Chapter 6 Electrolysis

Question 1

What happens to the ions in ionic compounds when they are dissolved in water?

Answer 1

When ionic substances dissolve the ions are free to move about.

Question 2

What word do we use to describe ionic substances that have been dissolved in water?

Answer 2

We describe ionic substances dissolved in water as aqueous (aq).

Question 3

What happens to the ions in ionic compounds when the compound is melted?

Answer 3

When ionic substances are molten the ions are free to move about.

Question 4

What is electrolysis?

Answer 4

Electrolysis breaks down a substance using electricity.

Question 5

What needs to be placed in molten or aqueous ionic substances for electrolysis to take place?

Answer 5

Electrodes are placed in a solution of an ionic compound or in a molten ionic compound.

Question 6

What is the positive electrode called?

Answer 6

The positive electrode is called the anode.

Question 7

What is the negative electrode called?

Answer 7

The negative electrode is called the cathode.

Question 8

When electricity is passed through molten or aqueous ionic substances how do the ions behave?

Answer 8

Positive ions move towards the cathode (negative electrode) and negative ions move towards the anode (positive electrode).

Question 9

What are electrodes usually made from and why?

Answer 9

The electrodes are usually made of graphite. Graphite conducts electricity but will not react with the ions.

Question 10

What is an electrolyte?

Answer 10

The substance being broken down by electrolysis is called the electrolyte.

Question 11

What happens to ions when they discharge?

Answer 11

Ions become discharged (lose their charge) at the electrodes and become elements.

Question 12

Write the half equation for the discharge of chloride ions. Where does this happen? (HIGHER ONLY)

Answer 12

At the anode (positive electrode) non-metals lose electrons and become elements. We write this as a ‘half equation”.
Example:

Question 13

What word do we use to describe the loss of electrons by non-metal ions?

Answer 13

When non-metal ions lose electrons we say they are ‘oxidised’.

Question 14

What is reduction?

Answer 14

Oxidation is the loss of electrons.
Reduction in the gain of electrons.
OIL RIG

Question 15

Write the half equation for the discharge of copper(II) ions. Where does this happen?

Answer 15

At the cathode (negative electrode) metals gain electrons and become elements. We write this as a ‘half equation”.
Example:

Question 16

What term do we use to describe metal ions gaining electrons?

Answer 16

When metal ions gain electrons we say they are ‘reduced’.

Question 17

Where in the reactivity series will you find the metals that are capable of being deposited on electrodes?

Answer 17

Only metals lower than hydrogen in the reactivity series will be deposited on the cathode when electrolysing a solution.

Question 18

What is produced at the cathode during the electrolysis of metal ions that are more reactive than hydrogen? What is formed at the anode?

Answer 18

Electrolysis involving metal ions more reactive than hydrogen in the reactivity series results in hydrogen gas being produced at the cathode (negative electrode) and oxygen being produced at the anode (positive electrode).
Example:

Question 19

Write the half equation for the formation of hydrogen at the cathode.
(HIGHER ONLY)

Answer 19

When the metal ions are more reactive than hydrogen the reaction at the cathode is:
2H+(aq) + 2e- → H2(g)

Question 20

Write the 2 half equations for the possible products at the anode when electrolysing ionic compounds with metals more reactive than hydrogen.
(HIGHER ONLY)

Answer 20

When the metal ions are more reactive than hydrogen the reaction at the anode depends on the concentration of halide ions in the electrolyte. If the concentration is high then a halide gas will form if it is low then oxygen gas will form.
2Cl-(aq) → Cl2(g) +2e-
Or
4OH-(aq) → 2H2O(l) + O2(g) + 4e-

Question 21

If there are 2 possible products as an electrode how do you know which one is actually formed?

Answer 21

If there are 2 elements that could be produced at an electrode it is always the less reactive element formed.

Question 22

What is the order at which ions are discharged (lose their charge) at the anode?

Answer 22

The order at which ions are discharged (lose their charge) an the anode is:
halide ion > hydroxide (OH-) > all other negatively charged ions

Question 23

Why is oxygen produced at the anode?

Answer 23

Oxygen is produced at the anode due to the breakdown of water by electrolysis.
H2O(l) → H+ + OH-
4OH-(aq) → 2H2O(l) +O2(g) + 4e-

Question 24

How are reactive metals extracted from their ores?

Answer 24

Electrolysis can be used to extract metals from their ores.

Question 25

Give 5 uses for aluminium.

Answer 25

Aluminium is an important metal. It is used to make: pans, overhead power cables, aeroplanes, cooking foil, drink cans, window and door frames, bicycles frames and car bodies.

Question 26

In what form is aluminium found in bauxite?

Answer 26

Aluminium is found in the form of aluminium oxide in the ore bauxite.

Question 27

Why does bauxite need to be processed?

Answer 27

Bauxite has impurities in it so once mined it must be processed to remove the impurities.

Question 28

Why can we not extract aluminium from its oxide using carbon?

Answer 28

Aluminium is more reactive than carbon so cannot be extracted from aluminium oxide using carbon.

Question 29

Can metals less reactive than carbon be extracted using electrolysis?

Answer 29

Metals less reactive than carbon can also be extracted using electrolysis but they don’t have to be.

Question 30

What is the formula for aluminium oxide?

Answer 30

Aluminium oxide has the formula Al2O3.

Question 31

Why do we mix aluminium oxide with cryolite during extraction?

Answer 31

Aluminium oxide is mixed with cryolite for electrolysis to lower the melting point from 2050°C to 850°C.

Question 32

Describe the electrolysis of aluminium oxide in detail. Include a diagram and the products at the electrodes.

Answer 32

Molten aluminium oxide is electrolysed to produce aluminium at the cathode. At the anode oxygen gas is produced. This oxygen gas reacts with the carbon electrode to produce carbon dioxide gas.

Question 33

Write the half reactions for the electrolysis of aluminium oxide. (HIGHER ONLY)

Answer 33

The half reactions for the electrolysis of aluminium oxide are:

Question 34

Why do the carbon anodes in the extraction of aluminium need to be regularly replaced?

Answer 34

At the anode the oxygen produced is constantly reacting with the carbon that the anode is made from. This means that the carbon anodes gradually burn away and need to be replaced regularly.

Question 35

Why is it worth using so much energy to extract aluminium from its ore?

Answer 35

The electrolysis of aluminium oxide requires huge amounts of energy and is only done due to the value of the aluminium metal produced.

Question 36

How can we test for the presence of chlorine gas at the cathode?

Answer 36

Blue litmus paper can be used to test for the presence of chlorine gas (Cl2) at the anode. If chlorine is present it will change from blue to colourless. We say it has ‘bleached’ the litmus paper.

Question 37

How can we test for the production of oxygen gas at the anode?

Answer 37

We can test for the presence of oxygen at the anode by seeing if it relights a glowing splint. If it does then oxygen gas has been produced.

Question 38

What is the formula for carbonate?

Answer 38

Question 39

What is the formula for the sulfate ion?

Answer 39

Question 40

What is the formula for the nitrate ion?

Answer 40

Question 41

What is the formula for the ammonium ion?

Answer 41

Question 42

What is the formula for hydroxide?

Answer 42