Electrolysis

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レッスンノート

What is Electrolysis?

Electrolysis is the breakdown of an ionic compound, when molten or in aqueous solution, by passing an electric current through it.
Liquids and solutions that conduct electricity are called electrolytes.
Covalent compounds cannot conduct electricity, so they do not undergo electrolysis.
An electrolytic cell is the apparatus used for electrolysis; it contains two electrodes (rods of metal or graphite) dipping into the electrolyte.
The anode is the positive electrode; the cathode is the negative electrode.
Cations (positive ions) are attracted to the cathode; anions (negative ions) are attracted to the anode.
When a metal conducts, electrons move through it; when a salt solution conducts, ions move towards the electrodes.

Apparatus for the electrolysis of copper(II) sulfate (graphite electrodes)

Why Ionic Compounds Must Be Molten or Dissolved

Solid ionic compounds cannot conduct electricity because their ions are fixed in position and cannot move.
When melted or dissolved, the ions become free to move and can carry charge.
Ionic compounds must be molten or dissolved in solution (usually aqueous) to undergo electrolysis.
In the molten or dissolved state, the ions migrate towards the electrodes when a current is passed.

Electrical conductivity of ionic compounds

Electrolysis of Molten Compounds

Binary ionic compounds contain just two elements joined by ionic bonding.
When heated beyond their melting point, they become molten and their ions can move freely, so they conduct electricity.
The positive ion migrates to the cathode and the negative ion migrates to the anode.
The product at the cathode is always the metal; the product at the anode is always the non-metal.
For example, molten lead(II) bromide produces lead at the cathode and bromine at the anode.
Zinc chloride produces zinc at the cathode and chlorine at the anode.
Electrodes must be inert (e.g., graphite or platinum) so they do not react with the electrolyte.

Electrolysis of molten lead(II) bromide

Electrolysis of Aqueous Solutions

Aqueous solutions always contain water molecules, which split into H⁺ and OH⁻ ions.
These ions take part in electrolysis along with ions from the dissolved compound.
At the cathode, H⁺ and metal ions are attracted; hydrogen is produced if the metal is above hydrogen in the reactivity series, otherwise the metal is produced.
At the anode, OH⁻ and non-metal ions are attracted; if halide ions (Cl⁻, Br⁻, I⁻) are present, the halogen is produced, otherwise oxygen is produced.
The reactivity series helps predict which ion is discharged at each electrode.
The other ion remains in solution.

Electrolysis of aqueous sodium chloride (brine)

Predicting Products in Aqueous Solutions

Sodium chloride solution: chlorine at the anode, hydrogen at the cathode.
Copper(II) chloride solution: chlorine at the anode, copper at the cathode.
Sodium sulfate solution: oxygen at the anode, hydrogen at the cathode.
Dilute sulfuric acid: oxygen at the anode, hydrogen at the cathode.
Copper(II) sulfate solution (with inert electrodes): oxygen at the anode, copper at the cathode.
The products depend on the relative reactivity of the elements involved.

Using Electrolysis to Extract Metals

Metals above carbon in the reactivity series are too reactive to be extracted by reduction with carbon; they are extracted by electrolysis of their molten compounds.
Metals below carbon can be extracted by heating with a reducing agent such as carbon or carbon monoxide.
Electrolysis is expensive because large amounts of energy are needed to melt the ore and produce the electric current.
Aluminium is extracted from bauxite (aluminium oxide) by electrolysis.
Aluminium oxide is dissolved in molten cryolite to lower the melting point and reduce energy costs.
The cathode produces aluminium; the anode produces oxygen.
The carbon anodes react with oxygen to form carbon dioxide, so they wear away and must be replaced regularly.

Electrolysis and Redox

Oxidation is the loss of electrons; reduction is the gain of electrons.
At the anode, negative ions lose electrons and are oxidised.
At the cathode, positive ions gain electrons and are reduced.
Half equations show the electron transfer at each electrode.
For molten lead(II) bromide: cathode: Pb²⁺ + 2e⁻ → Pb (reduction); anode: 2Br⁻ → Br₂ + 2e⁻ (oxidation).
For aqueous sodium chloride: cathode: 2H⁺ + 2e⁻ → H₂; anode: 2Cl⁻ → Cl₂ + 2e⁻.
For dilute sulfuric acid: cathode: 2H⁺ + 2e⁻ → H₂; anode: 2H₂O → O₂ + 4H⁺ + 4e⁻.
For aqueous copper(II) chloride: cathode: Cu²⁺ + 2e⁻ → Cu; anode: 2Cl⁻ → Cl₂ + 2e⁻.

Core Practical: Electrolysis of Copper(II) Sulfate

Part 1 – Inert (graphite) electrodes: copper is formed at the cathode and oxygen at the anode.
Part 2 – Active (copper) electrodes: the cathode increases in mass and the anode decreases in mass.
This happens because copper atoms at the anode are oxidised to Cu²⁺ ions, and Cu²⁺ ions at the cathode are reduced to copper atoms.
The gain in mass at the cathode equals the loss in mass at the anode, so the concentration of Cu²⁺ ions remains constant.
Hazards: copper(II) sulfate is corrosive and harmful; propanone is flammable.
Precautions: wear safety goggles, avoid skin contact, keep propanone away from naked flames.

Copper Refining by Electrolysis

Electrolysis can purify copper using a pure copper cathode and an impure copper anode.
The electrolyte is an aqueous solution of a soluble salt of the pure metal, e.g., copper(II) sulfate.
At the anode, copper atoms lose electrons and go into solution as Cu²⁺ ions: Cu → Cu²⁺ + 2e⁻.
Impurities fall to the bottom as anode sludge, which may contain valuable metals like silver.
At the cathode, Cu²⁺ ions gain electrons to form pure copper atoms: Cu²⁺ + 2e⁻ → Cu.
The cathode gradually becomes thicker as pure copper is deposited.

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練習問題

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1.Draw one line from each electrolysis keyword to its correct definition.
Medium
- electrolyte
- anode
- cathode
- anion
- cation
- positive electrode
- negative electrode
- negatively charged ion attracted to the anode
- positively charged ion attracted to the cathode
- ionic compound in molten or dissolved solution that conducts electricity
2.What is another name for the copper sulfate solution in an electrolysis experiment?
Medium
- AElectric
- BElectrolyte
- CElectrode
- DElectron
3.What is the name of the positive electrode?
Easy
- AAnion
- BAnode
- CCation
- DCathode
4.Which row correctly names the H+ ion and the OH− ion?
Easy
- AH+ ion: Hydrate; OH− ion: Oxide
- BH+ ion: Hydrate; OH− ion: Hydroxide
- CH+ ion: Hydrogen; OH− ion: Hydroxide
- DH+ ion: Hydrogen; OH− ion: Oxide
5.Place ticks by the two statements that are correct.
Medium
- APositive ions move towards the negative electrode
- BNegative ions move towards the cathode
- CNegative ions remain in solution
- DPositive ions move towards the anode
- ENegative ions move towards the anode
6.Which of these solutions are electrolytes?
Medium
- AK only
- BJ and L only
- CK and L only
- DJ, K and L
7.During the electrolysis of copper sulfate solution, copper ions move to the negative electrode. What is the reason for this?
Medium
- ACopper is less reactive than hydrogen and consists of negatively charged ions
- BCopper is more reactive than hydrogen and consists of negatively charged ions
- CCopper is less reactive than hydrogen and consists of positively charged ions
- DCopper is more reactive than hydrogen and consists of positively charged ions
8.Which statement explains why the mass of the negative electrode increases during the electrolysis of copper sulfate solution with copper electrodes?
Medium
- ACopper atoms are attracted to the anode
- BCopper ions are attracted to the anode
- CCopper atoms are attracted to the cathode
- DCopper ions are attracted to the cathode

もっと問題

9.During the electrolysis of copper sulfate solution, oxygen gas is produced at the positive electrode. The unbalanced half equation is: ___ OH− → O2 + ___ H2O + ___ e−. Which numbers correctly balance the half equation?
Medium
- A2, 2, 2
- B2, 4, 2
- C4, 2, 4
- D4, 4, 2
10.Negative ions are oxidised at the anode during electrolysis.
Easy
True or false?
11.Covalent compounds can conduct electricity and undergo electrolysis.
Easy
True or false?
12.Place the steps in the correct order for the electrolysis of molten lead(II) bromide using graphite electrodes.
Medium
- Heat the lead(II) bromide until it becomes molten
- Insert two graphite rods as electrodes
- Connect the electrodes to a power pack or battery
- Turn on the power pack to start electrolysis
- Observe brown bromine gas at the anode and grey lead metal at the cathode
13.Which of the following statements about the electrolysis of molten lead(II) bromide are true? (select all that apply)
Medium
- ALead is produced at the cathode
- BBromine is produced at the anode
- CBromide ions lose electrons at the anode
- DLead ions gain electrons at the anode
- EHydrogen gas is produced at the cathode
14.What are the products formed when molten zinc bromide is electrolysed?
Medium
- AZinc and oxygen
- BZinc and bromine
- CHydrogen and bromine
- DHydrogen and oxygen
15.Why is cryolite used in the extraction of aluminium by electrolysis?
Medium
- AIt reacts with aluminium oxide to produce aluminium
- BIt lowers the melting point of aluminium oxide so less energy is needed
- CIt increases the melting point of aluminium oxide so the reaction is faster
- DIt acts as a reducing agent to extract aluminium
16.Why does the positive electrode need to be continually replaced during the extraction of aluminium?
Medium
- AThe carbon anode reacts with oxygen to form carbon dioxide and wears away
- BThe carbon anode melts at the high temperature
- CThe aluminium produced deposits on the anode
- DThe cryolite dissolves the anode
17.Which statement is true for what happens at the negative electrode during the electrolysis of molten lead bromide?
Medium
- ALead atoms are oxidised
- BLead atoms are reduced
- CLead ions are oxidised
- DLead ions are reduced
18.Which of the following are true about electrolysis? (select all that apply)
Medium
- AOxidation is loss of electrons
- BReduction is gain of electrons
- CAt the anode, positive ions gain electrons
- DAt the cathode, positive ions gain electrons
- EAt the anode, negative ions lose electrons
19.Match each ion to the electrode it is attracted to during electrolysis.
Medium
- Cation (positive ion)
- Anion (negative ion)
- Cathode (negative electrode)
- Anode (positive electrode)
20.Which of the following statements about the movement of charge during electrolysis are correct? (select all that apply)
Medium
- AElectrons flow through the external circuit from the anode to the cathode
- BPositive ions in the electrolyte migrate towards the cathode
- CNegative ions in the electrolyte migrate towards the cathode
- DElectrons flow through the electrolyte from the cathode to the anode
- EIons in the electrolyte carry charge by moving to the electrodes
21.A student electrolyses aqueous sodium sulfate solution using inert graphite electrodes. Which statements about the products are correct? (select all that apply)
Medium
- AOxygen gas is produced at the anode
- BHydrogen gas is produced at the cathode
- CSodium metal is produced at the cathode
- DSulfate ions are discharged at the anode
- EThe solution contains H+ and OH− ions from water
22.Place the following stages of the electrolysis of molten lead(II) bromide into the correct order.
Medium
- Lead(II) bromide is heated until it melts
- Graphite electrodes are placed in the molten lead(II) bromide
- The power supply is switched on
- Bromide ions lose electrons at the anode to form bromine gas
- Lead ions gain electrons at the cathode to form lead metal
23.In the industrial extraction of aluminium by electrolysis, which statements are correct? (select all that apply)
Hard
- AAluminium is produced at the cathode
- BOxygen is produced at the anode
- CThe carbon anodes react with the oxygen produced and wear away
- DCryolite is added to raise the melting point of the electrolyte
- EThe electrolyte is an aqueous solution of aluminium oxide
24.During the electrolysis of aqueous copper(II) sulfate with copper electrodes, the mass gained by the cathode equals the mass lost by the anode.
Easy
True or false?
25.During the electrolysis of aqueous sodium chloride solution, chlorine gas is produced at the cathode.
Medium
True or false?
26.Match each aqueous solution to the product formed at the anode when it is electrolysed with inert electrodes.
Medium
- Sodium chloride solution
- Copper(II) sulfate solution
- Dilute sulfuric acid
- Chlorine gas
- Oxygen gas
27.Which of the following statements about electrolysis are correct? (select all that apply)
Medium
- AElectrolysis breaks down an ionic compound when a current is passed through it
- BAn electrolyte is a liquid or solution that can conduct electricity
- CCovalent compounds are good electrolytes
- DElectrolysis of a solid ionic compound works because the ions can move
- EPositively charged ions are called cations
28.A student sets up an electrolytic cell with two graphite rods in a beaker of copper(II) sulfate solution. The rod connected to the negative terminal of the power supply is called the cathode. Which of the following correctly describes the movement of ions in this cell?
Medium
- APositive ions move to the cathode and negative ions move to the anode.
- BPositive ions move to the anode and negative ions move to the cathode.
- CBoth positive and negative ions move to the cathode.
- DPositive ions remain in solution while negative ions move to the anode.
29.When molten zinc bromide is electrolysed, what are the products formed at the anode and cathode?
Medium
- AAnode: bromine, Cathode: zinc
- BAnode: zinc, Cathode: bromine
- CAnode: oxygen, Cathode: hydrogen
- DAnode: bromine, Cathode: hydrogen
30.During the electrolysis of aqueous copper(II) sulfate using inert graphite electrodes, which product is formed at the positive electrode?
Medium
- AOxygen gas
- BCopper metal
- CChlorine gas
- DHydrogen gas
31.In the electrolysis of aqueous sodium chloride solution using inert electrodes, which of the following statements about the products is correct?
Medium
- AHydrogen is produced at the cathode and chlorine at the anode.
- BSodium is produced at the cathode and chlorine at the anode.
- CHydrogen is produced at the anode and chlorine at the cathode.
- DOxygen is produced at the anode and hydrogen at the cathode.
32.During the electrolysis of molten lead(II) bromide, which half-equation correctly represents the reaction at the anode?
Hard
- A2Br⁻ → Br₂ + 2e⁻
- BPb²⁺ + 2e⁻ → Pb
- C2H⁺ + 2e⁻ → H₂
- D4OH⁻ → O₂ + 2H₂O + 4e⁻
33.In the electrolysis of aqueous copper(II) sulfate with copper electrodes, the mass of the anode decreases. Which half-equation explains this change?
Medium
- ACu → Cu²⁺ + 2e⁻
- BCu²⁺ + 2e⁻ → Cu
- C2H⁺ + 2e⁻ → H₂
- D4OH⁻ → O₂ + 2H₂O + 4e⁻
34.Which statement best describes the process of electrolysis?
Medium
- AThe decomposition of an ionic compound, when molten or in aqueous solution, by the passage of an electric current.
- BThe formation of an ionic compound by the reaction of a metal with a non-metal.
- CThe separation of a mixture of liquids by heating and condensation.
- DThe movement of electrons through a metal wire to produce a magnetic field.
35.Which of the following statements about the electrolysis of aqueous solutions using inert electrodes are correct? (select all that apply)
Hard
- AAt the cathode, hydrogen is produced if the metal is more reactive than hydrogen.
- BAt the anode, oxygen is produced if halide ions are present.
- CAt the cathode, the metal is produced if it is less reactive than hydrogen.
- DAt the anode, a halogen is produced if halide ions are present.
- EWater molecules do not take part in the electrolysis of aqueous solutions.
36.Which of the following are true about the extraction of aluminium by electrolysis? (select all that apply)
Medium
- AAluminium is extracted by electrolysis because it is more reactive than carbon.
- BThe electrolyte is a mixture of aluminium oxide and cryolite.
- CThe positive electrode is made of carbon and reacts with oxygen, so it needs regular replacement.
- DAluminium is produced at the anode.
- ECryolite is added to increase the melting point of the electrolyte.
37.Which of the following statements about the electrolysis of molten lead(II) bromide are correct? (select all that apply)
Medium
- ALead ions are reduced at the cathode.
- BBromide ions are oxidised at the anode.
- CBromine gas is produced at the cathode.
- DLead metal is produced at the anode.
- EThe electrolyte must be molten for the ions to move freely.
38.During the electrolysis of an aqueous solution, the products formed at the electrodes are always the same as those formed when the same compound is electrolysed in the molten state.
Medium
True or false?
39.In electrolysis, oxidation occurs at the anode because negatively charged ions lose electrons.
Easy
True or false?
40.Match each aqueous solution to the product formed at the cathode when it is electrolysed with inert electrodes.
Medium
- Copper(II) chloride solution
- Sodium chloride solution
- Copper(II) sulfate solution
- Copper
- Hydrogen
41.Complete the sentences about electrolysis using the words in the box: aqueous, liquid, solid, no free electrons, no free ions.
Easy
Ionic compounds cannot conduct electricity when they are ____ . This is because they have ____ that can move and carry charge. To conduct electricity, an ionic compound must be ____ or dissolved in solution.
42.Name the electrolyte used in the electrolysis of molten lead(II) bromide.
Easy
43.Calculate the mass of copper deposited on the cathode during the electrolysis of copper sulfate solution using copper electrodes. The cathode mass increases from 1.28 g to 1.57 g.
Medium
44.Complete the sentences about the extraction of aluminium by electrolysis.
Medium
Aluminium is extracted from its ore, bauxite, which contains aluminium oxide. Aluminium oxide is dissolved in molten ____ to lower its melting point. Aluminium is produced at the ____ electrode, and oxygen is produced at the ____ electrode.
45.An electrolysis of copper(II) sulfate solution is carried out for 15 minutes using copper electrodes. The anode has a mass of 2.40 g before electrolysis and 1.95 g afterwards. Calculate the mass of copper deposited on the cathode, in grams.
Medium
46.In the electrolysis of aqueous copper(II) sulfate with inert graphite electrodes, name the gas produced at the positive electrode.
Medium
47.Complete the sentences about electrolysis using the words below.
Medium
During electrolysis, positively charged ions move towards the ____ electrode, where they gain electrons and are ____. Negatively charged ions move towards the ____ electrode, where they lose electrons and are ____.
48.During the electrolysis of copper(II) sulfate solution using copper electrodes, the mass of the cathode increases from 1.28 g to 1.57 g. Calculate the mass of copper deposited on the cathode in grams.
Medium
49.In the electrolysis of molten sodium fluoride, the overall reaction is: 2NaF → 2Na + F₂. Calculate the maximum mass of sodium that could be formed from 168 g of sodium fluoride. (Relative atomic masses: Na = 23, F = 19; relative formula mass of NaF = 42)
Hard
50.In an electrolysis experiment using copper electrodes, the anode loses 0.31 g and the cathode gains 0.31 g. If the cathode initially had a mass of 2.50 g, what is its final mass in grams?
Hard
51.Complete the sentences about the electrolysis of aqueous solutions using the words below: hydrogen, oxygen, chlorine, metal, water.
Medium
In the electrolysis of aqueous solutions, at the cathode, if the metal is more reactive than hydrogen, ____ is produced. At the anode, if halide ions are present, ____ is produced; otherwise, ____ is produced.
52.Complete the sentence about the extraction of aluminium by electrolysis using the words below: cryolite, bauxite, carbon, oxygen.
Medium
Aluminium is extracted from its ore, ____, by electrolysis. The ore is dissolved in molten ____ to lower the melting point. The positive electrode is made of ____ and reacts with the ____ produced, so it must be replaced regularly.
53.Explain why sodium chloride solution, rather than solid sodium chloride, must be used in an electrolysis experiment.
Hard
54.Explain how a student could modify the apparatus used for the electrolysis of sodium chloride solution to carry out the electrolysis of molten lead bromide instead.
Hard
55.Explain why hydrogen gas is formed at the negative electrode when water is electrolysed.
Hard
56.Explain why a mixture of aluminium oxide and cryolite is used as the electrolyte in the extraction of aluminium.
Hard
57.Explain why the products formed at the electrodes when aqueous copper(II) sulfate is electrolysed with graphite electrodes are different from those formed when copper electrodes are used.
Hard
58.Calculate the mass of copper deposited on the cathode during the electrolysis of copper sulfate solution using copper electrodes. The cathode mass increases from 1.28 g to 1.57 g.
Medium
- A0.29 g
- B1.29 g
- C2.29 g
- D-0.71 g
59.An electrolysis of copper(II) sulfate solution is carried out for 15 minutes using copper electrodes. The anode has a mass of 2.40 g before electrolysis and 1.95 g afterwards. Calculate the mass of copper deposited on the cathode, in grams.
Medium
- A1.45 g
- B-0.55 g
- C0.45 g
- D2.45 g
60.During the electrolysis of copper(II) sulfate solution using copper electrodes, the mass of the cathode increases from 1.28 g to 1.57 g. Calculate the mass of copper deposited on the cathode in grams.
Medium
- A0.29 g
- B2.29 g
- C1.29 g
- D-0.71 g
61.In the electrolysis of molten sodium fluoride, the overall reaction is: 2NaF → 2Na + F₂. Calculate the maximum mass of sodium that could be formed from 168 g of sodium fluoride. (Relative atomic masses: Na = 23, F = 19; relative formula mass of NaF = 42)
Hard
- A46 g
- B92 g
- C184 g
- D69 g
62.In an electrolysis experiment using copper electrodes, the anode loses 0.31 g and the cathode gains 0.31 g. If the cathode initially had a mass of 2.50 g, what is its final mass in grams?
Hard
- A2.81 g
- B4.81 g
- C1.81 g
- D3.81 g

フラッシュカード

What is electrolysis?

Electrolysis is the decomposition (breaking down) of a molten or aqueous ionic compound when an electric current is passed through it.

Why can't solid ionic compounds conduct electricity?

In the solid state, the ions are fixed in position and cannot move, so they cannot carry charge.

What is an electrolyte?

An electrolyte is an ionic compound in molten or dissolved (aqueous) form that can conduct electricity.

What is the anode?

The anode is the positive electrode.

What is the cathode?

The cathode is the negative electrode.

What is an anion?

An anion is a negatively charged ion, which is attracted to the anode (positive electrode).

What is a cation?

A cation is a positively charged ion, which is attracted to the cathode (negative electrode).

During electrolysis, in which direction do electrons flow in the external circuit?

Electrons flow from the anode (positive terminal) to the cathode (negative terminal) through the external circuit.

What is produced at the cathode and anode when molten lead(II) bromide is electrolysed?

Lead metal is produced at the cathode (negative electrode) and bromine gas at the anode (positive electrode).

When a binary molten compound is electrolysed, what is formed at each electrode?

The metal is formed at the cathode and the non-metal at the anode.

Why must an ionic compound be molten or dissolved to undergo electrolysis?

Because its ions must be free to move and carry the charge; in the solid state the ions are fixed in place.

In the electrolysis of aqueous solutions, what ions do water molecules provide?

Water molecules dissociate to give hydrogen ions (H⁺) and hydroxide ions (OH⁻).

At the cathode, when is hydrogen produced instead of a metal?

Hydrogen is produced if the metal is more reactive than hydrogen (i.e. above hydrogen in the reactivity series).

At the anode, when is a halogen produced instead of oxygen?

A halogen is produced if halide ions (Cl⁻, Br⁻, I⁻) are present; otherwise oxygen is produced.

What are the products of the electrolysis of aqueous sodium chloride?

Hydrogen gas at the cathode and chlorine gas at the anode.

What are the products of the electrolysis of aqueous copper(II) chloride?

Copper metal at the cathode and chlorine gas at the anode.

What are the products of the electrolysis of aqueous sodium sulfate?

Hydrogen gas at the cathode and oxygen gas at the anode.

What are the products of the electrolysis of dilute sulfuric acid?

Hydrogen gas at the cathode and oxygen gas at the anode.

Why is cryolite added to aluminium oxide in the extraction of aluminium?

Cryolite lowers the melting point of the mixture, so less energy is needed and the process is cheaper.

Why does the positive electrode (anode) need to be replaced regularly in the extraction of aluminium?

The carbon in the graphite anode reacts with the oxygen produced to form carbon dioxide, so the anode wears away.

Define oxidation and reduction in terms of electrons.

Oxidation is the loss of electrons; reduction is the gain of electrons.

What happens at the anode and cathode in terms of electron transfer?

At the anode, negative ions lose electrons (oxidation); at the cathode, positive ions gain electrons (reduction).

Write the half equation for the reaction at the cathode when molten lead(II) bromide is electrolysed.

Pb²⁺ + 2e⁻ → Pb (reduction).

Write the half equation for the reaction at the anode when molten lead(II) bromide is electrolysed.

2Br⁻ → Br₂ + 2e⁻ (oxidation).

Write the half equation for the reaction at the cathode when aqueous sodium chloride is electrolysed.

2H⁺ + 2e⁻ → H₂ (reduction).

Write the half equation for the reaction at the anode when aqueous sodium chloride is electrolysed.

2Cl⁻ → Cl₂ + 2e⁻ (oxidation).

Write the half equation for the reaction at the cathode when aqueous copper(II) chloride is electrolysed.

Cu²⁺ + 2e⁻ → Cu (reduction).

Write the half equation for the reaction at the anode when aqueous copper(II) sulfate is electrolysed.

4OH⁻ → O₂ + 2H₂O + 4e⁻ (oxidation).

In the electrolysis of copper(II) sulfate using copper electrodes, what happens to the mass of the anode and cathode?

The anode decreases in mass (copper atoms are oxidised and go into solution) and the cathode increases in mass (copper ions are reduced and deposited).

Why does the concentration of copper(II) ions remain constant during the electrolysis of copper(II) sulfate with copper electrodes?

The copper deposited at the cathode is the same copper lost from the anode, so the concentration of Cu²⁺ ions stays the same.

What is the test for hydrogen gas?

A lit splint gives a squeaky 'pop'.

What is the test for oxygen gas?

A glowing splint relights.

What is the test for chlorine gas?

Damp blue litmus paper turns red and is then bleached white.

Why are graphite or platinum electrodes used in electrolysis?

They are inert, so they do not react with the electrolyte or interfere with the electrolysis reactions.

What is the product at the cathode when molten zinc chloride is electrolysed?

Zinc metal.

What is the product at the anode when molten zinc chloride is electrolysed?

Chlorine gas.

In the electrolysis of aqueous copper(II) sulfate with inert electrodes, what is produced at the cathode and anode?

Copper metal at the cathode and oxygen gas at the anode.

Why is electrolysis used to extract aluminium rather than reduction with carbon?

Aluminium is more reactive than carbon, so it cannot be extracted by reduction with carbon; electrolysis is used instead.

What is the electrolyte in the extraction of aluminium?

A molten mixture of aluminium oxide (Al₂O₃) and cryolite.

What is the product at the cathode in the extraction of aluminium?

Aluminium metal.

What is the product at the anode in the extraction of aluminium?

Oxygen gas.

What is the half equation for the reaction at the cathode in the electrolysis of molten aluminium oxide?

Al³⁺ + 3e⁻ → Al (reduction).

What is the half equation for the reaction at the anode in the electrolysis of molten aluminium oxide?

2O²⁻ → O₂ + 4e⁻ (oxidation).

In the electrolysis of aqueous solutions, why is hydrogen produced at the cathode when the metal is reactive?

The more reactive metal ions remain in solution, so the less reactive hydrogen ions are discharged instead.

What is meant by 'discharge' at an electrode?

An ion loses or gains electrons at the electrode and forms a neutral substance (element) that is released as a product.

Why do covalent compounds not undergo electrolysis?

They do not contain ions and cannot conduct electricity.

In the electrolysis of aqueous copper(II) sulfate with copper electrodes, what happens at the anode?

Copper atoms lose electrons and form copper(II) ions: Cu → Cu²⁺ + 2e⁻.

In the electrolysis of aqueous copper(II) sulfate with copper electrodes, what happens at the cathode?

Copper(II) ions gain electrons and form copper atoms: Cu²⁺ + 2e⁻ → Cu.

What is the purpose of electrolysis in copper refining?

To purify copper: impure copper is the anode, pure copper is the cathode, and the electrolyte is copper(II) sulfate solution.

What is the anode sludge in copper refining?

The impurities from the impure copper anode that fall to the bottom of the cell; it may contain valuable metals like silver.

Why does the anode in copper refining become thinner?

Copper atoms at the anode lose electrons and go into solution as copper(II) ions, so the anode loses mass.

Why does the cathode in copper refining become thicker?

Copper(II) ions from the solution gain electrons at the cathode and are deposited as pure copper atoms.

What is the half equation for the reaction at the cathode when water acidified with sulfuric acid is electrolysed?

2H⁺ + 2e⁻ → H₂ (reduction).

What is the half equation for the reaction at the anode when water acidified with sulfuric acid is electrolysed?

4OH⁻ → O₂ + 2H₂O + 4e⁻ (oxidation).

In the electrolysis of aqueous sodium sulfate, why is oxygen produced at the anode?

Hydroxide ions are discharged in preference to sulfate ions, producing oxygen gas.

What is the product at the cathode when molten sodium chloride is electrolysed?

Sodium metal.

What is the product at the anode when molten sodium chloride is electrolysed?

Chlorine gas.

What is the product at the cathode when molten potassium iodide is electrolysed?

Potassium metal.

What is the product at the anode when molten potassium iodide is electrolysed?

Iodine.

What is the product at the cathode when molten zinc bromide is electrolysed?

Zinc metal.

What is the product at the anode when molten zinc bromide is electrolysed?

Bromine gas.

What is the product at the cathode when molten sodium fluoride is electrolysed?

Sodium metal.

What is the product at the anode when molten sodium fluoride is electrolysed?

Fluorine gas.

In the electrolysis of aqueous magnesium iodide, what is produced at the cathode and anode?

Hydrogen gas at the cathode and iodine at the anode.

Why is hydrogen produced at the cathode in the electrolysis of aqueous magnesium iodide?

Magnesium is more reactive than hydrogen, so hydrogen ions are discharged instead of magnesium ions.

Why is iodine produced at the anode in the electrolysis of aqueous magnesium iodide?

Iodide ions are halide ions, so they are discharged in preference to hydroxide ions, forming iodine.

What is the half equation for the reaction at the cathode when aqueous copper(II) sulfate is electrolysed with inert electrodes?

Cu²⁺ + 2e⁻ → Cu (reduction).

What is the half equation for the reaction at the anode when aqueous copper(II) sulfate is electrolysed with inert electrodes?

4OH⁻ → O₂ + 2H₂O + 4e⁻ (oxidation).

What is the half equation for the reaction at the cathode when aqueous sodium sulfate is electrolysed?

2H⁺ + 2e⁻ → H₂ (reduction).

What is the half equation for the reaction at the anode when aqueous sodium sulfate is electrolysed?

4OH⁻ → O₂ + 2H₂O + 4e⁻ (oxidation).

What is the half equation for the reaction at the cathode when concentrated hydrochloric acid is electrolysed?

2H⁺ + 2e⁻ → H₂ (reduction).

What is the half equation for the reaction at the anode when concentrated hydrochloric acid is electrolysed?

2Cl⁻ → Cl₂ + 2e⁻ (oxidation).

What is the half equation for the reaction at the cathode when aqueous copper(II) chloride is electrolysed?

Cu²⁺ + 2e⁻ → Cu (reduction).

What is the half equation for the reaction at the anode when aqueous copper(II) chloride is electrolysed?

2Cl⁻ → Cl₂ + 2e⁻ (oxidation).

What is the half equation for the reaction at the cathode when aqueous sodium chloride is electrolysed?

2H⁺ + 2e⁻ → H₂ (reduction).

What is the half equation for the reaction at the anode when aqueous sodium chloride is electrolysed?

2Cl⁻ → Cl₂ + 2e⁻ (oxidation).

What is the half equation for the reaction at the cathode when molten lead(II) bromide is electrolysed?

Pb²⁺ + 2e⁻ → Pb (reduction).

What is the half equation for the reaction at the anode when molten lead(II) bromide is electrolysed?

2Br⁻ → Br₂ + 2e⁻ (oxidation).

What is the half equation for the reaction at the cathode when molten aluminium oxide is electrolysed?

Al³⁺ + 3e⁻ → Al (reduction).

What is the half equation for the reaction at the anode when molten aluminium oxide is electrolysed?

2O²⁻ → O₂ + 4e⁻ (oxidation).

What is the half equation for the reaction at the cathode when water acidified with sulfuric acid is electrolysed?

2H⁺ + 2e⁻ → H₂ (reduction).

What is the half equation for the reaction at the anode when water acidified with sulfuric acid is electrolysed?

4OH⁻ → O₂ + 2H₂O + 4e⁻ (oxidation).

What is the half equation for the reaction at the cathode when aqueous sodium sulfate is electrolysed?

2H⁺ + 2e⁻ → H₂ (reduction).

What is the half equation for the reaction at the anode when aqueous sodium sulfate is electrolysed?

4OH⁻ → O₂ + 2H₂O + 4e⁻ (oxidation).

What is the half equation for the reaction at the cathode when concentrated hydrochloric acid is electrolysed?

2H⁺ + 2e⁻ → H₂ (reduction).

What is the half equation for the reaction at the anode when concentrated hydrochloric acid is electrolysed?

2Cl⁻ → Cl₂ + 2e⁻ (oxidation).

What is the half equation for the reaction at the cathode when aqueous copper(II) chloride is electrolysed?

Cu²⁺ + 2e⁻ → Cu (reduction).

What is the half equation for the reaction at the anode when aqueous copper(II) chloride is electrolysed?

2Cl⁻ → Cl₂ + 2e⁻ (oxidation).

What is the half equation for the reaction at the cathode when aqueous sodium chloride is electrolysed?

2H⁺ + 2e⁻ → H₂ (reduction).

What is the half equation for the reaction at the anode when aqueous sodium chloride is electrolysed?

2Cl⁻ → Cl₂ + 2e⁻ (oxidation).

What is the half equation for the reaction at the cathode when molten lead(II) bromide is electrolysed?

Pb²⁺ + 2e⁻ → Pb (reduction).

What is the half equation for the reaction at the anode when molten lead(II) bromide is electrolysed?

2Br⁻ → Br₂ + 2e⁻ (oxidation).

What is the half equation for the reaction at the cathode when molten aluminium oxide is electrolysed?

Al³⁺ + 3e⁻ → Al (reduction).

What is the half equation for the reaction at the anode when molten aluminium oxide is electrolysed?

2O²⁻ → O₂ + 4e⁻ (oxidation).

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