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IB DemystifiedMYP Sciences
Electrolysis
Pass a current through a solution and it splits: metal coats one electrode, gas bubbles from the other. The same idea makes aluminium, plates jewellery and may fuel the steelworks of the future.
Recommended for MYP 5 · eAssessment priority · About 4 lessons · Criteria A, B, C and D
Figure 1. Electrolysis of copper(II) chloride solution.
describe electrolysis, electrolytes, anodes and cathodes
explain the movement and discharge of ions at the electrodes
predict the products of electrolysing molten compounds and aqueous solutions
interpret half equations as oxidation and reduction
explain aluminium extraction, electroplating and copper purification
relate the mass of product to current and time
Before you start
You will use these skills. If any feel shaky, review them first.
ions and ionic compounds, and the charges on common ions
the reactivity series of metals
current in circuits (see Electrical power and electricity in the home)
Key vocabulary
Electrolyte
A molten or dissolved ionic compound that conducts and is decomposed by electricity.
Anode / cathode
The positive electrode / the negative electrode.
Cation / anion
A positive ion, attracted to the cathode / a negative ion, attracted to the anode.
Discharge
When an ion gains or loses electrons at an electrode and becomes a neutral atom or molecule.
Oxidation / reduction
Loss of electrons (at the anode) / gain of electrons (at the cathode). Remember OIL RIG.
Inert electrode
An electrode, such as carbon or platinum, that does not react during electrolysis.
Understanding electrolysis
What is it?
Electrolysis uses a direct current to decompose an ionic compound that is molten or dissolved. Positive ions move to the cathode and gain electrons; negative ions move to the anode and lose electrons.
at the cathode: Cu²⁺ + 2e⁻ → Cu at the anode: 2Cl⁻ → Cl₂ + 2e⁻
Why does it happen?
The power supply pumps electrons into the cathode and pulls them out of the anode. Opposite charges attract, so the ions move through the liquid, carrying the current. At the electrodes, the ions exchange electrons and become neutral atoms or molecules.
How do we know?
We see the products: metal coating the cathode, bubbles of gas at the anode, and colour changes in the solution. In the 1830s Michael Faraday showed that the mass of product is proportional to the charge passed, which supports the idea of ions carrying fixed amounts of charge.
Why does it matter?
Electrolysis produces aluminium, chlorine and sodium hydroxide, purifies copper for wiring, and electroplates objects to stop corrosion. It may also produce the green hydrogen needed to cut emissions from heavy industry.
What does it connect to?
Electrolysis links to ionic bonding, the reactivity series and redox reactions in chemistry; to current and charge in physics; and to energy, resources and recycling in environmental science.
Predicting products
Molten compounds: the metal forms at the cathode and the non-metal at the anode (molten lead bromide gives lead and bromine).
Solutions: water also provides H⁺ and OH⁻ ions, so there is competition.
Cathode: the metal forms if it is less reactive than hydrogen (copper, silver); otherwise hydrogen forms (sodium, potassium).
Anode: a halogen forms from concentrated halide solutions (chloride gives chlorine); otherwise oxygen forms (sulfates, nitrates).
Active electrodes: a copper anode dissolves instead, which is used to purify copper and to electroplate.
Electrolysis in industry
Aluminium is made by electrolysing aluminium oxide dissolved in molten cryolite, using enormous amounts of electricity. The chlor-alkali industry electrolyses brine to make chlorine for water treatment and PVC, hydrogen, and sodium hydroxide for soap and paper. Chrome and nickel plating protect car parts and taps from corrosion.
Worked examples
Example 1: predicting products
Predict the products of electrolysing potassium iodide solution with carbon electrodes.
Cathode: potassium is more reactive than hydrogen, so hydrogen forms.
Anode: iodide is a halide, so iodine forms (the solution turns brown).
Potassium and hydroxide ions remain, forming potassium hydroxide solution.
Example 2: mass and current
A current of 0.4 A deposits 0.079 g of copper in 10 minutes. What mass would 0.4 A deposit in 25 minutes?
Mass is proportional to time at a fixed current.
Mass per minute = 0.079 ÷ 10 = 0.0079 g/min.
Mass in 25 minutes = 0.0079 × 25 ≈ 0.20 g.
In the eAssessment
Electrolysis questions often show a cell diagram or a results table and ask you to predict, explain and evaluate. Expect:
Predict: products for molten and aqueous electrolytes, using reactivity and the halide rule.
Explain: ion movement and discharge in terms of charges and electrons, and link to oxidation and reduction.
Interpret: mass–time and mass–current data, spotting proportional relationships and anomalies.
Discuss: aluminium production and recycling, or green hydrogen, weighing energy, cost and environment.
Common ways to lose marks: saying electrons flow through the electrolyte (ions carry the current); swapping anode and cathode; predicting sodium metal from a solution; and unbalanced half equations such as Cl⁻ → Cl + e⁻.
Check your understanding
Quick questions on the ideas above. Try each one before using a hint.
Practice questions
Show
Investigation: how does time affect the mass of copper deposited?
Partially guided investigation · about 60 minutes · pairs
Research question
How does the time of electrolysis (5 to 25 minutes) affect the mass of copper deposited on the cathode in copper(II) sulfate solution at a constant current of 0.50 A?
Scientific background
Each copper ion needs two electrons to become a copper atom. At a constant current, electrons are supplied at a steady rate.
Hypothesis
Write your own hypothesis, with a justification based on ions and electrons.
Variables
Identify the independent and dependent variables, and at least three control variables, explaining how and why you will control each.
Clean the cathode with emery paper, rinse, dry and weigh it.
Set up the circuit and adjust the variable resistor to give 0.50 A; keep it there throughout.
After 5 minutes, switch off, gently rinse and dry the cathode, and reweigh it.
Repeat for 10, 15, 20 and 25 minutes, ideally with a fresh cathode each time.
Plot increase in mass against time.
Safety. Wear eye protection: copper(II) sulfate is harmful and irritates eyes and skin. Use only a low-voltage supply. Dispose of the solution as your teacher instructs, not down the sink.
Results from one student are used in the Criterion C questions below. Then evaluate: why is it better to use a fresh cathode for each time?
Criterion-linked questions
Criterion B: inquiring and designing
Criterion C: processing and evaluating
Criterion D: reflecting on the impacts of science
Challenge questions
Harder problems in unfamiliar contexts. Plan before you calculate.
Topic check
Five questions picked at random from the whole topic. Take a new set whenever you like.
Review your mistakes
Questions you got wrong on this device appear here so you can try them again. Answer one correctly and it leaves the list.
Your progress
Tracked separately for each skill, on this device only.
