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Chemistry (other) > C6: Electrolysis > Flashcards

C6: Electrolysis Flashcards

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1
Q

What is electrolysis?

A

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

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2
Q

What is the name of the positive electrode?

A

Anode

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3
Q

What is the name of the negative electrode?

A

Cathode

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4
Q

What is an electrolyte?

A

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

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5
Q

Where are metals formed?

A

Cathode

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6
Q

Where are non-metals formed?

A

Anode

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7
Q

How can ionic substances be electrolysed?

A

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

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8
Q

Why can ionic substances not be electrolysed?

A

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

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9
Q

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

A

When the metal is more reactive than hydrogen.

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10
Q

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

A

Halogen.

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11
Q

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

A

Oxygen.

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12
Q

What is the half equation for the ionisation of water?

A

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

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13
Q

What metals are extracted from ionic compounds by using electrolysis?

A

Metals that are more reactive than carbon.

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14
Q

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

A

To lower the melting point.

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15
Q

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

A

Graphite.

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16
Q

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

A

They react with the oxygen being formed.

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17
Q

During electrolysis, which electrodes are positive ions attracted to?

A

The Cathode.

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18
Q

Why can ionic solids not undergo electrolysis?

A

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

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19
Q

Do ions get reduced or oxidised at the anode?

A

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

20
Q

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

A

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

21
Q

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

A

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

22
Q

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

A

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).

23
Q

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

A

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.

24
Q

Explain why electrolysis is used to extract aluminium from compounds ?

A

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

25
Q

Describe the electrolysis of aluminium oxide?

A

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.

26
Q

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

A

Al3+ + 3e– → Al (reduction – gain electrons).

O2 + 4e– → 2O2– (oxidation – lose electrons).

27
Q

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

A

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.

28
Q

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

A

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.

29
Q

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

A

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

30
Q

Describe how to electrolyse brine in terms of ions moving?

A

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

31
Q

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

A

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-.

32
Q

Describe electrolysis in terms of movement of ions?

A

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.

33
Q

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

A

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.

34
Q

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

A

Pb2+ +2e- —-> Pb(s)

2Br- -2e- —-> 2Br(l)

35
Q

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 ?

A

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

36
Q

1 mole of any gas will occupy what volume?

A

24,000cm^3

37
Q

What is the unit of electrical charge?

A

Coulombs

38
Q

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

A

96,500

39
Q

96,500 coulombs per mole is called what?

A

A Faraday

40
Q

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

A

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

41
Q

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

A

Amps (A)

42
Q

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

A

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

43
Q

What does t stand for?

A

Time

44
Q

What does n stand for?

A

Moles of electrons

45
Q

What does F stand for?

A

Faradays

46
Q

Define the term ‘Molten’?

A

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

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