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Cambridge O Level Chemistry Your notes
4.1 Electrolysis
Contents
Electrolysis Principles
Electrolysis of Molten Compounds
Electrolysis of Aqueous Sodium Chloride & Dilute Sulfuric Acid
Electrolysis of Aqueous Solutions
Ionic Half Equations
Electroplating
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Electrolysis Principles
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Electrolysis: General Principles
When an electric current is passed through a molten ionic compound the compound decomposes or
breaks down
The process also occurs for aqueous solutions of ionic compounds
Covalent compounds cannot conduct electricity hence they do not undergo electrolysis
Ionic compounds in the solid state cannot conduct electricity either since they have no free ions that
can move and carry the charge
Particles in ionic compounds are in fixed position in the solid state but can move around when molten or
in solution
Key terms used in a simple electrolytic cell
Electrode is a rod of metal or graphite through which an electric current flows into or out of an
electrolyte
Electrolyte is the ionic compound in a molten or dissolved solution that conducts the electricity
Anode is the positive electrode of an electrolysis cell
Anion is a negatively charged ion which is attracted to the anode
Cathode is the negative electrode of an electrolysis cell
Cation is a positively charged ion which is attracted to the cathode
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Your notes
The basic set-up of an electrolytic cell
Metals and hydrogen form positively charged ions and so either a metal or hydrogen gas is formed at
the cathode
Non-metals form negatively charged ions and so non-metals (except hydrogen) are formed at the
anode
Exam Tip
Use the PANIC mnemonic to remember which electrode is the positive and which is the negative:
Positive (is) Anode Negative Is Cathode
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Electrolysis: Charge Transfer
During electrolysis, current needs to flow around the circuit Your notes
In order for this to occur, charge must be transferred around the circuit (current is a measure of the rate
of flow of charge) by charge carriers
The power supply provides the cathode with a supply of electrons, causing it to become negatively
charged
Positive ions (cations) in the electrolyte move towards the cathode where they gain electrons
Negative ions (anions) in the electrolyte move towards the anode where they lose electrons
The electrons move from the anode back towards the power supply
So, in a complete circuit:
Electrons are the charge carriers in the external circuit
Ions are the charge carriers in the electrolyte
Diagram showing the direction of movement of electrons and ions in the electrolysis of NaCl
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Electrolysis of Molten Compounds
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Electrolysis of Molten Compounds
A binary ionic compound is one consisting of just two elements joined together by ionic bonding
When these compounds undergo electrolysis they always produce their corresponding elements
To predict the products made at each electrode, first identify the ions
The positive ion will migrate towards the cathode and the negative ion will migrate towards the anode
Therefore, the cathode product will always be the metal, and the product formed at the anode will
always be the non-metal
Example: Electrolysis of molten lead(II) bromide
Method:
Add lead(II) bromide into a beaker and heat it so it will turn molten, allowing ions to be free to move and
conduct an electric charge
Add two graphite rods as the electrodes and connect this to a power pack or battery
Turn on the power pack or battery and allow electrolysis to take place
Negative bromide ions move to the positive electrode (anode) and each loses one electron to form
bromine molecules. There is bubbling at the anode as brown bromine gas is given off
Positive lead ions move to the negative electrode (cathode) and gain electrons to form a grey lead
metal which deposits on the surface of the electrode
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Your notes
Diagram showing the electrolysis of lead(II) bromide
Worked example
Identify the product formed at the anode and cathode during the electrolysis of molten potassium
chloride.
Answer
The ions present are potassium (K+) and chloride (Cl-)
The chloride ions are attracted to the anode and form chlorine gas
The potassium ions are attracted to the cathode and form potassium metal
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Exam Tip
Your notes
Remember electrodes need to be inert such as graphite or platinum so that they don’t participate in a
side reaction with the electrolyte.
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Electrolysis of Aqueous Sodium Chloride & Dilute Sulfuric Acid
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Electrolysis of Aqueous Sodium Chloride & Dilute Sulfuric Acid
Aqueous sodium chloride
Brine is a concentrated solution of aqueous sodium chloride
It can be electrolysed using inert electrodes made from platinum or carbon/graphite
When electrolysed, it produces bubbles of gas at both electrodes as chlorine and hydrogen are
produced, leaving behind sodium hydroxide solution
These substances all have important industrial uses:
Chlorine is used to make bleach
Hydrogen is used to make margarine
Sodium hydroxide is used to make soap and detergents
Product at the Negative Electrode:
The H+ ions are discharged at the cathode as they are less reactive than sodium ions
The H+ ions gain electrons to form hydrogen gas
Product at the Positive Electrode:
The Cl– ions are discharged at the anode
They lose electrons and chlorine gas forms
The Na+ and OH– ions remain behind and form the NaOH solution
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Diagram showing the products of the electrolysis of aqueous sodium chloride
Dilute sulfuric acid
Dilute sulfuric acid can be electrolysed using inert electrodes made from platinum or carbon/graphite
Bubbles of gas are seen at both electrodes
Product at the Negative Electrode
H+ ions are attracted to the cathode, gain electrons and form hydrogen gas
Product at the Positive Electrode
OH- ions are attracted to the anode, lose electrons and form oxygen gas and water
Determining what gas is produced
If the gas produced at the anode relights a glowing splint dipped into a sample of the gas then the gas
is oxygen
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Your notes
Diagram showing the test for oxygen gas
If the gas produced at the anode bleaches damp litmus paper then the gas is chlorine
Diagram showing the test for chlorine gas
If the gas produced at the cathode burns with a 'pop' when a sample is lit with a lighted splint then the
gas is hydrogen
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Your notes
Diagram showing the test for hydrogen gas
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Electrolysis of Aqueous Solutions
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Electrolysis of Aqueous Solutions
Aqueous solutions will always have water present (H2O)
In the electrolysis of aqueous solutions, the water molecules dissociate producing H+ and OH– ions:
H2O ⇌ H+ + OH–
These ions are also involved in the process and their chemistry must be considered
We now have an electrolyte that contains ions from the compound plus ions from the water
Which ions get discharged and at which electrode depends on the relative reactivity of the elements
involved
Concentrated and dilute solutions of the same compound give different products
For anions, the more concentrated ion will tend to get discharged over a more dilute ion
Positive Electrode (anode)
Negatively charged OH– ions and non-metal ions are attracted to the positive electrode
If halide ions (Cl-, Br-, I-) and OH- are present then the halide ion is discharged at the anode, loses
electrons and forms a halogen (chlorine, bromine or iodine)
If no halide ions are present, then OH- is discharged at the anode, loses electrons and forms oxygen
gas
In both cases the other negative ion remains in solution
The concentration of the solution also affects which ion is discharged:
If a concentrated halide solution is being electrolysed, the halogen forms at the anode
If a dilute halide solution is being electrolysed, oxygen is formed
For example:
For a concentrated solution of barium chloride, the Cl- ions are discharged more readily than the
OH- ions, so chlorine gas is produced at the anode
If the solution is dilute however only the OH- ion is discharged and so oxygen would be formed
Negative Electrode (cathode)
Positively charged H+ and metal ions are attracted to the negative electrode but only one will gain
electrons
Either hydrogen gas or metal will be produced
If the metal is above hydrogen in the reactivity series, then hydrogen will be produced and bubbling
will be seen at the cathode
This is because the ions of the more reactive metal will remain in the solution, causing the ions of the
least reactive metal to be discharged
Therefore, at the cathode, hydrogen gas will be produced unless the positive ions from the ionic
compound are less reactive than hydrogen, in which case the metal is produced
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Your notes
The reactivity series of metals including hydrogen and carbon
Products formed for Common Aqueous Solutions
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Your notes
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Electrolysis of Aqueous Copper(II) Sulfate
Electrolysis of Aqueous Copper Sulfate Your notes
Aqueous copper sulfate contains the following ions:
Cu2+, SO42-, H+ and OH-
Using graphite electrodes:
Apparatus for the electrolysis of copper(II)sulfate using inert / passive graphite electrodes
Product at the Cathode:
Cu2+ and H+ will both be attracted to the cathode but the less reactive ion will be discharged
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In this case, copper is less reactive than hydrogen
Copper ions are discharged at the cathode, gain electrons and are reduced to form copper metal
The half equation for the reaction at the electrode is: Your notes
Cu2+ + 2e- → Cu
Product at the Anode:
SO42- and OH- are both attracted to the anode
OH- ions lose electrons more readily than SO42-
OH- lose electrons and are oxidised to form oxygen gas
The half equation for the reaction at the anode is
4OH– ⟶ O2 + 2H2O + 4e–
Using copper electrodes:
Apparatus for the electrolysis of copper(II)sulfate using active copper electrodes
Observations at the anode and cathode
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The cathode increases in mass while the anode decreases
This occurs as copper atoms are oxidised at the anode and form ions while copper ions are reduced at
the cathode, forming copper atoms Your notes
The gain in mass by the negative electrode is the same as the loss in mass by the positive electrode
Therefore the copper deposited on the negative electrode must be the same copper ions that are lost
from the positive electrode
That implies that the concentration of the Cu2+ ions in the solution remains constant
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Ionic Half Equations
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Ionic Half Equations
In electrochemistry we are mostly concerned with the transfer of electrons, hence the definitions of
oxidation and reduction are applied in terms of electron loss or gain rather than the addition or removal
of oxygen
Oxidation is when a substance loses electrons and reduction is when a substance gains electrons
As the ions come into contact with the electrode, electrons are either lost or gained and they
form neutral substances
These are then discharged as products at the electrodes
At the anode, negatively charged ions lose electrons and are thus oxidised
At the cathode, the positively charged ions gain electrons and are thus reduced
Ionic half equations show the oxidation and reduction of the ions involved
It is important to make sure the charges are balanced
Table of Reduction and Oxidation Reactions at the Electrodes
Exam Tip
To help you remember the definitions of oxidation and reduction use OIL RIG
Oxidation Is Loss (of electrons) Reduction Is Gain (of electrons)
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Electroplating
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Electroplating
Electroplating is a process where the surface of one metal is coated with a layer of a different metal
The anode is made from the pure metal you want to coat your object with
The cathode is the object to be electroplated
The electrolyte is an aqueous solution of a soluble salt of the pure metal at the anode
Example: coating a strip of iron metal with tin:
A piece of iron being electroplated with tin. The electrolyte is tin(II) chloride, a water-soluble salt of tin
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At the anode: Tin atoms lose electrons to form tin ions in solution
At the cathode: Tin ions gain electrons to form tin atoms which deposit on the strip of iron metal,
coating it with a layer of tin Your notes
Uses of electroplating
Electroplating is done to make metals more resistant to corrosion or damage
e.g, chromium and nickel plating
It is also done to improve the appearance of metals,
e.g. coating cutlery and jewellery with silver
Exam Tip
You may be asked to write the ionic half equations for the reaction at each electrode. For the example
above, these would be:
At the anode: Sn (s) → Sn2+ (aq) + 2e-
At the cathode: Sn2+ (aq) + 2e- → Sn (s)
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