6
Electrochemistry
Q uestions
•
•
•
•
renewable if
produced using
solar or wind
energy
•
lower flammability
than petrol
•
virtually emissionfree
large fuel tank
required
•
as yet there are very
few 'filling stations',
where a car could
be topped up
with hydrogen
zero emissions of
CO2
•
non-renewable if
generated using
nuclear energy
or energy from
fossil fuels
non-toxic
What is currently the main source of hydrogen for
hydrogen-powered vehicles?
9
For a future carbon-neutral approach, what is the
preferred method of generating hydrogen? What
sources of energy could be used in this approach?
10 In one type of hydrogen-oxygen fuel cell the
electrode reactions are:
•
engine redesign
needed or a fuel
cell system
•
currently expensive
Table 6 .7: Advantages and disadvantages of hydrogen as a
-
8
fuel for motor vehicles.
141
HiCg) ... _ W(aq) + _ e·
0 2(g) + 4W(aq) + 4e- ... 2Hp(1)
_
Balance the first of these half-equations so that
together they add up to the overall equation:
2Hi(g) + Oz<g) ... 2Hp(l)
>
CAMBRIDGE IGCSE™ CHEMISTRY: COURSEBOOK
~i•iiil4id
Metals can act as electricaJ conductors and non-metallic materials (e.g. plastics and ceramics) as
non-conducting insulators.
Electrolysis is the decomposition of an ionic compound by the passage of an electric current when molten or
dissolved is aqueous solution.
A simple electrolytic cell has different components, such as the cathode, anode, liquid electrolyte and a
power supply.
The electrode products of molten lead(II} bromide during electrolysis using inert graphite electrodes
are lead and bromine, and it is possible to predict the products of electrolysis of other molten binary
ionic compounds.
The products of the electrolysis of concentrated sodium chloride solution a re hydrogen and chlorine, and of
dilute sulfuric acid solution are hydrogen and oxygen.
During electrolysis, metals or hydrogen are formed at the negative electrode (cathode), and non-metals are
formed at the positive electrode (anode).
Ionic half-equations can be constructed to show the reactions taking place at the anode and cathode during
different examples of electrolysis.
To the electrolysis products of dilute and concentrated solutions of metal halides can be predicted provided
the concentration of the solution is known.
The transfer of charge around the circuit during electrolysis consists of three distinct stages, including the
movement of electrons in the external wires, the transfer of electrons at the electrodes and the movement of
ions in the electrolyte.
Different electrolysis products are formed when copper(Il) sulfate solution is electrolysed using
graphite or copper electrodes, and these differences are shown in the electrolyte solution as well as the
electrode reactions.
..
Electrolysis can be used to electroplate metal objects with another metal to improve their appearance and
protect against corrosion.
A hydrogen-oxygen fuel cell can be used to generate electricity from the reaction between hydrogen and
oxygen, with water as the only product.
The hydrogen fuel cell has advantages and disadvantages as an alternative to petrol engines in vehicles.
...,
, .f
142
6
EJectrochemistry
PROJECT
Generating 'green' hydrogen for modern transport systems
You work for a scientific consultancy and have been asked to explain the process of electrolysis to
members of the government to help inform their decisions to establish hydrogen-generating plants in
several regions of your country.
Work in groups to prepare a presentation on the process of electrolysis and its environmental impacts.
In your presentation, include the following:
•
An explanation of the process of electrolysing water to produce hydrogen. Use the structure pro.v ided
by the explanatory triangle shpwn in Figure 6.19. This helps to explain the meaning of the word
electrolysis and the process involved. lJse the example of the electrolysis of dilute sulfuric acid ir:i
your explanation.
ELECTRO LYSI S
'-lysis'
to split
'electro-'
uses elect ricity
-
ELECTRO DE {4H ' + 4e- -+ 2H2}
EQUATIONS 202- -+ 0 2 + 4e-
CONDITIONS
- ions must b e
able t o move
O BSERVATIONS
~ - two gases formed
in ratio 2 : 1
Figure 6.19: The electrolysis of water.
•
Identify the sources of clean energy needed to carry out the electrolysis.
•
Suggest how t he hydrogen is used to power different types of transport system, and their
environmental advantage.
Share your presentations with t he whole class.
143
>
CAMBRIDGE IGCSE™ CHEMISTRY: COURSEBOOK
EXAM-STYLE QUESTIONS
1 There are certain requirements for electrolysis to take place. One of these
is the nature of the substance between the electrodes. Which of the diagrams
A- D in the figure shows a beaker in which electrolysis takes place?
A
B
~~
~~
.,.::::.~ .,.::::.:,~
ethanol
distilled
water
C
D
~~r
~~f
carbo~
electrodes ~
aqueous
sodium chloride
carbo~
electrodes ~
mercury
(1)
144
6
CONTINUED
2 Electroplating is a common industrial process. In which set of apparatus
A- D in the figure would the metal key be electroplated with copper?
B
A
fl cJ
aqueous
copper(II)
sulfate
aqueous
copper(II)
s-ulfate
C
D
c:r-0
aqueous
copper(II)
sulfate
■
•
aqueous
copper(II)
sulfate
piece of copper
c:r-() • key
(1)
3 Eight substances A- G that all conduct electricity are listed here.
A aluminium
B chromium
C copper
D dilute sulfuric acid
E platinum
F molten sodium bromide
G sodium chloride solution
Name the substance in A-G that:
a is used to plate steel cutlery
b is used for inert electrodes in some electrolytic cells
c produces a metal at the cathode when electrolysed
d produces oxygen at the anode when electrolysed
e is used in household electrical wiring
145
(1I
(1)
(1)
(1)
(1 I
[Total: SJ
Electrochem istry
>
CAMBRIDGE IGCSE™ CHEMISTRY: COURSEBOOK
CONTIN UED
4 The figure shows an electrolytic cell.
power supply
a Complete the labels for the following:
the electrode indicated
(1]
ii the contents of the beaker.
(1]
b Draw arrows on the connecting wires to show the direction of the
(1 I
movement of electrons in the circuit.
c Which of these solutions would give a colourless gas at both
electrodes during electrolysis?
dilute sulfuric acid
copper sulfate solution
(1 I
sodium chloride solution
s uggest: apply
d The electrodes shown are inert. S uggest a suitable substance that they
could be made from.
(1]
[Total: SJ
146
COMMAND WORD
knowledge and
understanding t o
situations where
there is a range
of valid ·responses
in order to make
proposals/
put forward
considerations
6
COMMAND WORDS
CONTINUED
c
Electrochemistry
Write an ionic half-equation for the reaction at the positive electrode.
(2)
The electrolysis is repeated using copper electrodes.
d What difference would be observed in:
(4)
the changes in the masses of the electrodes?
ii the change in colour of the solution?
Explain your answer in each case.
e Describe how the charge is transferred between the electrodes.
(2)
[2]
[Total: 13)
SELF-EVALUATION CHECKLIST
explain: set o ut
purposes or reasons/
make the relationships
between thi ngs evident/
p rovide why and/or
how and support with
re levant evid ence
describe: state the
po ints of a topic / g ive
characte ristics and
main features
.
After studying this chapter, think about how confident you are with the different topics. This will help you see any
gaps in your knowledge and help you to learn more effectively.
Needs
See
! Almost
Topic ... _ more work there
-
--
describe metals as electrical c0nductors and non-metallic
materials as non-cond acting insulators
6.1
define electrolysis and identify the components of a
simple electrolytic cell
6.1
describe the electrolysis products of molten lead(ll)
bromide using graphite electrodes and predict the
electrolysis products of other molten binary compounds
6.1
describe the electrolysis of concentrated sodium ehloride
solution and dilute sulfuric acid using inert electrodes
6.2
predict the products of electrolysis of dilute and
concentrated halide solutions
6.2
>
identify the products of the electrolysis of copper(ll)
sulfate solution using carbon or copper electrodes
6.2
>
describe how charge is transferred around the circuit
involved in electrolysis and construct ionic half-equations
for the ionic reactions taking place at the anode
and cathode
6.2
describe how electrolysis can be used to electroplate a
metal object
6.2
consider how a hydrogen--0xygen fuel cell
generates electricity
6.3
describe the advantages and disadvantages of hydrogenoxygen fuel cells as a means of powering road vehicles
6.3
. >
>
J~
I
I
-'
I can
147
-
-