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Cambridge (CIE) IGCSE Your notes
Chemistry
Electrolysis
Contents
Electrolysis Principles
Electrolysis of Molten Compounds
Electrolysis of Aqueous Sodium Chloride & Dilute Sulfuric Acid
Electrolysis of Aqueous Solutions
Ionic Half Equations
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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
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Anion is a negatively charged ion which is attracted to the anode
Cathode is the negative electrode of an electrolysis cell Your notes
Cation is a positively charged ion which is attracted to the cathode
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
Examiner Tips and Tricks
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Use the PANIC mnemonic to remember which electrode is the positive and which is the negative:
Positive (is) Anode Negative Is Cathode Your notes
Electrolysis: charge transfer
Extended tier only
During electrolysis, current needs to flow around the circuit
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
Page 4 of 21
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Your notes
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
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
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The potassium ions are attracted to the cathode and form potassium metal
Your notes
Examiner Tips and Tricks
Remember: Electrodes need to be inert such as graphite or platinum so that they don’t participate
in a side reaction with the electrolyte.
Page 8 of 21
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Electrolysis of Aqueous Sodium Chloride & Dilute Sulfuric Acid
Your notes
Electrolysis of aqueous sodium chloride & dilute
sulfuric acid
Electrolysis of 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
The ions in brine are:
Na+ and Cl– ions from the brine / aqueous sodium chloride
H+ and OH– ions from the water
When electrolysed, it produces bubbles of gas at both electrodes
The gases chlorine and hydrogen are produced
Sodium hydroxide solution is the product remaining in the electrolysis chamber / container
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
Page 9 of 21
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Your notes
Diagram showing the products of the electrolysis of aqueous sodium chloride
Electrolysis of dilute sulfuric acid
Dilute sulfuric acid can be electrolysed using inert electrodes made from platinum or carbon / graphite
The ions in dilute sulfuric acid are:
H+ and SO42– ions from the sufuric acid
H+ and OH– ions from the water
When electrolysed, it produces bubbles of gas at both electrodes
The gases oxygen and hydrogen are produced
Page 10 of 21
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Your notes
Electrolysing dilute sulfuric acid in a Hoffman voltameter shows that twice as much hydrogen is
produced, compared to oxygen
Product at the negative electrode:
H+ ions are attracted to the cathode
The H+ ions gain electrons to form hydrogen gas
Product at the positive electrode:
OH- ions are attracted to the anode
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They lose electrons and form oxygen gas and water
Your notes
Examiner Tips and Tricks
When a gas is produced during electrolysis, you should be able to give the appropriate gas test:
Oxygen - if a glowing splint is dipped into a sample of the gas, then the splint will relight
Hydrogen - if a lit splint is dipped into a sample of the gas, then a squaeky pop will be heard
Chlorine - if damp litmus paper is dipped into a sample of the gas, it will turn red and then
bleach to a white colour
Page 12 of 21
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Electrolysis of Aqueous Solutions
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Electrolysis of aqueous solutions
Extended tier only
Aqueous solutions always have water, H2O, present
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–) are present, the halogen is produced at the anode
The halide ions lose electrons and forms the halogen (chlorine, bromine or iodine)
If there are no halide ions but OH– ions are present, oxygen is produced at the anode
The hydroxide ions lose electrons and forms oxygen gas (and water)
In both cases, the other negative ion remains in solution
How concentration affects products at the anode
The concentration of the solution affects the ion being discharged:
If a concentrated halide solution is being electrolysed, the halogen forms at the anode
If a dilute halide solution is being electrolysed, oxygen forms at the anode
For example:
For concentrated barium chloride solution :
Cl– ions are discharged more readily than the OH– ions
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So, chlorine gas is produced at the anode
For dilute barium chloride solution:
Your notes
Only OH– ions are discharged
So, oxygen is produced at the anode
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:
The ions of the more reactive metal remain in solution
This causes the less reactive hydrogen ions, H+, to be discharged
So, hydrogen will be produced and bubbling will be seen at the cathode
If the metal is below hydrogen in the reactivity series:
The less reactive metal ions are discharged
So, the metal is produced and this will be seen plating onto the cathode
Page 14 of 21
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Your notes
The reactivity series of metals including hydrogen and carbon
Electrolysis of aqueous copper sulfate
Aqueous copper sulfate contains the following ions:
Cu2+ and SO42– from the copper sulfate
H+ and OH– from the water
Using graphite electrodes:
Page 15 of 21
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Your notes
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
In this case, copper is less reactive than hydrogen
Copper ions are discharged at the cathode
They gain electrons and are reduced to form copper metal
The half equation for the reaction at the electrode is:
Cu2+ + 2e– → Cu
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Product at the anode:
SO42– and OH– are both attracted to the anode Your notes
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:
The cathode increases in mass
This is because copper ions, Cu2+, are reduced at the cathode and form copper atoms
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The anode decreases in mass
This is because copper atoms are oxidised at the anode and form copper ions, Cu2+ 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
This implies that the concentration of the Cu2+ ions in the solution remains constant
Products formed for common aqueous solutions
Aqueous solution - ions present Product at the anode Product at the cathode
Concentrated sodium chloride, NaCl Chlorine gas Hydrogen gas
Dilute sodium chloride, NaCl Oxygen gas Hydrogen gas
Concentrated aqueous copper(II) sulfate, CuSO4 Oxygen gas Copper
Dilute sulfuric acid, H2SO4 Oxygen gas Hydrogen gas
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Ionic Half Equations
Your notes
Ionic half equations
Extended tier only
Electrochemistry is concerned with the transfer of electrons
This is why the definitions of oxidation and reduction are in terms of losing or gaining electrons, not
oxygen
Oxidation is the loss of electrons
Reduction is the gain of electrons
As the ions come into contact with the electrode:
Electrons are lost or gained
The ions form neutral substances
These substances are discharged as products at the electrodes
At the anode, negatively charged ions lose electrons
So, oxidation occurs at the anode
At the cathode, the positively charged ions gain electrons
So, reduction occurs at the cathode
Ionic half-equations only show half of what is happening in a reaction involving electron transfer
The ionic half-equation for oxidation shows the loss of electrons
The ionic half-equation for reduction shows the gain of electrons
Ionic half-equations must have the atoms and charges balanced
Writing ionic half-equations
Metals
Metals are positive ions, e.g. Li+, Cu2+, Al3+
If a metal is produced during electrolysis:
The metal ions will gain electrons to form metal atoms
The metal ions will be reduced
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The charge on the metal ion indicates the number of electrons that will be gained
For example: Your notes
Li+ + e– → Li
Cu2+ + 2e– → Cu
Al3+ + 3e– → Al
Non-metals
Non-metals are typically negative ions, e.g. Cl–, Br–, OH–
The hydrogen ion, H+, is an exception to this
Hydrogen ion / hydrogen half-equation:
If hydrogen is formed during electrolysis:
Two hydrogen ions will gain 2 electrons to form hydrogen, H2
Hydrogen ions are reduced
2H+ + 2e– → H2
Halide ion / halogen half-equation:
If a halogen is formed during electrolysis:
Two halide ions will lose 2 electrons to form the halogen
Halide ions are oxidised
2X– → X2 + 2e–
Hydroxide ion / oxygen half-equation:
The ionic half-equation for hydroixde ions forming oxygen is more challenging to balance
Hydroxide ions lose electrons to form oxygen, O2, and water, H2O
Hydroxide ions are oxidised
4OH– → O2 + 2H2O + 4e–
Table of reduction and oxidation reactions at the electrodes
Electrolysis of Anode reaction Cathode reaction
Molten lead(II) bromide, PbBr2 2Br– → Br2 + 2e– Pb2+ + 2e– → Pb
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Concentrated aqueous sodium chloride, NaCl 2Cl– → Cl2 + 2e– 2H+ + 2e– → H2
Your notes
Dilute sulfuric acid, H2SO4 4OH– → O2 + 2H2O + 4e– 2H+ + 2e– → H2
Examiner Tips and Tricks
To help you remember the definitions of oxidation and reduction, use OIL RIG
Oxidation Is Loss (of electrons) Reduction Is Gain (of electrons)
To help you remember where oxidation and reduction take place, use a RED CAT and AN OX
REDuction at the CAThode
ANode for OXidation
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