5 Electrolysis

Question 1

How can you test and show that a substance is an electrolyte?

Answer 1

Use a circuit with a cell, lamp, wires and two graphite electrodes.

Place the two electrodes into the solution.

If the bulb lights then the solution is an electrolyte.

Question 2

State why the electrodes used in electrolysis are usually made from an inert material.

Answer 2

So that they do not take part in the reactions that occur during electrolysis.

Question 3

During electrolysis, anions move towards the….

Answer 3

ANODE

or postively charged electrode.

Question 4

During electrolysis, cations move towards the….

Answer 4

CATHODE

or negatively charge electrode.

Question 5

What are the products of electrolysis of magnesium chloride solution?

Answer 5

CATHODE - No Copper, silver, gold - therefore hydrogen forms

ANODE- Halide present! Chlorine forms

Question 6

What are the products of electrolysis of a copper sulphate solution?

Answer 6

CATHODE - Copper, silver, gold present - therefore copper forms

ANODE- No halide present- oxygen forms

Question 7

What are the products of the electrolysis of a sodium chloride solution?

Answer 7

CATHODE - No copper, silver, gold present - therefore hydrogen forms

ANODE- halide present- chlorine forms

Question 8

What are the products of electrolysis of potassium sulphate solution?

Answer 8

CATHODE - No copper, silver, gold present - therefore hydrogen forms

ANODE- No halide present- therefore oxygen forms

Question 9

Describe the movement of ions in an electrolytic cell.

Answer 9

Cations/positive ions move towards the cathode/negative electrode

Anions/negative ions move towards the anode/postive electrode

Question 10

In the electrolysis of molten lead iodide, iodine gas forms at the anode.

The I^- ion is attracted to the anode. iodide ion is oxidised/ loses an electron

2I^- –> I₂ + 2e-

Predict the reduction processes that occurs at the cathode.

Answer 10

CATHODE- attracts the cation - lead ion to form lead

Pb²⁺ + 2e^- –> Pb

Lead ion is reduced- gains two electrons

Question 11

In the electrolysis of molten sodium bromide, sodium metal forms at the cathode.

The Na+ ion is attracted to the cathode. Sodium ion is reduced/ gains an electron

Na⁺ + e^- –> Na⁰

Predict the oxidation processes that occurs at the anode.

Answer 11

ANODE- attracts the anion - bromide ion to form bromine gas

2Br- –> Br₂ + 2e-

Bromide ion is oxidised- loses electrons

Question 12

Electrolysis was carried out on an aqueous solution of X

Product at CATHODE- hydrogen gas

Product at ANODE- oxygen gas

Suggest the identity of compound X

Answer 12

Any soluble salt which is not a copper, silver or gold salt AND not a halide salt.

Such as sodium sulfate OR potassium nitrate OR ammonium sulphate

Question 13

Define electrolysis

Answer 13

The breaking down of a substance using electricity.

An electric current is passes through an electrolyte, causing it to decompose.

Question 14

How do you know if a substance is reduced during electrolysis

Answer 14

OIL RIG

Reduction is gain of electrons.

Therefore, a cation is reduced when it gains electrons

Question 15

How do you know if a substance is oxidised during electrolysis

Answer 15

Oxidation is loss of electrons.

Therefore, an anion is oxidised when it loses electrons

Question 16

Which of the following compounds can be used as an electrolyte?

A - solid ionic compound

B- solid simple covalent compoound

C- soluble ionic compound in water

D- molten ionic compound

Answer 16

C and D only

both have ions which can move freely and carry charge

Question 17

Half reactions show how electrons are transferred during reactions. They are really useful for showing what happens at each electrode during electrolysis.

Write the half reaction for O^2- at the ANODE

Answer 17

2O^2- –> O₂ + 4e^-

Question 18

Half reactions show how electrons are transferred during reactions. They are really useful for showing what happens at each electrode during electrolysis.

Write the half reaction for Cl^- at the ANODE

Answer 18

2Cl^- –> Cl₂ + 2e^-

Question 19

Half reactions show how electrons are transferred during reactions. They are really useful for showing what happens at each electrode during electrolysis.

Write the half reaction for Br^- at the ANODE

Answer 19

2Br^- –> Br₂ + 2e^-

Question 20

Half reactions show how electrons are transferred during reactions. They are really useful for showing what happens at each electrode during electrolysis.

Write the half reaction for H⁺ at the CATHODE

Answer 20

2H⁺ + 2e^- –> H₂

Question 21

Half reactions show how electrons are transferred during reactions. They are really useful for showing what happens at each electrode during electrolysis.

Write the half reaction for Cu²+ at the CATHODE

Answer 21

Cu²⁺ + 2e^- –> Cu⁰

Question 22

Half reactions show how electrons are transferred during reactions. They are really useful for showing what happens at each electrode during electrolysis.

Write the half reaction for Pb²⁺ at the CATHODE

Answer 22

Pb²⁺ + 2e^- –> Pb⁰

Question 23

Half reactions show how electrons are transferred during reactions. They are really useful for showing what happens at each electrode during electrolysis.

Write the half reaction for Na⁺ at the CATHODE

Answer 23

Na⁺ + e^- –> Na⁰

Question 24

Why is it more difficult to predict what products you get when you electrolyse SOLUTION substances?

Answer 24

SOLUTION substances have two possible CATIONS and two possible ANION to choose from.

NOTE: Water provides two other ions (H+ and OH-)

CATHODE- if the electrolyte contains copper, silver or gold, it forms, if not hydrogen gas forms

ANODE- if the the electrolyte contains a halide- halogen forms, if not oxygen forms.

Question 25

Why is it easy to predict what products you get when you electrolyse MOLTEN substances?

Answer 25

MOLTEN substances only have one CATION and one ANION to choose from, there are no other possible products.

Question 26

Which of the following are reduction reactions?

A 2H⁺ + 2e^- –> H₂

B 2Cl^- –> Cl₂ + 2e^-

C 2O^2- –> O₂ + 4e-

D Pb²⁺ + 2e^- –> Pb⁰

Answer 26

OIL RIG

Reduction is gain of electrons

Both A and D as both the ions gain electrons

Question 27

Use the rules to decide what forms at both the CATHODE and ANODE in each example

CATHODE- Is there copper, silver or gold? if yes, it forms, if not hydrogen forms

ANODE - Is there a halide? if yes the halogen forms- if not oxygen forms

Answer 27

1st Row - hydrogen and oxygen

2nd Row - silver and bromine

3rd Row- hydrogen and oxygen

Question 28

In the electrolysis of water, a small amount of sulfuric acid is added to the water to make the water more conductive.

hydrogen forms at the cathode

oxygen forms at the anode

Why is double the volume of hydrogen formed?

Answer 28

The formula of water is H₂O

Therefore there is double the number of moles of hydrogen ions than oxygen ions forming double the volume of hydrogen than oxygen.

Question 29

What are the name of the electrodes in electrolysis?

Answer 29

negative electrode is called the CATHODE because it attracts the CATIONS (positive ions)

positive electrode is called the ANODE because it attracts the ANIONS (negative ions)