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GCSE Chemistry > C6 - Electrolysis > Flashcards

C6 - Electrolysis Flashcards

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1
Q

What does the word electrolysis actually mean?

A

It means ‘breaking down using electricity’.

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2
Q

What is electrolysis?

A

The decomposition of an ionic substance, by passing an electric current through it.

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3
Q

What is the ionic substance that is being electrolysed called?

A

The electrolyte

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4
Q

Why are all ionic compounds strong electrolytes?

A

Because when they dissolve, they break apart into ions which are then able to conduct a current.

Even insoluble ionic compounds can still conduct electricity in a molten state.

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5
Q

In what condition can an ionic substance be electrolysed?

A

When in molten or dissolved state.

The ions become free to move around and can carry the charge to the electrode.

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6
Q

What electrode do positive ions move to?

A

The negative electrode

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7
Q

What electrode to negative ions move to?

A

The positive electrode

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8
Q

What is the negative electrode called?

A

Cathode (-)

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9
Q

What is the positive electrode called?

A

Anode (+)

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10
Q

What are the positive ions that move to the cathode called?

A

Cations

“Cations are pussitive”

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11
Q

What are the negative ions that move to the anode called?

A

Anions

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12
Q

What happens to the the ions when they reach their electrode?

A

The ions lose their charge and become elements.

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13
Q

How do the cations become elements at the cathode (-)?

A

Reduction - they gain electrons (to become stable) and are reduced.

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14
Q

How do the anions become elements at the anode (+)?

A

Oxidation - they lose electrons (to become stable) and are oxidised.

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15
Q

What type of reaction is electrolysis - considering it involves oxidation and reduction?

A

A redox reaction

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16
Q

What happens at the cathode (-)?

A

Reduction

RIG - Reduction is gain.

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17
Q

What happens at the anode (+)?

A

Oxidation

OIL - Oxidation is loss.

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18
Q

Write the word and chemical equation for the electrolysis of molten zinc chloride?

A

Zinc chloride ———> Zinc + Choride

ZnCl2 (l)* ———> Zn (s/l) + Cl2 (g)

*Because the symbol is (l), you know that the solution is molten and NOT AQUEOUS. This is very important as aqueous solutions are more complicated.

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19
Q

Explain and write the half equation for the electrolysis of zinc chloride at the negative electrode (cathode):

A

At the cathode (-) the cations are positive and therefore gain electrons. This means they are reduced.

1) Is the equation balanced: Yes

Zn2+ ———> Zn

2) Need to make the side without the lowest charge (Zn2+) equal to the side with the lowest charge (Zn). We do this by adding 2 electrons to the left hand side.

Zn2+ + 2e- ———> Zn

Reduction ALWAYS takes place at the cathode and half-equations for reduction at the cathode will always have the electrons (e-) before the arrow.

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20
Q

Explain and write the half equation for the electrolysis of zinc chloride at the positive electrode (anode):

A

At the anode (-) the anions are negative and therefore lose electrons. This means they are oxidised.

1) Is the equation balanced: No

Cl- ———> Cl2

Make it:

2Cl- ———> Cl2

2) We need to make the side without the lowest charge (Cl2) equal to the side with the lowest charge (2Cl-). We do this by adding 4 electrons onto the right hand side.

2Cl- ———> Cl2 + 2e-

Could also be written as:

2Cl- -2e- ———> Cl2

Oxidation ALWAYS takes place at the anode and half equations for oxidation at the anode will always have the electrons (e-) after the arrow.

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21
Q

Write the word, chemical and half equations at each electrode for the electrolysis of molten calcium fluoride?

A

Calcium Fluoride ———> Calcium + Fluorine

CaF2 ———> Ca + F2

Cathode: Ca2+ +2e- ———> Ca

Anode: F- ———> F2

Not Balanced:

2F- ———> F2 + 2e-

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22
Q

Why is electrolysis more complicated in aqueous (aq) solutions than in molten (l) solutions?

A

Because water ionises and splits into OH- and H+ ions.

Water ———> Hydrogen ions + Hydroxide ions

H2O (l) ———> H+ (aq) + OH- (aq)

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23
Q

What is the rule for working out what element will be produced at the CATHODE (-) during the electrolysis of an AQUEOUS solution?

A

There are 2 possible products as the positively charged H+ ions as well as the positively charged metal ions will both go to the anode. Either:

1) Hydrogen gas - in the form of bubbles - test with lighted splint (squeaky pop)
2) The metal in the solution - coats the electrode

Rule: The less reactive element out of hydrogen and the metal will be produced at the cathode.

E.g -if hydrogen is less reactive than the metal, it will be produced.
-if the metal is less reactive than hydrogen, it will be produced.

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24
Q

What is the rule for working out what element will be produced at the ANODE (+) during the electrolysis of an AQUEOUS solution?

A

2 possible products:

1) A gas of a halide (chlorine, fluorine, bromine)
2) Oxygen bubbles of gas

Rule: If a halide is present in the solution, the gas will be that of the halide. If a halide is not present, oxygen gas will be produced.

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25
Q

What would be produced at each electrode with the following aqueous solutions?

a) sodium chloride
b) copper (II) chloride
c) sodium sulfate

A

a) Cathode (-) = Hydrogen (because Hydrogen is less reactive than sodium)

Anode (+) = chlorine

b) Cathode (-) = copper (less reactive than hydrogen)

Anode (+) = chlorine gas

c) Cathode (-) = Hydrogen (less reactive than sodium)

Anode (+) = oxygen (no halide present)

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26
Q

What substance are left behind in the solution after the electrolysis of an aqueous solution?

A

The substances that are not reduced or oxidised at the elctrodes are left behind in the solution.

E.g sodium chloride - hydrogen gas is produced at the cathode (instead of sodium) and chlorine gas is produced at the anode (instead of the OH- ions that are then released as oxygen).

Therefore, Sodium hydroxide is left behind.

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27
Q

Why is Alimunium such an important metal?

A

Aluminium is used to make pans, aeroplanes, cooking foil, drink cans, bike frames and car bodies.

28
Q

How reactive is Aluminium?

A

Aluminium is pretty reactive and is high up in the reactivity series.

29
Q

What compound acts as the electrolyte for the extraction of aluminium by electrolysis?

A

Aluminium Oxide (Al2O3) is the electrolyte.

30
Q

Where do you find Aluminium Oxide?

A

The ore ‘bauxite’

31
Q

How is bauxite ore retrieved?

A

It is mined directly from the surface.

32
Q

What is the main impurity that bauxite ore contains?

A

Iron (III) oxide - this makes the waste solution from the separating process a rusty brown colour.

33
Q

How is the aluminium oxide prepared for electrolysis once purified?

A

It is melted so that the ions are free to move around.

34
Q

What is the melting point of aluminium oxide?

A

2050°C

Enormous amount of energy needed.

35
Q

What is aluminium oxide mixed with to reduce its melting point? What does it reduce the melting point to?

A

Molten cryolite

This reduces the melting point to 850°C - saving energy and money.

36
Q

What is the overall word and chemical equation for the electrolysis of molten aluminium oxide?

A

Aluminium oxide ———> Aluminium + oxygen

2Al2O3 (l) ———> 4Al (l) + 3O2 (g)

37
Q

What forms at the cathode for the extraction of Aluminium? What takes place?

A

Aluminium forms at the cathode and reduction takes place.

Al3+ +3e- ———> Al

38
Q

What forms at the anode for the extraction of Aluminium? What takes place?

A

Oxygen forms at the anode and oxidation takes place.

2O2- ———> O2 + 4e-

39
Q

What are the electrodes used in electrolysis of aluminium oxide made of? Why?

A

Graphite - (a form of carbon) - ia good conductor of electricity and heat.

40
Q

What is the problem with the electrodes used in the electrolysis of aluminium oxide?

A

The oxygen produced at the anode reacts with the hot graphite anode itself - releasing carbon dioxide and burning away the electrode.

The anodes must be changed regularly.

41
Q

What is the chemical name for brine/salt water?

A

Sodium chloride

42
Q

Why is the electrolysis of brine (aqueous) an important industrial process?

A

Produces 3 usesful product:

1) Chlorine gas - at the anode
2) Hydrogen Gas - at the cathode
3) Sodium Hydroxide

43
Q

What is the word equation for the electrolysis of brine?

A

Sodium chloride solution ———> hydrogen gas + chlorine gas + sodium hydroxide solution

44
Q

What are the chemical tests for chlorine, hydrogen and oxygen gas?

A

Hydrogen Gas - Squeaky pop test with lighted splint.

Chlorine - Use litmus paper. Should bleach.

*Also, oxygen - glowing splint (oxygen not produced in brine)

45
Q

How do the products differ at the cathode and anode when electrolysing molten brine?

A

Cathode = Sodium formed

Anode = Chlorine formed.

46
Q

Where is most copper extracted from?

A

Copper-rich ores - finite.

Supplies of these ores are becoming scarcer and scarcer.

47
Q

What are the 2 main methods of extracting copper from the ore?

A

1) Sulfuric acid used to produce a copper sulfate solution, before extracting the copper metal.
2) Smelting (roasting). Copper ore is heated to a high temperature in a furnace to produce molten impure copper. Then, the molten IMPURE COPPER is USED as the ANODE (+) in electrolysis cells to make pure copper at the cathode (-).

At the cathode, copper ions are are reduced to form pure copper.

Cu2+(aq) + 2e- ———> Cu

At the anode, impure copper is oxidised. (See Kerboodle page 213).

Cu(s) ———> Cu2+(aq) + 2e-

48
Q

Why is smelting bad?

A
  • Highly expensive.
  • Causes a lot of pollution.
  • Requires a huge amount of electrical energy.
49
Q

What 2 new ways are scientists developing to extract copper from low grade ores?

A

1) Bioleaching - the extraction of valuable metals from a low-grade ore using microorganisms such as bacteria.
2) Phytomining - the production of a ‘crop’ of a metal by growing high-biomass plants that accumulate high metal concentrations.

50
Q

What is the corrosion of iron called?

A

Rusting

51
Q

What metals does rust form on?

A

Iron and Iron alloys (steel) - on the surface

52
Q

What chemical substance is rust a form of?

A

Hydrated iron (III) oxide.

Fe2O3 with water loosely bonded in its structure.

53
Q

What 2 things are needed for iron to rust?

A

1) Air (oxygen)
2) Water

Iron + oxygen + water ———> Hydrated Iron (III) oxide

54
Q

How can iron be protected from rusting?

A

Providing a barrier between the iron and air + water:

  • Paint
  • Oil
  • Grease
  • Plastic
  • A less reactive metal
  • A more reactive metal (e.g zinc coating).
55
Q

When coating a metal with a less/more reactive metal, what is this called?

A

Sacrificial protection

56
Q

What are the properties of pure metals? How are they bonded?

A
  • Relatively soft and malleable.
  • Due to their regular layers of positive ions in their giant lattices, they can easily slide over each other when force is applied.
57
Q

What are alloys? What are their properties? How are they bonded?

A

Mixtures of metals

  • Within the alloy, differently-sized metal ions make it difficult for the layers to slide over eachother.
  • They are JAMMED together - so alloys are much harder than pure metals.
58
Q

What are the 2 common copper alloys?

A

1) Bronze - the first alloy made by humans about 5500 years ago. Made by alloying copper with tin. Bronze is used to make decorative items and statues.
2) Brass - made my alloying copper with zinc. It’s workable charactersitic means it can be used to make musical instruments like trumpets.

59
Q

How many alloys of aluminium are available?

A

Over 300

60
Q

What are lightweight aluminium alloys used for?

A

Building aircratfs

61
Q

What are heavier aluminium alloys used for?

A

Military vehicles and tanks.

62
Q

What is gold usually alloyed with to make jewellery?

A

Copper

63
Q

How is the purity of gold expressed?

A

In ‘carats’

24-carat gold is 99.9% gold.

An 18-carat gold ring is 18/24 = 75% gold.

64
Q

What are steels?

A

Alloys of iron and carbon with other elements.

65
Q

What are carbon steels used for?

A

They are the cheapest steels and are used for car bodywork, ships and containers.

66
Q

What are alloy steels?

A

More expensive than carbon steels, they are made with 1%-5% of other metals. E.g chromium-nickel steel is used as stainless steel.

GCSE Chemistry (17 decks)

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