Electrolysis

Question 1

How could the students separate the silver from the silver nitrate solution?

Answer 1

by filtration

Question 2

The products at the electrodes are lead and bromine.
Why should the teacher do the demonstration in a fume cupboard?

Answer 2

toxic / poisonous fumes

Question 3

Explain the difference between the processes in electrolysis and in a
chemical cell.

Answer 3

electrolysis uses electricity to produce a chemical reaction
(but) cells use a chemical reaction to produce electricity

Question 4

Some of the copper produced did not stick to the negative electrode but fell
to the bottom of the beaker.
Suggest how the students could find the total mass of copper produced.

Answer 4

filter the mixture

wash and dry the copper / residue

weigh the copper collected

add to the increase in mass of the electrode

Question 5

Copper nitrate solution is blue.
Suggest why the blue colour of the copper nitrate solution fades during the
electrolysis

Answer 5

copper ions are discharged (from the solution)

Question 6

‘The electrolysis of a salt solution produces a metal at the negative electrode and
a gas at the positive electrode.’ What observation would be made at each electrode if the hypothesis is
correct?

Answer 6

(negative electrode) solid produced
(positive electrode) bubbles / fizzing / effervescence

Question 7

Give two reasons why graphite is used for the electrodes.

Answer 7

(graphite) conducts (electricity)
(graphite) is inert (unreactive)

Question 8

The purple crystal contained:
* colourless positive ions
* purple coloured negative ions.
The purple crystal dissolved in the electrolyte solution.
(d) What happens to the purple coloured ions?

Answer 8

the ions move towards the positive electrode
bc
the electrode attracts ions of the opposite charge

Question 9

What is meant by an ‘alloy’?

Answer 9

a mixture of metals

Question 10

Bauxite contains between 15% and 25% aluminium.
Aluminium oxide always contains 53% aluminium.
How does this show that bauxite is a mixture and not a compound?

Answer 10

bauxite contains a variable percentage of aluminium

Question 11

Suggest two possible problems with storing the waste material in lakes of
mud.

Answer 11

leakage of toxic substances from mud to environment
* danger of dam bursting
* water pollution
* damage to habitats
* visual pollution
* (dam) blocks light
* reduces the value of houses

Question 12

The formula of cryolite is Na3AlF6
Give the total number of atoms in the formula

Answer 12

10

Question 13

What is the reason for adding cryolite to the aluminium oxide?

Answer 13

to lower the melting point of the mixture

Question 14

When the molten aluminium oxide and cryolite mixture is electrolysed the
product at
the positive electrode is:

Answer 14

oxygen

Question 15

oxygen reacts with the positive electrode because the positive electrode is made of:

Answer 15

carbon

Question 16

Name the substance in the solution that provides the hydroxide ions.

Answer 16

water

Question 17

Describe how the solid forms at the negative electrode. (copper chromate)

Answer 17

copper ions gain two electrons
(to) form copper (atoms)
Cu2+ + 2e- → Cu

Question 18

Name the product at each electrode when potassium iodide solution is
electrolysed.
Negative electrode:
positive elecctrode:

Answer 18

hydrogen

iodine

Question 19

Aluminium is produced by electrolysing a molten mixture of aluminium oxide and
cryolite.
Explain why a mixture is used as the electrolyte instead of using only
aluminium oxide.

Answer 19

mixture has a lower melting point
(so) less energy needed

Question 20

What happens at the negative electrode during the production of
aluminium?

Answer 20

aluminium ions gain electrons

Question 21

Complete the balanced half-equation for the process at the positive
electrode.

Answer 21

2 O2- → O2 + 4 e

Question 22

Explain why the positive electrode must be continually replaced.

Answer 22

the electrode reacts with oxygen

the electrode is carbon / graphite

(so) carbon dioxide is produced

Question 23

Sodium metal and chlorine gas are produced by the electrolysis of molten
sodium chloride.
Explain why sodium chloride solution cannot be used as the electrolyte to
produce sodium metal.

Answer 23

hydrogen (gas) would be produced (instead of sodium)

(because) sodium is more reactive than hydrogen

Question 24

The teacher demonstrates the electrolysis of:
* molten zinc chloride
* potassium bromide solution.
Product at cathode
(negative electrode)
Product at anode
(positive electrode) is what

Answer 24

zinc , hydrogen at cathode

chlorine , bromine at anode

Question 25

Explain why electrolysis would not take place in solid zinc chloride

Answer 25

solid (zinc chloride) does not conduct (electricity) (because) ions cannot move in the solid

Question 26

No gas escapes from the apparatus.
Suggest one reason for the difference in volume of each gas collected.

Answer 26

chlorine reacts with water

Question 27

Explain why the product at the negative electrode is not always a metal

Answer 27

(sometimes) hydrogen is produced

(because) the metal is more reactive than hydrogen

Question 28

Predict the product at the positive electrode in the electrolysis of:
* sodium chloride solution
* copper sulfate solution.

Answer 28

chlorine

oxygen

Question 29

Predict the boiling point of bromine

Answer 29

anything in range +30 to +120

Question 30

Alloys can be bent. Some alloys return to their original shape when
heated.
What name is used for these alloys?

Answer 30

shape memory

Question 31

Explain why magnesium chloride must be molten or dissolved in
water to be electrolysed.

Answer 31

because ions can move
to the electrodes

Question 32

Suggest why magnesium is not produced at the negative electrode
in Experiment 2.

Answer 32

magnesium is more reactive than hydrogen

Question 33

Explain why metals can be bent and shaped.

Answer 33

layers (of particles/atoms/ions)

(particles/atoms/ions/layers) can slide