1. Introduction to electrolysis – electrolytes and
Electrolysis is the process of
electrically inducing chemical changes in a conducting melt or solution e.g.
splitting an ionic compound into the metal and non–metal or producing gases like
hydrogen, oxygen and chlorine from salt solutions.
(a) A summary of common electrical conductors
- What makes up a circuit in cells –
batteries or electrolysis? What carries the current?
- Conductors are materials (solid or
liquid/solution) that carry an electric current
via freely moving electrically charged particles, when a potential
difference (voltage!) is applied across them, and they include:
- All metals (molten or solid), non–metal carbon (graphite) and some recently developed 'smart
materials' (which get no further mention in this section of notes).
- This conduction involves the movement of
free or delocalised electrons (e– charged particles) and does
not involve any chemical change.
- That doesn't mean to say these
'electrical currents' can't promote chemical change, which happens as we
will see later in the process we call electrolysis.
- Any compound, molten or dissolved in
solution, in which
the liquid contains free moving ions is called an
electrolyte and can conduct an electrical current. (see
- Ions are electrically charged particles e.g.
Na+ sodium ion, or Cl– chloride ion, and their movement or flow
constitutes an electric current, in other words the electrolyte is a solution containing mobile ions that conduct
electrical energy as a
stream of moving charged particles.
An electrolyte may consist of ...
- (i) a molten ionic compound i.e. on
melting the ions are free to move to carry the current, or
- (ii) any compound that dissolves in a
liquid to give a solution of ions that are free to move.
- The compound is usually ionic and the
liquid is usually water, so in most of the examples described, the
electrolyte is an aqueous solution of ions with a few molten salt
- Note: Aqueous means to do with water, so
an aqueous solution is a solution made by dissolving something in
the solvent water.
- When an electric current is passed through
such an electrolyte chemical changes can occur on the electrical contacts
(called electrodes) and chemical changes happen to break down the
compound in a process called electrolysis.
- Water is very poor conductor because it
is a covalent compound and only minute amounts of it ionises to form
hydrogen and hydroxide ions, so water it is not an effective
- What doesn't conduct and why? why can't solid ionic
compounds not conduct electricity?
- Apart from metals, most solids do not conduct
electricity because there are no free electrons or ions to carry an electric
- However, although molten ionic compounds and
solutions in of ionic compounds in water can carry a current (and undergo
electrolysis - chemical change) solid ionic compounds cannot conduct
electricity and undergo electrolysis. This is because the ions are tightly
held in the crystal lattice and cannot move around and migrate to any
electrical contact placed on the solid.
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Apparatus for investigations - the electrolysis cell
- When an appropriate d.c. current is
passed through an electrolyte, chemical changes occur where the external
circuit connections (electrodes) are dipped into the electrolyte.
- These chemical changes ONLY occur on the surface of the electrodes where
they are in contact with the electrolyte solution and (usually) elements are released as the compound is
broken down by the process called electrolysis.
What does a
simple, but complete, electrical
circuit for electrolysis consist of?
simple cell system for electrolysis
electrolyte container can be made from a short piece of wide glass/plastic tubing with a rubber bung base with two holes drilled in it to take to
carbon rods as electrodes.
- Small test tubes filled with electrolyte are
inverted over the carbon electrodes.
- This even simpler set-up is recommended by the RSC and consists of two wire
electrodes bent in a S shape so the gases can be collected in little
test tubes filled with the electrolyte and inverted over the electrodes
in the beaker of electrolyte.
- The diagrams above illustrate simple
electrolysis experiments you will see or (hopefully) do in a school
- The electrolyte solution (in this
case sulfuric acid, can be sodium chloride etc.) is contained within the
electrolysis cell (e.g. section of wide plastic piping).
- Two electrical connectors called electrodes
(e.g. graphite/carbon rods) protrude upwards into the electrolyte
solution pushed through two holes drilled in a larger rubber bung. This is
the same function as the two wires in the other simpler electrolysis set-up
- The circuit is completed when connected
to an external electrical power supply of d.c. current, and usually a
voltage of 2-3 V is quite sufficient to give a good rate of electrolysis.
- So, in sequence from the negative
terminal, through the external copper wire electrons flow clockwise from
the positive electrode to
the negative electrode (cathode), then ions (NOT
electrons) carry the current through the electrolyte across to the
positive electrode (anode), and then
electrons again carry the current through another external wire to the
positive terminal of the battery or other power supply.
- When you switch on the d.c. power on, or
connect the battery, the electrolysis process should start. Often,
but not always, you will see bubbles of gas appearing on the electrode surface, because that's where the
we call electrolysis take place!
- Flowing in one direction only, the
electrons carry the current in the external copper wires BUT not in the
electrolyte solution. However in the electrolyte solution there are two ion currents flowing in opposite directions,
and it is important that this is understood because no chemical change can
take place if the ions are not attracted to their oppositely charged
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(c) What happens in electrolysis? and what products can be
- All salts and many acids are good
electrolytes, good electrical conductors of a d.c. current because
they provide high concentrations of positive and negative ions.
- Positive ions (cations) e.g.
hydrogen H+, copper Cu2+, sodium
are attracted to the negative electrode (cathode).
migrate to the negative cathode.
- Negative ions (anions) e.g.
chloride Cl– , sulfate SO42–,
bromide Br–, are attracted to the positive
- Negative anions
migrate to the positive
- It is possible to
demonstrate this flow using a coloured
ion experiment (see diagram and text below).
- Remember no electrons flow in
the solution, but they do flow in the external metal wires or carbon
(graphite)/metal electrodes of the external circuit.
So, what is the chemistry of electrolysis?
- When an ion meets its oppositely
charged electrode, one of two things can happen. Either the ion
hangs around the electrode and does nothing OR the ion undergoes
chemical change, sometimes referred to as 'the ion is discharged'.
- The chemical changes that occur
on the surface of an electrode are either a
the negative cathode electrode) or an
OXIDATION (on the
positive anode electrode).
- Each of the oxidation or
reduction changes is written as a half-equation, so you
see the electrons lost or gained
- At the
electrode, positive ions (cations) are attracted and these
positive ions may gain electrons and are reduced to
some chemical product e.g. typical half-reactions ...
- Either hydrogen gas or a metal
deposit is formed on the negative cathode electrode.
+ 2e– ==> H2(g)
(colourless hydrogen gas from acid solutions)
+ 2e– ==> Cu(s) (copper
deposit from copper sulfate solution)
+ 2e– ==> Pb(l) (lead formed
from a hot molten salt)
+ 3e– ==> Al(l) (aluminium formed
from molten oxide)
- Note that ...
- (i) reduction = electron
gain, at the negative cathode electrode,
- (ii) hydrogen and metals
are formed at the negative cathode electrode,
- (iii) not all the
positive ions will be discharged i.e. reduced, so in a
mixture of H+ and Na+ ions in aqueous
solution, the hydrogen ions are preferentially reduced to
hydrogen, leaving the sodium ions unchanged in solution,
AND generally speaking, the
less reactive a metal, the more easily its ion is reduced to
the metal on the electrode surface e.g. in a mixture of
positive ions the preference order is
- Cu2+ (==>
Cu) > H+ (==> H2) > Na+
- A general rule with reference to the
reactivity series of metals:
- If the metal in the salt solution
is more reactive than hydrogen, then hydrogen the
hydrogen ion is most likely to be discharged at the negative cathode giving
- If a reactive metal like sodium was
discharged, it would immediately react with water giving hydrogen
- If the metal in the salt solution
is less reactive than hydrogen it is the metal ion that
is likely to be discharged forming a deposit of the metal
on the electrode surface.
- In practice you can get a deposit of lead
from a lead nitrate solution.
- At the
electrode, negative ions (anions) are attracted and these
negative ions may lose electrons and are oxidised
to some chemical product e.g. typical half-reactions ...
- A non-metal like oxygen or
chlorine is released at the positive anode electrode.
– 2e– ==> Cl2(g) (chloride ion
green chlorine gas from NaCl)
– 2e– ==> Br2(g) (bromide ion
bromine vapour from molten bromide)
– 4e– ==>
O2(g) (oxide ion ==> colourless oxygen gas from a molten oxide)
– 4e– ==>
+ O2(g) (hydroxide ==> colourless oxygen gas, from
alkaline hydroxide solution or traces of OH- from water)
+ O2(g) + 4e–
- Note that ...
oxidation = electron
loss, at the positive anode electrode,
- (ii) apart from hydrogen, other non–metals are formed at the positive anode electrode,
- (iii) AND not all the
negative ions will be discharged i.e. oxidised, so in a
mixture of OH– and Cl– ions in aqueous
solution, the chloride ions are preferentially oxidised to
chlorine, leaving most of the hydroxide ions unchanged in solution.
- Oxygen is usually produced unless a halide
ion is present,
- so chloride would give chlorine, bromide
gives bromine and iodide gives iodine at the positive anode.
- All the above electrode
equations showing the formation of the electrolysis products
are fully described in the context of a laboratory experiment or
- Three 'connected' sub–notes on
- As well as investigating the products of electrolysis, you can also
vary experimental conditions e.g. changes in voltage p.d. or
electrolyte concentration can be studied. Possible investigations
can show ....
- (i) The greater the concentration of the electrolyte
ions, the lower the electrical resistance of the solution.
This is because there are more ions present to carry the current
e.g. if the voltage (V, volts) is kept constant, the
current flowing (I, amps) will steadily increase as the
concentration of the electrolyte is increased.
- (ii) If the
electrolyte (ion) concentration is kept constant, the current
will steadily increase with increase in voltage
just like any other electrical circuit because the increase
in electrical field effect from the increased p.d. (voltage)
will force the ion flow at a greater rate.
- (iii) So,
increase in ion concentration (salts, acids etc.) OR
increase in voltage will increase the speed of electrolysis
i.e. the electrode reactions, whether it involves gas formation
or electroplating metals etc.
- The greater the voltage, the
faster the rate of electrolysis, but don't over do it!
- The molten or dissolved materials are
usually acids, alkalis or salts and their electrical conduction
is usually accompanied by chemical changes
- The chemical changes occur at the
electrodes which connect the electrolyte liquid containing ions with the
external d.c. electrical supply.
- If the current is switched off, the
electrolysis process stops.
are liquids or solutions that do not contain ions, do not conduct
electricity readily and cannot undergo the process of electrolysis
e.g. ethanol (alcohol), sugar solution etc. and are usually covalent
molecule liquids or solutions of covalent compounds. Even if a covalent
compound dissolves in water, if no ions are formed, there will be no
- How do we know that ions move to the
electrodes when a d.c. current is applied?
- The coloured ion experiment described below
illustrates how you can show the slow movement of ions when a voltage is
applies across a conducting solution of ions – the electrolyte.
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simple experiment to show the
movement of coloured ions
rectangle of filter paper is soaked in an ammonia–ammonium chloride
solution and mounted on a microscope slide. The paper is connected to a
d.c. supply with clips. A 'line' of copper chromate solution (*) is placed
in the middle of the filter paper and the current switched on. (* Using
a mixture of copper(II) sulfate and potassium chromate(VI).
copper chromate is
green–brown in solution but gradually it disappears and
separates, in different directions, into a yellow and blue bands.
The yellow band is due to negative chromate ions, CrO42––,
moving towards the positive electrode.
band is due to positive copper ions, Cu2+, moving towards the
negative electrode. All due to opposite charges attracting
in the electric field produced by the potential difference (the
electrolyte, as with any solution, ALL the particles are moving around
at random, but they are not distributed at random as the electrolysis
electrolysis positive ions on average move towards the negative cathode
AND the balancing
negative ions on average move towards the positive anode
streams of moving ions constitute the electric current flowing in the
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(e) Examples of diagrams to explain electrolysis
and the formation of products on the electrodes
examples are illustrated below, but the full electrolysis description
and explanation is given on individual web pages - see
Diagram showing the net direction of ion movement in water acidified with
Diagram showing the net direction of ion movement in molten sodium chloride
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Summary of the criteria for electrolysis to take place and the splitting of a
- Liquids that conduct must contain freely
moving ions to carry the current and complete the circuit.
- You can't do electrolysis with an ionic
solid!, the ions are too tightly held by chemical bonds and can't
flow from their ordered situation!
- The particle theory of solids still
applies even if you try to pass electricity through it (apart from
graphite and metals).
- When an ionically bonded substances are
melted or dissolved in water the ions are free to move about.
- However some covalent substances
dissolve in water and form ions.
- e.g. hydrogen chloride HCl, dissolves
in water to form 'ionic' hydrochloric acid H+Cl–(aq)
- The solution of ions (e.g. salts, acids etc.)
or melt of ions (e.g. chlorides, oxides etc.) is called the
electrolyte which forms part of the circuit. The circuit is
completed by e.g. the external copper wiring and the (usually) inert
electrodes like graphite (form of carbon) or platinum AND electrolysis can
only happen when the current is switched on and the circuit complete.
ELECTROLYSIS SPLITS a molten
- When substances which are made of ions are
dissolved in water, or melted material, they can be broken down
(decomposed) into simpler substances by passing an electric current
- This process is called electrolysis and is
used extensively in the chemical industry for extracting elements like
sodium and chlorine from their naturally occurring compounds.
- Since it requires an 'input' of energy, it
is an endothermic process and costly to pay for the electrical
- During electrolysis in the electrolyte
(solution or melt of free moving ions) ...
- ... positive metal or hydrogen ions move
to the negative electrode (cations attracted to cathode), e.g. in the
diagram, sodium ions Na+ , move to the negative electrode
- and negatively charged ions move to the
positive electrode (anions attracted to anode), e.g. in the diagram,
chloride ions Cl–, move to the positive electrode (+ve).
- The diagram shows the industrial electrolysis
process (in a Down's Process Cell) to extract sodium metal from sodium
chloride (common salt). This is an example of how electrolysis is used in
the chemical industry.
- During electrolysis, gases may be given off,
or metals dissolve or are deposited at the electrodes.
- Metals and hydrogen are formed at the
negative electrode from positive ions by electron gain (reduction), e.g.
in molten sodium chloride
- sodium ions change to silvery grey
- Na+ + e–
Na (a reduction electrode reaction)
- and non–metals e.g. oxygen, chlorine,
bromine etc. are formed from negative ions changing on the positive
electrode by electron loss (oxidation), e.g. in molten sodium chloride
- chloride ions change to green chlorine
- 2Cl– – 2e–
==> Cl2 + 2e–
oxidation electrode reaction)
- The electrons released by the oxidation at
the positive anode, flow round through the anode and wire to the
positive cathode and so bring about the reduction i.e. of the sodium
- In a chemical reaction, if an oxidation
occurs, a reduction must also occur too (and vice versa) so these reactions
'overall' are called
- You need to be able to complete and
balance electrode equations or recognise them and maybe have to derive an overall
equation for the electrolysis.
- e.g. for the electrolysis of molten sodium
chloride described above, the overall chemical change due to
electrolysis can be written as ...
- 2NaCl(l) ==> 2Na(l) + Cl2(g)
- At this point it is appropriate and very
important to mention and use state symbols in all electrode
equations and overall chemical change equations.
- Reminder: (g) = gas, (l) =
liquid, (s) = solid, (aq) = aqueous solution in water
- e.g. for sodium chloride,
NaCl(l) for the molten salt,
- NaCl(aq) or
Na+ + Cl– for two possible
expressions of the aqueous solution
- When dealing with the electrolysis of
aqueous solutions of salts and acids in water, things can be more
complicated and sometimes several competing electrode reactions can occur at
the same time, and sometimes products differ depending on the nature of the
cathode and anode electrodes.
- There are
links from the
electrochemistry index page which describe
in great detail particular examples of the process of electrolysis and all
can be done as college or school student experiments or teacher
demonstrations, your pupils can have great fun with these experiments but
take great care with the production of chlorine!
Electrolysis Quiz (GCSE 9-1 HT Level (harder)
Electrolysis Quiz (GCSE 9-1 FT Level (easier)
keywords and phrases:
revision study notes for AQA Edexcel OCR IGCSE/GCSE chemistry
topics modules on electrolysis introduction on how to do electrolysis investigations,
examples of what are electrolytes?, examples of what are
non-electrolytes, what are electrodes? how to you write electrode equations,
examples of common electrical conductors, descriptions of apparatus
needed to do electrolysis investigations, diagrams of electrolysis
cells, What happens in electrolysis? and what products can be formed on
the electrode surface, A simple experiment to show the movement of
coloured ions using copper chromate salt solution, Examples of diagrams
explaining electrolysis and product formation on the electrode surface,
what is needed for electrolysis to take place and explaining the
splitting up of a compound into its elements by electrolysis
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