C6: Electrolysis

Question 1

What is electrolysis?

Answer 1

The process of using electricity to extract elements from a compound.

Question 2

What is the name of the positive electrode?

Answer 2

Anode

Question 3

What is the name of the negative electrode?

Answer 3

Cathode

Question 4

What is an electrolyte?

Answer 4

Liquid or solution that contains ions and so can conduct electricity.

Question 5

Where are metals formed?

Answer 5

Cathode

Question 6

Where are non-metals formed?

Answer 6

Anode

Question 7

How can ionic substances be electrolysed?

Answer 7

By melting or dissolving them, and then passing a direct current through them.

Question 8

Why can ionic substances not be electrolysed?

Answer 8

They do not conduct electricity, or the ions cannot move.

Question 9

In the electrolysis of solutions, when is the metal not produced at the cathode?

Answer 9

When the metal is more reactive than hydrogen.

Question 10

In the electrolysis of a metal halide solution, what is produced at the anode?

Answer 10

Halogen.

Question 11

In the electrolysis of a metal sulphate solution, what is produced at the anode?

Answer 11

Oxygen.

Question 12

What is the half equation for the ionisation of water?

Answer 12

H20(l) produced from H+(aq)+OH-(aq)

Question 13

What metals are extracted from ionic compounds by using electrolysis?

Answer 13

Metals that are more reactive than carbon.

Question 14

In the electrolysis of aluminium oxide, why is the aluminium oxide mixed with cryolite?

Answer 14

To lower the melting point.

Question 15

In the electrolysis of aluminium oxide, what are the anodes made of ?

Answer 15

Graphite.

Question 16

In the electrolysis of aluminium oxide, why do the anodes need to be replaced ?

Answer 16

They react with the oxygen being formed.

Question 17

During electrolysis, which electrodes are positive ions attracted to?

Answer 17

The Cathode.

Question 18

Why can ionic solids not undergo electrolysis?

Answer 18

They cannot conduct electricity as they are solid so therefore no current can run through the solution which is the main part of electrolysis.

Question 19

Do ions get reduced or oxidised at the anode?

Answer 19

Oxidation happens at the positive anode because this is where negative ions lose electrons.

Question 20

During the manufacture of aluminium from bauxite, which electrode is aluminium formed at?

Answer 20

Aluminium is formed at the cathode. Aluminium ions are reduced by gaining 3 electrons.

Question 21

In what situation will hydrogen gas be given out during the electrolysis of an aqueous solution of an ionic solid?

Answer 21

Hydrogen will be produced if the metal is more reactive than hydrogen.

Question 22

If halide ions are present in an aqueous solution of an ionic solid will oxygen gas be released?

Answer 22

Oxygen is produced from hydroxide ions, unless halide ions (chloride, bromide or iodide ions) are present. In that case, the negatively charged halide ions lose electrons and form the corresponding halogen (chlorine, bromine or iodine).

Question 23

What test could you use to determine if hydrogen gas has been produced in the electrolysis reaction?

Answer 23

The burning split test. A splint is lit and held near the opening of the tube, then the stopper is removed to expose the splint to the gas. If the gas is flammable, the mixture ignites.

Question 24

Explain why electrolysis is used to extract aluminium from compounds ?

Answer 24

Aluminium is more reactive than carbon so it must be extracted from its compounds using electrolysis.

Question 25

Describe the electrolysis of aluminium oxide?

Answer 25

It is dissolved in molten cryolite (an aluminium compound with a lower melting point than aluminium oxide). This reduces the energy needed so therefore reduces the cost of electrolysis. Both the negative electrode (cathode) and positive electrode (anode) are made of graphite, which is a form of carbon. Aluminium ions receive electrons at the negative electrode and are reduced to aluminium atoms. Oxide ions lose electrons at the positive electrodes and are oxidised to oxygen gas. This oxygen reacts with the carbon of the positive electrodes, forming carbon dioxide. As a result, the positive electrodes have to be replaced frequently. This adds to the cost of the process.

Question 26

Write the word equation to describe the electrolysis of aluminium oxide?

Answer 26

Al3+ + 3e– → Al (reduction – gain electrons). | O2 + 4e– → 2O2– (oxidation – lose electrons).

Question 27

Explain why electrolysis is an expensive metal extraction method and illustrate this with the extraction of aluminium?

Answer 27

Electrolysis is an expensive metal extraction method as it takes a lot of energy to melt and you have to replace the positive electrodes often as they react with the oxygen that is being formed.

Question 28

Explain why cryolite is added to aluminium oxide in the industrial extraction of aluminium?

Answer 28

Cryolite must be added as it lowers the melting point of the aluminium oxide therefore reducing the amount of energy needed and the cost of electrolysis.

Question 29

State the products of the electrolysis of brine and a use for each?

Answer 29

Chlorine - disinfectant and purifier. Sodium Hydroxide - removing pollutants from water. Hydrogen- it is used as a fuel.

Question 30

Describe how to electrolyse brine in terms of ions moving?

Answer 30

Sodium ions migrate to the cathode. Chloride ions migrate the other way, toward the anode.

Question 31

Explain the electrolysis of brine using half equations, classifying reactions at the electrode as oxidation or reduction?

Answer 31

Sodium ions go to the cathode, where electrons enter the melt and are reduced to sodium metal. H+ cations are reduced when they gain electrons: 2H+(aq) + 2e- → H2(g). Chloride ions go toward the anode. Cl- anions are oxidised when they lose electrons: 2Cl-(aq) → Cl2(g) + 2e-.

Question 32

Describe electrolysis in terms of movement of ions?

Answer 32

Positively charged ions move to the negative electrode during electrolysis. They receive electrons and are reduced. Negatively charged ions move to the positive electrode during electrolysis. They lose electrons and are oxidised. The substance that is broken down is called the electrolyte.

Question 33

Explain why electrolysis can only occur when an ionic compound is molten or in an aqueous solution?

Answer 33

Electrolysis can only occur when a solution is melted or in an aqueous solution as otherwise the ions will be not free to move and not conduct electricity and the key part of electrolysis is that electricity can run through the solution.

Question 34

Write a balanced symbol equation including state symbols for the overall electrolysis of lead bromide?

Answer 34

Pb2+ +2e- ----> Pb(s) | 2Br- -2e- ----> 2Br(l)

Question 35

Explain how hydrogen ions and hydroxide ions can be present in solutions, including a balanced symbol equation with state symbols, for the reversible reaction which water ionsies ?

Answer 35

H20(l) produced from H+(aq) +OH-(aq)1 mole of any gas will occupy what volume?

Question 36

1 mole of any gas will occupy what volume?

Answer 36

24,000cm^3

Question 37

What is the unit of electrical charge?

Answer 37

Coulombs

Question 38

One mole of electrons is the same as how many coulombs?

Answer 38

96,500

Question 39

96,500 coulombs per mole is called what?

Answer 39

A Faraday

Question 40

Electrical current (i) is the flow of electric charge which is equivalent of what?

Answer 40

1 coulomb per second (1 A=1C/s).

Question 41

The rate of flow in electrical charge in measured in what?

Answer 41

Amps (A)

Question 42

One amp is one coulomb per second (show the letter equation)?

Answer 42

Q=Ixt=nxF (Charge is equal to current times by time) (This means that the number of coulombs is going to be the same as the number of moles of charge that flow*Faradays

Question 43

What does t stand for?

Answer 43

Time

Question 44

What does n stand for?

Answer 44

Moles of electrons

Question 45

What does F stand for?

Answer 45

Faradays

Question 46

Define the term 'Molten'?

Answer 46

A term used to describe a liquid substance (eg rock, glass or metal) formed by heating a solid.