentration of Ag ions
+
increases, Ag ions may also start to be reduced, forming silver metal on the cathode. 2.
An electrolytic cell is set up to nickel-plate a key, using a solution of acidified copper
nickel (II) sulfate.
a)
Identify the materials that you would use for the electrodes.
cathode = key
b)
anode = nickel metal strip à Ni(s)
Write the equations that occur at each electrode.
2+
–
cathode:
Ni (aq) + 2e à Ni(s)
anode:
Ni(s) à Ni (aq) + 2e
2+
–
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Chemistry 12 (HL)
c)
Unit 4 / IB Topics 9.5 and 19.2
State and explain the effect of each of the following on the amount of copper
metal that forms:
i) decreasing the current
à less copper metal forms because the charge transferred decreases
when the current decreases
ii) increasing the time
à more copper metal forms because the charge transferred increases
when the time increses
d)
Calculate the mass of nickel that forms if the cell operates at 25.0 mA for 3
hours.
q = I x t = 0.0250 A x (3 x 3600) s = 270 C
–
4
moles of e = q / F = 270 C / (9.65 x 10 ) = 0.00280 mol
–
–
moles of Ni formed = 0.00280 mol e x ( 1 mol Ni / 2 mol e ) = 0.00140 mol Ni
mass of Ni = 0.00140 mol x 58.7 g/mol = 0.082 g
3.
Which electrolytic cell would produce a higher mass of solid zinc metal at the
cathode, given the same conditions of current and time. Explain.
Cell A:
electroplating zinc
Cell B:
electroplating silver
Note the
change in
wording of
the question!
This cell! The charge on silver ions (+1) is
less than the charge on zinc ions (+2). The lower
the ion charge, the higher the amount of metal
formed at the cathode.
The silver cell has a 1:1 electron to metal ratio, but
the zinc cell has a 2:1 electron to metal ratio.
4.
Aqueous solutions of AgNO3, Cu(NO3)2 and Cr(NO3)3 are electrolyzed using
the same quantity of electricity. How do the number of moles of metal formed
compare?
A.
Ag = Cu = Cr
B.
Ag > Cu > Cr
C.
Ag < Cu < Cr
D.
Cu > Ag > Cr
5.
Which processes occur during the electrolysis of molten sodium chloride?
I.
Sodium and chloride ions move through the electrolyte.
II.
Electrons move through the external circuit.
III.
Oxidation takes place at the positive electrode (anode).
A.
I and II only
B.
I and III only
C.
II and III only
D.
I, II and III
6.
What happens at the positive electrode in a voltaic cell and in an electrolytic cell?
Voltaic cell
Electrolytic cell
voltaic cell: + = cathode = reduction
A.
Oxidation
Reduction
electrolytic cell: + = anode = oxidation
B.
Reduction
Oxidation
C.
Oxidation
Oxidation
D.
Reduction
Reduction
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Chemistry 12 (HL)
7.
Unit 4 / IB Topics 9.5 and 19.2
What happens when molten sodium chloride is electrolysed in an electrolytic cell?
A.
Chlorine is produced at the positive electrode.
Electrolysis of molten salts:
cathode = – = reduction of metal cations to metal solid
anode = + = oxidation of anions to a non-metal
B.
Sodium ions lose electrons at the negative electrode.
x Cations GAIN electrons.
C.
Electrons flow through the liquid from the negative electrode to the
positive electrode.
x Electrons only flow through the EXTERNAL CIRCUIT.
D.
Oxidation occurs at the negative electrode and reduction at the positive
electrode.
x This is true for voltaic cells. The opposite is true for electrolytic cells.
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