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GCSE Chemistry · GCSEChapter 11 · Electrolysis and oxidation and reduction
GCSE · Chapter 11

Electrolysis and oxidation and reduction

By the end of this chapter you will be able to

  • Explain what an electrolyte is, and what happens at the cathode and anode
  • Name the products of electrolysis of molten compounds and solutions
  • Explain selective discharge and predict the products of an aqueous solution
  • Identify oxidation and reduction in a reaction
  • [H] Write and check half equations for electrode reactions

You should already know

  • Ions and their charges (Chapter 4)
  • Reactivity series and displacement (Chapter 3)

Where this leads

  • Metal extraction and recycling (Chapter 19)
  • Electrolysis of brine and the chemical industry

GCSE Chemistry. Content follows the GCSE (9-1) subject content for England (Edexcel and AQA). Content references: Describe electrolysis as the decomposition of an ionic compound by electricity, and name the products at each electrode, Predict the products of electrolysis of aqueous solutions (selective discharge), Identify oxidation and reduction in terms of electrons and of oxygen, [H] Write half equations for the reactions at the electrodes, and balance electrons, Describe the uses of electrolysis, including aluminium extraction and electroplating.

Exercise P  Do you remember?

1
FWhat is the charge of a chloride ion? Write the sign and number.
2
FWhich ion do acids produce in water? Write the symbol.
3
FWhich group do the halogens belong to? Write the number.
4
FWhat is 2 x 2 x 2? Write the number.
5
FWhich metal is more reactive: copper or magnesium? Write the name.
11.1

Electrolysis of molten compounds

GCSE
Describe electrolysis as the decomposition of an ionic compound by electricity, and name the products at each electrode
Video lesson: Electrolysis of molten compoundsComing soon
Context. In a lecture demonstration, a teacher passes electricity through melted salt and metal appears at one electrode. “How does electricity break the salt apart?” asks a pupil. Electricity causes the ions to move to electrodes, where they gain or lose electrons.

Electrolysis is the breaking down of an ionic compound by electricity. The compound must be molten or dissolved, so that its ions are free to move. The electrolyte is the substance being broken down. The positive electrode is the anode (it attracts negative ions); the negative electrode is the cathode (it attracts positive ions). Positive ions (cations) move to the cathode and are reduced there; negative ions (anions) move to the anode and are oxidised there.

Electrolysis of molten compounds is a teacher demonstration only. The molten compounds are very hot and the products are hazardous.

Electrolysis of molten sodium chloride+ battery -molten sodium chloride (teacher demonstration only)anode (+)cathode (-)Na<sup>+</sup>Cl<sup>-</sup>
Electrolysis of molten sodium chloride: sodium forms at the cathode and chlorine at the anode (not to scale)

Key ideas

  • Electrolysis breaks down an ionic compound using electricity.
  • The ions must be free to move: molten or dissolved.
  • Cations (+) move to the cathode (-), where they gain electrons (reduction).
  • Anions (-) move to the anode (+), where they lose electrons (oxidation).
Worked example 11.1.1
In molten sodium chloride, which ion moves to the cathode? Write the symbol.

Method. Positive ions move to the negative electrode.

  1. Recall. Cathode is negative, so it attracts the positive ions.
    positive
  2. Decide. The positive ion in sodium chloride is Na+.
    Na+
  3. Write the answer.
    Na+

Check. Cl- moves to the anode. ✓

Worked example 11.1.2
In molten lead bromide, which product is formed at the cathode? Write the name.

Method. The cathode gains electrons from the metal ion.

  1. Ion. The cation is Pb2+.
    Pb2+
  2. Product. Pb2+ gains electrons at the cathode and forms lead metal.
    lead
  3. Write the answer.
    lead

Check. Bromine forms at the anode. ✓

Worked example 11.1.3
In molten sodium chloride, which gas is produced at the anode? Write the name.

Method. The anions are oxidised at the anode.

  1. Ion. The anion is Cl-.
    Cl-
  2. Product. Two Cl- lose electrons to form Cl2 gas.
    chlorine
  3. Write the answer.
    chlorine

Check. Chlorine is a green gas. ✓

Watch out

Watch out: in electrolysis, the metal is formed at the negative electrode (cathode), not the positive one. Cations go to the cathode.

Exam tip

Exam tip. Write “cathode: metal” and “anode: non-metal” for a molten binary compound, and check the charges.

Exercise 11A  Electrolysis of molten compounds

1
FWhat is electrolysis? Write the words.
2
FWhat is the name of the electrode that is negative? Write the word.
3
FWhat is the name of the electrode that is positive? Write the word.
4
FWhich ions move to the cathode? Write the word.
5
FIn molten sodium chloride, which metal is formed at the cathode? Write the name.
6
FIn molten sodium chloride, which gas is formed at the anode? Write the name.
7
FIn molten lead bromide, which product is formed at the anode? Write the name.
8
FWhy does electrolysis need the compound to be molten or dissolved? Write the words.
9
FIn molten lead bromide, which ion moves to the anode? Write the symbol.
10
PSWhich electrode does the lead form on: the anode or the cathode? Write the word.
11
PSPredict the product at the anode when molten potassium bromide is electrolysed. Write the name.
12
RExplain why electrolysis is a teacher demonstration only, in one sentence. Accept any sentence that says molten compounds are very hot and the products are hazardous.
13
RExplain why a solid sodium chloride crystal does not conduct electricity, but molten sodium chloride does, in one sentence. Accept any sentence that says the ions are fixed in the solid but free to move when molten.
11.2

Electrolysis of aqueous solutions

GCSE
Predict the products of electrolysis of aqueous solutions (selective discharge)
Video lesson: Electrolysis of aqueous solutionsComing soon
Context. A teacher electrolyses copper sulfate solution with carbon electrodes, and a pupil sees copper form on one electrode. “Why not hydrogen?” asks the pupil. In solution, water also has ions, and the product depends on which ion is discharged more easily.

In an aqueous solution, the water also provides ions: H+ and OH-. At the cathode, the ion that is discharged is the one that is less reactive: a metal less reactive than hydrogen is deposited, but a metal more reactive than hydrogen is not. This is selective discharge. At the anode, a halide ion (Cl-, Br-) is discharged to give a halogen, but in dilute solution oxygen can form instead.

SubstanceFormCathode (-): productAnode (+): product
molten lead bromide, PbBr2moltenleadbromine
molten sodium chloride, NaClmoltensodiumchlorine
aqueous sodium chloride (concentrated)solutionhydrogenchlorine
aqueous copper sulfate, CuSO4solutioncopperoxygen (and the solution becomes acidic)
Products of electrolysis (teacher demonstration only for all of these)

Key ideas

  • Selective discharge: the less reactive ion is discharged at the cathode (copper is deposited, not sodium).
  • Hydrogen is formed at the cathode when the metal is more reactive than hydrogen (for example sodium).
  • Concentrated NaCl solution: hydrogen at the cathode, chlorine at the anode.
  • Copper sulfate solution with inert electrodes: copper at the cathode, oxygen at the anode, and the solution becomes acidic.

Try it (with your teacher). Teacher demonstration only: electrolysis of copper sulfate solution with carbon electrodes. Pupils observe. A copper layer forms on the cathode, and bubbles of oxygen form at the anode. The blue colour of the solution fades as copper is removed. Wear goggles; keep the apparatus under the teacher’s control.

Worked example 11.2.1
In electrolysis of copper sulfate solution, which product forms at the cathode? Write the name.

Method. Copper is less reactive than hydrogen, so it is discharged.

  1. Ions. The cations are Cu2+ and H+.
    Cu2+ and H+
  2. Discharge. Copper is less reactive than hydrogen, so Cu2+ is discharged.
    copper
  3. Write the answer.
    copper

Check. Copper deposits as a red-brown layer. ✓

Worked example 11.2.2
In concentrated sodium chloride solution, which gas forms at the anode? Write the name.

Method. Chloride ions are discharged at the anode when concentrated.

  1. Ions. The anions are Cl- and OH-.
    Cl- and OH-
  2. Discharge. Concentrated chloride ions are discharged to form chlorine.
    chlorine
  3. Write the answer.
    chlorine

Check. Chlorine is made commercially this way. ✓

Worked example 11.2.3
In electrolysis of sodium chloride solution, why is hydrogen formed at the cathode and not sodium? Write one reason.

Method. Compare the reactivity of sodium and hydrogen.

  1. Compare. Sodium is more reactive than hydrogen, so water is reduced instead.
    sodium more reactive
  2. Conclude. So H+ from water is discharged and hydrogen gas forms.
    hydrogen
  3. Write the answer.
    sodium is more reactive than hydrogen, so hydrogen is discharged

Check. Water provides the H+ ions. ✓

Watch out

Watch out: in a solution, the metal is not always formed at the cathode. Only metals less reactive than hydrogen are deposited; a more reactive metal leaves hydrogen to form.

Exam tip

Exam tip. For an aqueous solution, list all the ions present (including H+ and OH- from water), then decide which is discharged at each electrode.

Exercise 11B  Electrolysis of aqueous solutions

1
FIn copper sulfate solution, what is deposited at the cathode? Write the name.
2
FIn copper sulfate solution, which gas forms at the anode? Write the name.
3
FWhat happens to the pH of copper sulfate solution during electrolysis? Write the word, higher or lower.
4
FIn concentrated sodium chloride solution, which gas is formed at the anode? Write the name.
5
FIn concentrated sodium chloride solution, which gas is formed at the cathode? Write the name.
6
FWhat ions are in an aqueous solution from water, in addition to the ions of the solute? Write the formulae.
7
PSWhich metal is NOT deposited on the cathode in electrolysis of its aqueous solution: copper or sodium? Write the name.
8
PSPredict the product at the anode in dilute sodium chloride solution, using oxygen as a possible answer. Write the name of the gas.
9
PSWhich product forms at the cathode when a solution of potassium chloride is electrolysed? Write the name.
10
RExplain why copper forms at the cathode in copper sulfate solution, in one sentence. Accept any sentence that says copper is less reactive than hydrogen, so its ions are discharged.
11
RExplain why the solution becomes more acidic during electrolysis of copper sulfate, in one or two sentences. Accept any answer that says OH- is discharged at the anode, leaving H+ ions behind.
11.3

Oxidation and reduction

GCSE
Identify oxidation and reduction in terms of electrons and of oxygen
Video lesson: Oxidation and reductionComing soon
Context. A pupil heats copper oxide with hydrogen and the black solid turns into a brown metal. “What has happened to the copper and the hydrogen?” asks a pupil. The copper has lost oxygen (it is reduced) and the hydrogen has gained oxygen (it is oxidised). Both ideas come from the same reaction.

Oxidation is the loss of electrons or the gain of oxygen. Reduction is the gain of electrons or the loss of oxygen. Reactions in which both happen are redox reactions. The mnemonic OIL RIG helps: oxidation is loss, reduction is gain (of electrons).

DefinitionOxidationReduction
in terms of electronsloss of electronsgain of electrons
in terms of oxygengain of oxygenloss of oxygen
mnemonicOIL (oxidation is loss)RIG (reduction is gain)
Oxidation and reduction: two ways to describe each

Key ideas

  • Oxidation: loss of electrons; gain of oxygen.
  • Reduction: gain of electrons; loss of oxygen.
  • Redox: a reaction where one substance is oxidised and another is reduced.
  • In electrolysis, the cathode is reduction and the anode is oxidation.
Worked example 11.3.1
In CuO + H2 -> Cu + H2O, which substance is reduced? Write the formula.

Method. Reduction is loss of oxygen.

  1. Look. CuO loses oxygen to form Cu.
    loses O
  2. Decide. So CuO is reduced.
    CuO
  3. Write the answer.
    CuO

Check. The hydrogen gains oxygen and is oxidised. ✓

Worked example 11.3.2
In Mg -> Mg2+ + 2e-, is magnesium oxidised or reduced? Write the word.

Method. Oxidation is loss of electrons.

  1. Look. Magnesium atoms lose 2 electrons to form Mg2+.
    loses 2e-
  2. Decide. Loss of electrons is oxidation.
    oxidised
  3. Write the answer.
    oxidised

Check. The magnesium atoms are the reducing agent. ✓

Worked example 11.3.3
In Cu2+ + 2e- -> Cu, is copper ion reduced or oxidised? Write the word.

Method. Gain of electrons is reduction.

  1. Look. Cu2+ gains 2 electrons to form Cu.
    gains 2e-
  2. Decide. Gain of electrons is reduction.
    reduced
  3. Write the answer.
    reduced

Check. Reduction is gain: RIG. ✓

Watch out

Watch out: a substance that is reduced gains electrons, or loses oxygen. Do not say it is reduced because its mass goes down, or because it is oxidised by the other substance.

Exam tip

Exam tip. Write the change in oxidation for each element, or the electrons gained and lost, and then name the process.

Exercise 11C  Oxidation and reduction

1
FWhat is oxidation in terms of electrons? Write the words.
2
FWhat is reduction in terms of electrons? Write the words.
3
FWhat is oxidation in terms of oxygen? Write the words.
4
FWhat is reduction in terms of oxygen? Write the words.
5
FIn CuO + H2 -> Cu + H2O, which substance is oxidised? Write the formula.
6
FIs the loss of electrons oxidation or reduction? Write the word.
7
FIs the gain of electrons oxidation or reduction? Write the word.
8
PSIn 2Mg + O2 -> 2MgO, which substance is reduced? Write the formula.
9
PSIn Zn + Cu2+ -> Zn2+ + Cu, which substance is oxidised? Write the formula.
10
PSIn the reaction in which iron is oxidised to Fe2+, what happens to the iron atoms? Write the words.
11
RExplain why a reaction that is oxidation is always accompanied by reduction, in one sentence. Accept any sentence that says the electrons lost by one substance are gained by another.
12
RExplain why the reduction of copper oxide is also a loss of oxygen, in one sentence. Accept any sentence that says the oxygen is removed from the copper oxide by the hydrogen.
11.4

Half equations [H]

GCSE
[H] Write half equations for the reactions at the electrodes, and balance electrons
Video lesson: Half equations [H]Coming soon
Context. A pupil writes an equation for the reaction at the cathode, but the charges do not balance. “How do I check?” she asks. A half equation shows only what happens at one electrode, including the electrons, so the charges must balance on both sides.

[H] Higher tier only. A half equation shows the reaction at one electrode. Electrons are shown explicitly: at the cathode, electrons are on the left (reduction); at the anode, electrons are on the right (oxidation). Both the atoms and the total charge must balance. Example: Cu2+ + 2e- -> Cu (cathode); 2Cl- -> Cl2 + 2e- (anode).

Key ideas

  • Cathode: gain of electrons (reduction), e.g. Cu2+ + 2e- -> Cu.
  • Anode: loss of electrons (oxidation), e.g. 2Cl- -> Cl2 + 2e-.
  • Check both atoms and charge balance on each side.
  • Electrons in a half equation are shown as e-.
Worked example 11.4.1
[H] Write the half equation for the formation of aluminium at the cathode, from Al3+. Write the equation.

Method. Aluminium ions gain three electrons.

  1. Ion. Al3+ has charge +3.
    +3
  2. Electrons. It gains 3 electrons to form Al with charge 0.
    3e-
  3. Equation. Al3+ + 3e- -> Al.
    Al3+ + 3e- -> Al
  4. Write the answer.
    Al3+ + 3e- -> Al

Check. Charge: +3 - 3 = 0. ✓

Worked example 11.4.2
[H] Write the half equation for chloride ions discharged at the anode to form chlorine. Write the equation.

Method. Two chloride ions lose one electron each to form Cl2.

  1. Atoms. Two Cl- form one Cl2, so 2Cl-.
    2Cl-
  2. Electrons. Each Cl- loses one electron; charge goes from -2 to 0, so 2e- are released.
    2e-
  3. Equation. 2Cl- -> Cl2 + 2e-.
    2Cl- -> Cl2 + 2e-
  4. Write the answer.
    2Cl- -> Cl2 + 2e-

Check. Charge: -2 on the left, and 0 + (-2) on the right. ✓

Worked example 11.4.3
[H] Are the electrons in the half equation for chloride at the anode on the right or the left? Write the word.

Method. At the anode, electrons are lost.

  1. Recall. Loss of electrons is oxidation at the anode.
    loss
  2. Decide. So the electrons appear on the right, as products.
    right
  3. Write the answer.
    right

Check. The cathode half equation has electrons on the left. ✓

Watch out

Watch out: a half equation must balance both the atoms and the charge. Count the electrons carefully: two chloride ions give two electrons, but one copper ion takes two electrons too.

Exam tip

Exam tip. Balance the atoms first, then add electrons until the charge on each side is equal.

Exercise 11D  Half equations [H]

1
F[H] How many electrons does a Cu2+ ion gain at the cathode? Write the number.
2
F[H] How many electrons does an Al3+ ion gain at the cathode? Write the number.
3
F[H] Write the half equation for lead ions gained at the cathode, Pb2+. Write the equation.
4
F[H] Write the half equation for bromide ions oxidised at the anode, 2Br-. Write the equation.
5
F[H] Which half equation is an oxidation: Cu2+ + 2e- -> Cu or 2Cl- -> Cl2 + 2e-? Write the formula of the species that loses electrons.
6
PS[H] Write the half equation for hydrogen ions gained at the cathode, forming H2. Write the equation.
7
PS[H] Write the half equation for O2- ions oxidised at the anode to O2. Write the equation.
8
PS[H] Balance the charge in Fe3+ + e- -> Fe2+. Is it balanced? Write yes or no.
9
R[H] Explain why both atoms and charge must balance in a half equation, in one sentence. Accept any sentence that says atoms and charge are conserved in every reaction.
11.5

Uses of electrolysis

GCSE
Describe the uses of electrolysis, including aluminium extraction and electroplating
Video lesson: Uses of electrolysisComing soon
Context. A car part is coated with chromium, and a copper wire is cleaned by electrolysis before it is used. “Why is electrolysis used for this?” asks a pupil. Electrolysis can deposit a metal in an even layer, or remove impurities from a metal.

Electrolysis is used to extract very reactive metals such as aluminium, which cannot be reduced with carbon. It is used in electroplating to coat objects with a metal, and to purify copper for electrical wires. The chlorine and hydrogen produced in the electrolysis of brine are useful chemicals.

UseWhat happensWhy electrolysis
extracting aluminiumaluminium oxide dissolved in molten cryolite is electrolysedaluminium is too reactive to extract with carbon
electroplatinga metal object is coated with another metala thin, even layer of metal forms on the cathode
purifying copperimpure copper is the anode; pure copper is deposited on the cathodegives very pure copper for wires
producing chlorine and hydrogenconcentrated brine is electrolysedproduces useful gases and sodium hydroxide
Uses of electrolysis and why it is used

Key ideas

  • Aluminium is extracted by electrolysis of aluminium oxide dissolved in molten cryolite.
  • Electroplating: the object is the cathode, coated with a thin, even layer of metal.
  • Purifying copper: impure copper is the anode; pure copper forms on the cathode.
  • Electrolysis of brine gives chlorine, hydrogen and sodium hydroxide.
Worked example 11.5.1
Why is electrolysis used to extract aluminium, instead of heating with carbon? Write one reason.

Method. Compare the reactivity of aluminium with carbon.

  1. Reactivity. Aluminium is more reactive than carbon.
    more reactive
  2. Conclude. So carbon cannot reduce aluminium oxide; electrolysis is needed.
    electrolysis needed
  3. Write the answer.
    aluminium is too reactive to be extracted by reduction with carbon

Check. Aluminium is in the reactivity series above carbon. ✓

Worked example 11.5.2
In electroplating a metal object with silver, which electrode is the object? Write the word.

Method. The object must be coated, so it is where the metal forms.

  1. Recall. The metal is deposited on the cathode.
    cathode
  2. Decide. So the object is the cathode.
    cathode
  3. Write the answer.
    cathode

Check. The silver is the anode and dissolves into the solution. ✓

Worked example 11.5.3
In purifying copper, which electrode is the impure copper? Write the word.

Method. Impure copper is dissolved away from the anode.

  1. Recall. Impure copper is the anode.
    anode
  2. Decide. Pure copper is then deposited on the cathode.
    pure copper
  3. Write the answer.
    anode

Check. Pure copper forms on the cathode. ✓

Watch out

Watch out: in electroplating, the object must be the cathode. The metal being plated comes from the anode.

Exam tip

Exam tip. Name the electrode and say what happens there: deposition at the cathode or dissolving at the anode.

Exercise 11E  Uses of electrolysis

1
FWhich metal is extracted by electrolysis of molten aluminium oxide? Write the name.
2
FWhich substance is the electrolyte in aluminium extraction, along with aluminium oxide? Write the name.
3
FIn electroplating, which electrode is the object being plated? Write the word.
4
FIn purifying copper, which electrode is pure copper formed on? Write the word.
5
FName one use of electroplating. Write the use. Accept any of: chromium plating, silver plating, gold plating of jewellery.
6
FName two products of the electrolysis of concentrated sodium chloride solution. Write the two names. Accept any two of: chlorine, hydrogen, sodium hydroxide.
7
PSWhy must electrolysis be used to extract aluminium? Write one reason.
8
PSIn electroplating a metal object with copper, which solution is used? Write the solute. Accept copper sulfate.
9
RExplain why pure copper is preferred for electrical wires, in one sentence. Accept any sentence that says impurities reduce the conductivity, so pure copper conducts better.
10
RExplain why electroplating is useful, in one or two sentences. Accept any answer that says it protects a metal from corrosion or improves its appearance.

Chapter summary

SkillMethod
ElectrolysisBreaking an ionic compound with electricity; the electrolyte must be molten or dissolved.
ElectrodesCathode (-): reduction, cations go here. Anode (+): oxidation, anions go here.
Aqueous solutionsSelective discharge: the less reactive ion is discharged; H+ and OH- from water matter.
Oxidation and reductionOxidation: loss of electrons or gain of oxygen. Reduction: gain of electrons or loss of oxygen.
[H] Half equationsShow the electrons; balance atoms and charge on each side.
UsesAluminium extraction (molten cryolite), electroplating, purifying copper, chlorine and hydrogen from brine.

Can you…?

  • I can name the electrode products of molten and aqueous electrolysis
  • I can predict products using selective discharge
  • I can identify oxidation and reduction in terms of electrons and oxygen
  • [H] I can write and check half equations
  • I can describe the uses of electrolysis, including aluminium extraction and electroplating

Key terms

electrolysis
the breaking down of an ionic compound by electricity
electrolyte
the molten or dissolved ionic compound that is electrolysed
cathode
the negative electrode, where positive ions gain electrons
anode
the positive electrode, where negative ions lose electrons
selective discharge
the ion that is discharged at an electrode is the one less reactive than the others
oxidation
the loss of electrons, or the gain of oxygen
reduction
the gain of electrons, or the loss of oxygen
half equation
[H] an equation showing the electrons gained or lost at one electrode

Exercise 11Z  Check-up

1
What is the name of the negative electrode in electrolysis? Write the word.
2
In molten sodium chloride, which product forms at the cathode? Write the name.
3
In copper sulfate solution, which product forms at the anode? Write the name.
4
What is oxidation in terms of electrons? Write the words.
5
In CuO + H2 -> Cu + H2O, which substance is reduced? Write the formula.
6
[H] Write the half equation for Al3+ gaining electrons to form Al. Write the equation.
7
Which product forms at the anode in molten lead bromide? Write the name.
8
What is electroplating? Write the words.
9
Explain why a solid ionic compound cannot be electrolysed, in one sentence. Accept any sentence that says its ions are fixed and cannot move to the electrodes.
10
Explain why aluminium is extracted by electrolysis, in one sentence. Accept any sentence that says aluminium is too reactive to be reduced by carbon.

Total 20 marks. Allow about 20 minutes.

Exercise 11X  Exam-style questions

1
FName the products at each electrode in molten sodium chloride. Write two names, cathode then anode.[2 marks]
2
RName the product formed at the cathode in copper sulfate solution, and explain why in one sentence. Accept copper, with a reason that copper is less reactive than hydrogen.[3 marks]
3
PSIdentify the substance that is oxidised in Zn + Cu2+ -> Zn2+ + Cu. Write the formula.[1 mark]
4
PSWrite the half equation for chloride ions oxidised at the anode. Write the equation.[2 marks]
5
RExplain why electrolysis is used for purifying copper, in one or two sentences. Accept any answer that says impure copper is the anode and pure copper is deposited on the cathode.[3 marks]
6
RExplain why aluminium oxide must be dissolved in molten cryolite, in one or two sentences. Accept any answer that says cryolite lowers the melting point, saving energy.[2 marks]
7
PS[H] Write the half equation for the reduction of Cu2+ at the cathode. Write the equation.[2 marks]
8
RExplain what happens at the anode in electrolysis, using the words oxidation and electrons, in one sentence. Accept any sentence that says ions lose electrons, which is oxidation.[2 marks]
9
RName one use of electrolysis, and describe what happens at the electrode in that use. Accept any use with a correct electrode statement, such as electroplating with the object at the cathode.[3 marks]
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