22: Electrolysis

Question 1

Explain the meaning of electrolysis.

Answer 1

Electrolysis is the process of decomposing an electrolyte with an electric current.

Question 2

Name the apparatus in which electrolysis is carried out.

Answer 2

Electrolytic cell

Question 3

State and explain the polarity of the cathode in an electrolytic cell.

Answer 3

At the cathode, reduction reactions occur, which means the cations gain electrons from the electrode. As the cations are attracted to the electrode, the polarity of the cathode in an electrolytic cell is negative.

Question 4

State and explain the polarity of the anode in an electrolytic cell.

Answer 4

At the anode, oxidation occurs, which means the anions lose electrons to the electrode. As the anions are attracted to the electrode, the polarity of the anode in an electrolytic cell is positive.

Question 5

State and explain whether the positive electrode is the cathode or the anode in an electrolytic cell.

Answer 5

At the positive electrode, anions are attracted; they lose electrons and are oxidised. Hence the positive electrode of the electrolytic cell is the anode.

Question 6

State and explain whether the negative electrode is the cathode or the anode in an electrolytic cell.

Answer 6

At the negative electrode, cations are attracted; they gain electrons and are reduced. Hence the negative electrode of the electrolytic cell is the cathode.

Question 7

State and explain the polarity of the cathode in an electrochemical cell.

Answer 7

At the cathode, reduction occurs, so the electrode gains electrons from the external circuit. As the electrons in the external circuit are attracted to the cathode, the polarity of the cathode in the chemical cell is positive.

Question 8

State and explain the polarity of the anode in an electrochemical cell.

Answer 8

At the anode, oxidation occurs, so the electrode loses electrons and releases them into the external circuit. As the electrons flow from the anode to the positive terminal in the external circuit, the polarity of the anode in the chemical cell is negative.

Question 9

State and explain whether the positive electrode is the cathode or the anode in an electrochemical cell.

Answer 9

At the positive electrode, electrons are received from the external circuit to reduce the oxidising agent. Hence the positive electrode of the chemical cell is the cathode.

Question 10

State and explain whether the negative electrode is the cathode or the anode in an electrochemical cell.

Answer 10

At the negative electrode, electrons are released into the external circuit and the reducing agent is oxidised. Hence the negative electrode of the chemical cell is the anode.

Question 11

Explain why aqueous solutions of electrolytes contain more than one type of cation and more than one type of anion.

Answer 11

There are hydrogen ions and hydroxide ions which arise from the dissociation of water.

Question 12

Explain why the electrolysis of water uses acidified water.

Answer 12

Pure water is a poor conductor of electricity. Adding a small amount of sulphuric acid can improve the conductivity of water.

Question 13

Name an apparatus to demonstrate the electrolysis of acidified water.

Answer 13

Hoffman voltameter

Question 14

Explain why no sulphur compounds are produced in the electrolysis of water (acidified with sulphuric acid).

Answer 14

The sulphur in the sulphuric acid exists as sulphate ions, which is very low in the electrochemical series / a very weak reducing agent. Meanwhile, hydroxide ion is a stronger reducing agent than sulphate ion and it is preferentially discharged at the positive electrode. Hence sulphate ions remain as ions in the solution and do not produce sulphur compounds in electrolysis of water.

Question 15

Acidified water is electrolysed. Describe how the product at the cathode can be tested.

Answer 15

Hydrogen gas is liberated at the cathode. It burns with a “pop” sound when a burning splint is inserted into it.

Question 16

Write the ionic equation at the cathode of the electrolysis of acidified water.

Answer 16

2H⁺ + 2e^- -> H₂

Question 17

Write the ionic equation at the anode of the electrolysis of acidified water.

Answer 17

4OH^- -> 2H₂O + O₂ + 4e^-

Question 18

Explain the ratio between the theoretical amounts of hydrogen gas and oxygen gas produced in the electrolysis of water.

Answer 18

When 4 electrons are gained in the cathode, 2 hydrogen gas molecules are formed. When 4 electrons are released in the anode, only 1 oxygen gas molecule is formed. Since electrons are released from the anode at the same rate as they are gained in the cathode, the volume ratio of hydrogen to oxygen is 2:1.

Question 19

Explain two reasons why the actual volume of oxygen gas collected is much smaller than that of hydrogen gas collected in the electrolysis of water.

Answer 19

1. The produced volume ratio of hydrogen to oxygen is 2:1.
2. Some oxygen dissolves in the water while hydrogen cannot dissolve in water.

Question 20

Very dilute sodium chloride solution (0.01M) is electrolysed with a few drops of universal indicator solution.
State the colour changes at the cathode and anode after a period of time.

Answer 20

Cathode: The universal indicator solution changes from green to blue.
Anode: The universal indicator solution changes from green to red.

Question 21

Very dilute sodium chloride solution (0.01M) is electrolysed with a few drops of universal indicator solution.
Explain the colour change at the cathode.

Answer 21

At the cathode, hydrogen ions are preferentially discharged. As electrolysis proceeds, water molecules dissociate to replace the hydrogen ions, but the hydroxide ions are not consumed and accumulate. Thus, the concentration of hydroxide ions rises above that of hydrogen ions, the solution there becomes alkaline, and the indicator changes from green to blue.

Question 22

Very dilute sodium chloride solution (0.01M) is electrolysed with a few drops of universal indicator solution.
Explain the colour change at the anode.

Answer 22

At the anode, hydroxide ions are preferentially discharged. As electrolysis proceeds, water molecules dissociate to replace the hydroxide ions, but the hydrogen ions are not consumed and accumulate. Thus, the concentration of hydrogen ions rises above that of hydroxide ions, the solution there becomes acidic, and the indicator changes from green to red.

Question 23

Very dilute sodium chloride solution (0.01M) is electrolysed with a few drops of universal indicator solution.
Explain any change in concentration of sodium chloride.

Answer 23

The concentration of the sodium chloride increases since water is consumed in the electrolysis.

Question 24

Very dilute sodium chloride solution (0.01M) is electrolysed.
Name all products after complete reaction.

Answer 24

Hydrogen gas, oxygen gas, more concentrated sodium chloride solution

Question 25

Concentrated sodium chloride solution (2M) is electrolysed. Name all products after complete reaction.

Answer 25

Hydrogen gas, chlorine gas, sodium hydroxide solution

Question 26

Concentrated sodium chloride solution (2M) is electrolysed with a few drops of universal indicator solution. Explain the colour change of the solution.

Answer 26

Hydrogen ions and chloride ions are preferentially discharged. At the cathode, hydrogen ions are consumed. Water molecules dissociate to replace the hydrogen ions, but the hydroxide ions are not consumed and **accumulate**. Thus, the concentration of hydroxide ions rises above that of hydrogen ions, the solution there becomes **alkaline**, and the indicator changes from green to blue.

Question 27

Concentrated sodium chloride solution (2M) is electrolysed. Describe a chemical test and expected result for the gas liberated at the cathode.

Answer 27

Hydrogen gas is liberated at the cathode. It burns with a "pop" sound when a burning splint is inserted into it.

Question 28

Concentrated sodium chloride solution (2M) is electrolysed. Describe a chemical test and expected result for the gas liberated at the anode.

Answer 28

Chlorine gas is liberated at the anode. When a **moist blue litmus** paper is held near it, the paper turns to red immediately and is slowly **bleached** to white colour.

Question 29

Explain the colour change when a moist blue litmus paper is held over chlorine gas.

Answer 29

Chlorine gas reacts with the water on the litmus paper to produce HCl (hydrochloric acid) and HOCl (hypochlorous acid). The hydrochloric acid immediately reacts with the litmus paper and changes it to **red**. The hypochlorous acid acts as **bleaching agent** and slowly bleaches the litmus paper to white.

Question 30

Name the ion responsible for bleaching in aqueous chlorine solution.

Answer 30

Hypochlorite ion (OCl^-), which can bleach by oxidation.

Question 31

Concentrated sodium chloride solution (2M) is electrolysed to obtain hydrogen gas. Name one suitable electrode and one unsuitable electrode for the reaction.

Answer 31

Suitable electrode: graphite Unsuitable electrode: platinum, since it is **attacked** by chlorine.

Question 32

Dilute copper(II) sulphate solution is electrolysed with carbon electrodes. State 3 expected observations.

Answer 32

1. The cathode is coated with a reddish brown solid. 2. A colourless gas is produced at the anode. 3. The blue colour of the solution fades.

Question 33

Dilute copper(II) sulphate solution is electrolysed with carbon electrodes. Name all products after complete reaction.

Answer 33

Oxygen gas, copper solid, sulphuric acid.

Question 34

Dilute copper(II) sulphate solution is electrolysed. Name and explain the material of electrode(s) which allows the colour intensity of the solution to be unchanged.

Answer 34

**Copper anode**. Copper is a stronger reducing agent than hydroxide ions and sulphate ions. Therefore, it **dissolves** to form copper(II) ions. Meanwhile, copper(II) ions are reduced into copper metal at the cathode at the same rate. The concentration of blue copper(II) ions remains the same so the colour intensity of the solution is unchanged.

Question 35

Dilute copper(II) sulphate solution is electrolysed. Name and explain the material of electrode(s) which allows the pH of the solution to be unchanged.

Answer 35

Copper or any metal higher than copper in the electrochemical series is used as **anode**. These reactive metals are stronger reducing agents than hydroxide ions and sulphate ions. Therefore, they are oxidised to form metal ions and hydroxide ions are **not discharged**. Copper(II) ions are reduced in the cathode and the reactive metal is oxidised in the anode. There is **no net change** in the concentration of hydrogen or hydroxide ions, so the pH of the solution is unchanged.

Question 36

Concentrated sodium chloride solution (2M) is electrolysed. Name the material of the electrode(s) which allows the discharging of metal ions instead of hydrogen ions.

Answer 36

Mercury cathode, it discharges the sodium ions and forms an **amalgam** with it.

Question 37

Name one device in the industry which uses mercury cathode in electrolysis.

Answer 37

Flowing mercury electrolytic cell

Question 38

Rank the reducing powers of hydroxide ion, halide ions, nitrate ion, and sulphate ion from high to low.

Answer 38

Highest: Hydroxide ion, **iodide ion**. bromide ion, **chloride ion**, nitrate ion, sulphate ion (lowest)

Question 39

Rank the oxidising powers of calcium ion, sodium ion, lead ion, copper ion, and hydrogen ion from high to low.

Answer 39

Highest: copper ion, **hydrogen ion**, lead ion, sodium ion, calcium ion (lowest)

Question 40

List 3 criteria for determining the preferential discharge of ions during electrolysis.

Answer 40

1. Position in electrochemical series. 2. Concentration of ions with similar position in the solution. 3. Presence of special electrodes (reactive metal anode or mercury cathode).

Question 41

Name 4 common metals for electroplating.

Answer 41

Copper, chromium, silver, tin

Question 42

Name the anode, cathode, and electrolyte in metal plating.

Answer 42

Anode: plating metal Cathode: object to be plated Electrolyte: solution with ions of the plating metal

Question 43

Name the anode, cathode, and electrolyte in aluminium anodisation.

Answer 43

Anode: Aluminium metal (to be anodised) Cathode: platinum Electrolyte: dilute sulphuric acid

Question 44

Explain the principle of aluminium anodisation using preferential discharge.

Answer 44

At the anode, **hydroxide ions** are discharged to oxygen, reacting with aluminium to form an aluminium oxide layer. Although aluminium is higher in the electrochemical series than hydroxide ions, its discharge creates an **insoluble aluminium hydroxide** layer which prevents aluminium from further discharging.

Question 45

List 5 requirements to obtain a good metal coating during electroplating.

Answer 45

1. The object must be clean and free of grease. 2. The object should be rotated for an even coating. 3. The current must not be too large of the coating will flake off. 4. The temperature of the electrolyte must be controlled for the rate of coating deposit. 5. The concentration of the electrolyte must be controlled for the rate of coating deposit.

Question 46

Name 3 major pollutants in the electroplating industry.

Answer 46

Acids and alkalis, heavy metal ions, cyanides

Question 47

List 7 ways to control pollution from the electroplating industry.

Answer 47

1. Design better rinsing systems and reduce water usage. 2. Replace cyanides with less toxic compounds. 3. Recover metals from waste solutions. 4. Reuse plating bath solutions and rinsing water. 5. Adjust pH of effluents to around neutral. 6. Remove heavy metal ions in waste water by precipitation using sodium hydroxide. 7. Remove chromium(IV) ions in waste water.

Question 48

Describe the process of removing chromium(IV) ions in waste water.

Answer 48

1. Reduce them to less toxic chromium(III) ions using a reducing agent (eg. sodium sulphite). 2. Add sodium hydroxide solution to precipitate the ions as **chromium(III) hydroxide**.