5.

Electrochemistry
Presented by Seracettin Haslak
5.1 Electricity and chemistry

• Electrolysis is the decomposition of a molten ionic compound, or an ionic

compound dissolved in water, by the passage of an electric current.
• The electricity is carried through the electrolyte by ions. Electrolytes must be in
the liquid state. Solid ionic compounds, such as sodium chloride, do not conduct
electricity because the oppositely charged ions are held together in the giant
ionic lattice by strong electrostatic attraction. As the ions are not moving, solid
sodium chloride does not conduct electricity.
• Substances that do not conduct electricity when in the molten state or in solution
are called non-electrolytes.
• Substances that conduct electricity can be subdivided into conductors and
electrolytes.
5.1 Electricity and chemistry
5.1 Electricity and chemistry

• The electric current enters and leaves the

electrolyte through electrodes, which are usually
made of unreactive metals, such as platinum or
the non-metal carbon (graphite). These are said
to be inert electrodes because they do not react
with the products of electrolysis.
• The two electrodes are the cathode, the negative
electrode which attracts cations (positively
charged ions), and the anode, the positive
electrode which attracts anions (negatively
charged ions).
5.2 Electrolysis of lead(II) bromide

• When the electrodes are first connected, the bulb

does not light because the solid compound does not
allow electricity to pass through it. However, when the
compound is heated until it is molten, the bulb does
light. The lead(II) bromide is now behaving as an
electrolyte.
• Lead(II) bromide is a binary compound. Binary
compounds are those that contain two elements
chemically combined.
• Generally, when molten binary compounds such as
lead(II) bromide are electrolysed, metals such as lead
are formed at the cathode and non-metals, such as
bromine, are formed at the anode.
5.2 Electrolysis of lead(II) bromide

• Half-equation:

• Two bromine atoms then combine to form a

bromine molecule: 2Br- à Br2(g)
5.2 Electrolysis of lead(II) bromide

• At the anode, negative ions are oxidised by losing electrons.

• At the cathode, positive ions are reduced by gaining electrons.
Worked example 1

1.
a. Predict the products of the electrolysis of molten magnesium fluoride (MgF2).
The metal element of the magnesium fluoride is produced at the cathode. The
magnesium ions (Mg2+) are attracted and deposited at this electrode. The non-
metal element of the magnesium fluoride is produced at the anode. The
fluoride ions (F−) are attracted and deposited at this electrode.
b. Write ionic half-equations for the substances produced at the different
electrodes you answered in 1a. Ions present in MgF2 are Mg2+ and F−. At the
cathode:
• Mg2+(l) + 2e− → Mg(l)
• At the anode:
• 2F−(l) → F2(g) + 2e−
Worked example 1

c. State whether the processes are oxidation or reduction.

• The cathode reaction is reduction (Mg2+ gains electrons). The anode reaction is
oxidation (F− loses electrons).
5.3 Electrolysis of aluminium oxide

• Hall–Heroult process:
1. Bauxite, an impure form of aluminium oxide, is first treated with sodium
hydroxide to obtain pure aluminium oxide, removing impurities such as iron(III)
oxide and sand. This improves the conductivity of the molten aluminium oxide.
2. The purified aluminium oxide is then dissolved in molten cryolite (Na3AlF6). The
advantages of this are that:
• the electrolyte can be maintained in the liquid state between 800°C and 1000°C,
a temperature considerably lower than 2071°C, which greatly reduces energy
costs.
• cryolite improves the conductivity of the electrolyte.
Test yourself - Question

1. Copy and complete the following sentence about electrolysis using words from
this list:
• Compound, electricity, inert, chemical, platinum, molten, breakdown
• Electrolysis is the________ of an ionic _________when __________ or in
aqueous solution by the passage of ____________. Electrodes are often made of
graphite or __________ and are generally used in electrolysis because they are
__________.
Test yourself - Question

2. Predict the products of the electrolysis of molten:

a)
i. potassium chloride, KCl
ii. lead oxide, PbO.
b. Write ionic half-equations for the substances produced at the different
electrodes for a i and a ii.
c. Use the answers to 2b to decide which processes are oxidation and which are
reduction.
5.3 Electrolysis of aluminium oxide

3. Aluminium oxide in molten cryolite

behaves in the same way as molten
aluminium oxide as far as the products of
electrolysis are concerned.
4. Electrolysis is carried out in a steel tank
using carbon (graphite) as electrodes. The
anodes are carbon (graphites) blocks
which are lowered into the electrolyte.
The cathode is the carbon (graphite) lining
of the tank.
5.3 Electrolysis of aluminium oxide

• During electrolysis, the negatively charged oxide ions are attracted to the anode
(the positive electrode), where they lose electrons (oxidation).

• The positive aluminium ions are attracted to the cathode (the negative
electrode). They gain electrons to form molten aluminium metal (reduction).

• A handy way of remembering what happens is OIL RIG – Oxidation Is Loss,

Reduction Is Gain of electrons.
5.3 Electrolysis of aluminium oxide

• The overall reaction which takes place in the cell is:

• Molten aluminium collects at the bottom of the tank and is siphoned off.
• The oxygen that is produced at the anode reacts, at the high temperature of the
cell, with the graphite anodes, producing carbon dioxide gas which escapes:
• C(s) + O2(g) à CO2(g)
• Thus, the anodes burn away and need to be replaced regularly.
5.3 Electrolysis of aluminium oxide

• The cost of electricity is the largest expense in this process, so it is carried out in
regions where cheap electricity is available, for example from hydroelectric
power.
• Aluminium is used in the manufacture of electrical/transmission cables. It is used
because of its low density, chemical inertness and good electrical conductivity. It
is also used for making cars, bikes, cooking foil and food containers, as well as in
alloys such as duralumin, which is used in the manufacture of aeroplane bodies.
5.3 Electrolysis of aluminium oxide – Recycling aluminium

• The collecting scrap metal and transport to the metal extraction plant add extra
costs to the overall recycling costs.
• There are serious environmental problems associated with the location of
aluminium plants including:
1. the effects of the extracted impurities, which form a red mud.
2. the fine cryolite dust, which is emitted through very tall chimneys so as not to
affect the surrounding area.
3. the claimed link between environmental aluminium and a degenerative brain
disorder called Alzheimer’s disease.
Test yourself - Question

3. Produce a flow chart to summarise the processes involved in the extraction of

aluminium metal from bauxite.
Test yourself - Question

4. Explain why the mixture of gases formed at the anode contains oxygen, carbon
dioxide and traces of fluorine.
Test yourself - Question

5. Complete the following passage about the extraction of aluminium using words
from the list below.
• Anode, cell, cryolite, replaced, burn, cathode, oxygen, bauxite, carbon.
• Aluminium is extracted from its molten ore, ___________by electrolysis in an
electrolytic ___________.Before the extraction process begins, __________is
added. The ____________ electrodes are then lowered into the cell. During the
process aluminium is produced at the ___________ and is siphoned off. At the
____________, oxygen is produced. Because of the high temperature required in
the process, the carbon electrodes ____________ away due to the reaction of
the anodes with ___________ produced at that electrode. Therefore the anodes
have to be ___________regularly.
5.4 Electrolysis of aqueous solutions

• Pure water is a very poor conductor of electricity because

there are so few ions in it. However, it can be made to
decompose if an electric current is passed through it in a
Hofmann voltameter.
• To enable water to conduct electricity better, some dilute
sulfuric acid (or sodium hydroxide solution) is added.
• For hydrogen to be collected in this way, the positively
charged hydrogen ions must have moved to the cathode.
5.4 Electrolysis of aqueous solutions

• If, during this process, the water molecules lose H+(aq), then the remaining
portion must be hydroxide ions, OH−(aq). These ions are attracted to the anode.

• Generally when aqueous solutions are electrolysed, hydrogen is produced at the

cathode and non-metals (other than hydrogen) are formed at the anode.
5.4 Electrolysis of aqueous solutions
5.4 Electrolysis of aqueous solutions

• Aqueous solutions of acids always produce hydrogen at the cathode. The H+ ion is
found in both the acidic substance and the water.
• During the electrolysis of any aqueous solution containing positive ions of a metal
above hydrogen in the reactivity series, hydrogen is produced at the cathode, not
the metallic element. For example, NaCl is dissolved in water. However, the H+
ions absorb electrons more easily than the Na+ ions and so hydrogen gas (H2) is
produced at the cathode. Since sodium is more reactive than hydrogen, it loses
electrons more easily.
• Very reactive metals that react with cold water (such as potassium, sodium and
calcium) cannot be produced by electrolysis of aqueous solutions containing ions
of these metals. These metals can only be extracted by electrolysis using a molten
electrolyte.
Worked example 2

1. Predict the products at the cathode and anode of the electrolysis of:
a. dilute aqueous potassium chloride. At cathode: hydrogen (from the water). At
anode: chlorine and oxygen (from the water).
b. concentrated hydrobromic acid (HBr).
• Concentrated acid contains very little or no water.
• So in the electrolysis of the acid, the ions present are H+ and Br−. Hence:
• At cathode: hydrogen
• At anode: bromine.
Test yourself - Question

6. Suggest a reason for only ‘roughly’ twice as much hydrogen gas being produced
at the cathode as oxygen gas at the anode in the electrolysis of water.
5.5 Electrolysis of copper(II) sulfate aqueous solution

• If aqueous copper(II) sulfate is electrolysed

using carbon or platinum electrodes (inert
electrodes), the products are copper at the
cathode and oxygen at the anode.
• Four ions are present in solution:
Øfrom the water: H+(aq) + OH−(aq)
Øfrom the copper(II) sulfate: Cu2+(aq) + SO42−(aq).
• The solution gradually loses ist blue colour and
eventually turns colourless. The blue colour is
due to Cu2+(aq) and the colour gradually fades
because the concentration of Cu2+(aq)
decreases.
5.5 Electrolysis of copper(II) sulfate aqueous solution

• H+(aq) and Cu2+(aq) ions are both attracted to the cathode, the Cu2+ ions
accepting electrons more readily than the H+ ions (preferential discharge). Copper
metal is therefore deposited at the cathode.

• OH−(aq) and SO42−(aq) ions are both attracted to the anode. The OH− ions release
electrons more easily than the SO42− ions, so oxygen gas and water are produced
at the anode.
5.5 Electrolysis of copper(II) sulfate aqueous solution
5.5 Electrolysis of copper(II) sulfate aqueous solution –
Purification of copper
• As copper is a very good conductor of electricity, it is used for electrical wiring
and cables. However, even small amounts of impurities cut down this
conductivity quite noticeably, whether in fine wires or larger cables.
• The impure copper is used as the anode in the electrolysis process. The cathode
is made from very pure copper. Because copper is itself involved in the
electrolytic process, the copper cathode is known as an ‘active’ electrode.
• The electrolyte is a solution of copper(II) sulfate (0.3 mol/dm3) acidified with a 2
mol/dm3 solution of sulfuric acid to help the solution conduct electricity.
• When the current flows, the copper moves from the impure anode to the pure
cathode. Any impurities fall to the bottom of the cell and collect below the anode
in the form of a slime.
5.5 Electrolysis of copper(II) sulfate aqueous solution –
Purification of copper
5.5 Electrolysis of copper(II) sulfate aqueous solution –
Purification of copper
• The ions present in the solution are:
Øfrom the water: H+(aq) + OH−(aq)
Øfrom the copper(II) sulfate: Cu2+(aq) + SO42−(aq).
• During the process, the impure anode loses mass because the copper atoms lose
electrons and become copper ions, Cu2+(aq).
5.5 Electrolysis of copper(II) sulfate aqueous solution –
Purification of copper
• The electrons released at the anode travel
around the external circuit to the cathode. There
the electrons are passed on to the copper ions,
Cu2+(aq), from the copper(II) sulfate solution and
the copper is deposited or copper plated on to
the cathode.
Test yourself - Question

8. Why do you think it is advantageous to use inert electrodes in the electrolysis

processes?
9. Predict the products of electrolysis of a solution of copper(II) sulfate if carbon
electrodes are used instead of those made from copper as referred to in the
purification of copper section.
10. Predict the products of the electrolysis of concentrated hydrochloric acid using
platinum electrodes. (Remember, concentrated acids have very little or no
water present.)
Test yourself - Question

11. Using your knowledge of electrolysis, predict the likely products of the
electrolysis of copper(II) chloride solution, using platinum electrodes. Write
ionic halfequations for the formation of these products at the electrodes.
5.6 Fuel cells

• Fuel cells are chemical cells except that the reagents are supplied continuously to
the electrodes. The reagents are usually hydrogen and oxygen.
• 2H2(g) + O2(g)→2H2O(l)
• The aqueous NaOH electrolyte is kept within the cell by electrodes which are
porous, allowing the transfer of O2, H2 and water through them.
• As O2 gas is passed into the cathode region of the cell, it is reduced:
• O2(g) + 2H2O(l) + 4e− → 4OH−(aq)
• The OH− ions formed are removed from the fuel cell by reaction with H2:
• H2(g) + 2OH−(aq) → 2H2O(l) + 2e−
5.6 Fuel cells
5.6 Fuel cells – Advantages and disadvantages of the
hydrogen fuel cell
• Fuel cell technology has its advantages; for example, a fuel cell:
1. uses hydrogen and oxygen and makes nonpolluting water in the process of
generating electricity, whereas petrol and diesel engines produce many
pollutants
2. is similar to a battery but does not require any external charging
3. is capable of producing electricity as long as hydrogen fuel and oxygen are
supplied.
5.6 Fuel cells – Advantages and disadvantages of the
hydrogen fuel cell
• However there are some disadvantages to using fuel cells. For example:
1. hydrogen is in the gas state at room temperature and pressure, so it is difficult
to store in a car
2. the infrastructure does not yet exist, as it does for fossil fuels, for example, the
number of refuelling stations
3. fuel cells and electric motors are less durable than petrol or diesel engines,
which means they do not last as long
4. fuel cells are very expensive at the moment.
Test yourself - Question

12. The fuel cell was discovered during electrolysis experiments with water. It is the
reverse process which produces the electricity. Write a balanced chemical
equation to represent the overall reaction taking place in a fuel cell.
5.7 Electroplating

• Electroplating is the process involving electrolysis where one metal is plated, or

coated, with another.
• The purposes are:
1. to improve appearance
2. to prevent corrosion, for example to prevent rusting of iron or steel
• Electroplating is carried out using:
1. The plating metal as the anode
2. The object to be plated as the cathode
3. An aqueous solution containing ions of the plating metal as the electrolyte
5.7 Electroplating

• The plating metal (anode) is silver.

• The object to be plated (cathode) is a metal
spoon.
• The electrolyte is an aqueous solution of silver
nitrate containing Ag+(aq).
• The silver produced at the cathode
electroplates the spoon.
• The silver ions in the electrolyte are discharged
at the cathode: Ag+(aq) + e- à Ag(s)
• The silver anode goes into solution as silver
ions: Ag(s) à Ag+(aq) + e-
Test yourself - Question

13. Explain why copper(II) chloride solution would not be used as an electrolyte in
the electrolyte cell used for copper plating.
Test yourself - Question

14. Write equations which represent the discharge at the cathode of the following
ions:
• a) K+ b) Pb2+ c) Al3+
• and at the anode of:
• d)Br- e) O2- f) F-
Key objectives

By the end of this section, you should be able to:

• Define electrolysis.
• In a simple electrolytic cell, identify the component parts of anode, cathode and
the electrolyte.
• Identify which electrode in a simple electrolytic cell the anions and cations are
attracted to.
• Describe the transfer of charge that takes place during electrolysis.
• Construct ionic half-equations for reactions at the anode (to show oxidation) and
at the cathode (to show reduction).
Key objectives

• Describe the electrode products as well as the observations made during the
electrolysis of the following substances: molten lead(II) bromide, concentrated
aqueous sodium chloride, dilute sulfuric acid.
• Describe the process of the extraction of aluminium from purified bauxite.
• State that metals or hydrogen are formed at the cathode and that non-metals
(other than hydrogen) are formed at the anode.
• Predict the identity of the products of electrolysis of a specified binary
compound, such as lead(II) bromide, in the molten state.
• Describe how the refining of copper takes place by the process of electrolysis
using an aqueous solution of copper(II) sulfate with carbon and copper
electrodes.
Key objectives

• Describe how and why metals are electroplated.

• State that hydrogen–oxygen fuel cells use hydrogen and oxygen to generate
electricity with water as the only substance produced.
• Describe the advantages and disadvantages of using hydrogen–oxygen fuel cells
in comparison with gasoline or petrol engines in vehicles.
Key terms

• Anion: A negative ion. Anions are attracted to the anode in electrolysis.

• Anode: The positive (+) electrode. It is positively charged because electrons are
drawn away from it.
• Binary compound: A compound containing two elements chemically combined.
• Cathode: The negative (−) electrode. It is negatively charged because an excess of
electrons move towards it.
• Cation: A positive ion. Cations are attracted to the cathode in electrolysis.
• Electrodes: The conducting rods by which electric current enters and leaves the
electrolyte.
• Electrolysis: The decomposition of an ionic compound, when molten or in
aqueous solution, by the passage of an electric current
Key terms

• Electrolyte: A liquid which will carry electric current and is chemically changed by
it.
• Inert electrode: An electrode that does not react with the electrolyte or the
products of electrolysis. Examples are carbon (graphite) and platinum.
Homework

1. This is a diagram of an experiment in which

electricity was passed through a mixture of
distilled water containing a little dilute sulfuric
acid.
a. Identify the gas that collects at A.
b. Identify the gas that collects at B.
c. If 100 cm3 of gas collects in A, how much
would there be in B?
d. Identify the metal usually used for X and Y.
Homework

e. X is called the .
f. Y is called the .
g. Give the formulae of the three ions present in the solution.
h. Give the ionic half-equations for the reactions that take place at both X and Y.
Homework

2. Explain the meaning of each of the following terms.

a. Anode
b. Cathode
c. Electrolysis
d. Electrolyte
e. Anion
f. Cation
g. Oxidation, in terms of electrons
h. Reduction, in terms of electrons
Homework

3. Copper is purified by electrolysis, as in the

example shown below.
a. Identify the materials used for the electrodes A
and B.
b. Identify the electrolyte C and substance D.
c. Explain why substance D is of economic
importance in respect of this process.
d. Give word and symbol equations for the reactions
which take place at the cathode and anode during
this process.
Homework

e. Explain why electrolyte C has to be acidified with dilute sulfuric acid.

f. Explain why copper has to be 99.99% pure for use in electrical cables.
Homework

4. Copy and complete the table below, which shows the results of the electrolysis
of four substances using inert electrodes.
Homework

a. Give the meaning of ‘inert electrodes’.

b. Explain why the sodium chloride solution becomes progressively more alkaline
during electrolysis.
c. Explain why solid lithium chloride is a nonconductor of electricity, whereas
molten lithium chloride and lithium chloride solution are good conductors of
electricity.
d. During the electrolysis of molten aluminium chloride (AlCl3) the carbon anodes
are burned away. Explain why this happens and write balanced chemical
equations for the reactions that take place.
Homework

5. Sodium hydroxide is made by the electrolysis of brine (concentrated sodium

chloride solution).
a. Give ionic half-equations for the reactions which take place at the cathode and
anode. State clearly whether a reaction is oxidation or reduction.
b. Give two large-scale uses of the products of this electrolytic process.
c. Analyse the following statement: ‘This electrolytic process is a very expensive
one.’
Homework

6. Electroplating is an important industrial process

a. Explain what electroplating is.
b. Explain why certain metals are electroplated.
c. Give two examples of the use of electroplating.