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chemistry > Electrolysis > Flashcards

Electrolysis Flashcards

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1
Q

Electrolyte

A

an Ionic compound, either molten, or aqueous that undergoes electrolysis and conducts electricity

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2
Q

Electrolysis

A

Breaking down ionic compounds in their molten or aqueous state by passage of electricity.

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3
Q

Hydrogen-oxygen fuel cells

A

Use hydrogen and oxygen to produce electricity
with water as the only chemical product

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4
Q

How is electricity conducted in external circucit

A

By electrons

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5
Q

How is electricity conducted in electrolye

A

By mobile ions

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6
Q

What happens during electrolysis

A

Ions are changed back to atoms by losing or gaining electrons

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7
Q

What energy change occurs in an electrolytic cell

A

electrical energy into chemical energy

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8
Q

What is at anode and cathode of molten electrolysis

A

Anode (+) = Non-Metals (except hydrogen gas)
Cathode(-) = Metals

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9
Q

In molten electrolysis, why does the bulb continue to flow after battery switched off?

A

metal cations reduced to molten metal atoms which fill bottom of test tube

after alot of metal at bottom, it makes contact with both electrodes and conducts, even if heat removed because metal can conduct as solids

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10
Q

Aqueous Electrolysis anode rules:

A

Concentrated:
1) Halogen (if halogen present)

2) Oxygen (if no halogen present)

Dilute:

Always oxygen

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11
Q

Aqueous Electrolysis cathode rules:

A

Concentrated:
1) Hydrogen gas (if metal MORE reactive than hydrogen)

2) Metal (If metal LESS reactive than H/ all transition metals except zinc and iron)

Dilute:
always hydrogen gas

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12
Q

Name of concentrated aqueous sodium chloride?

A

Brine

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13
Q

Why does pH of aqueous electrolyte increase (more alkaline)?

A

H+ ions are removed/ escape as hydrogen gas

Leave behind OH- ions

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14
Q

Why does pH of aqueous electrolyte decrease (acidic)?

A

OH- ions are removed/ escape as oxygen gas

Leave behind H+ ions

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15
Q

What colour is copper(II) sulfate electrolyte

A

Blue

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16
Q

Typical observations of electrolysis:

A
  • solid deposits at cathode
  • Bubbles of colourless/(coloured) gas at anode
  • Bulb lights up
  • Electrolye colour change
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17
Q

Dilute solution electrolysis anode and cathode results:

A

ALWAYS results in H+ at cathode (hydrogen)

ALWAYS OH- gas at anode (oxygen)

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18
Q

What is pH change in dilute electrolysis

A

Remains the same, because ions left over is the same as starting ions

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19
Q

Anode oxygen reaction:

A

4OH- → 2H₂O + O₂ + 4e-

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20
Q

Cathode Hydrogen reaction:

A

2H+ + 2e- → H₂

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21
Q

What are the two reactions of electrolysis using Active electrodes

A

1) Refining of metals (purification)

2) Electroplating

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22
Q

What is placed at the ANODE when refining a metal

A

the IMPURE metal

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23
Q

What is placed at the CATHODE when refining a metal

A

the PURE metal

24
Q

What is electrolyte of refining metals?

A

Aqueous
metal name
nitrate (/soluble salt)

25
Q

Why electrolye and electrode are same when refining metals?

A

So no different electrolyte metal is deposited at cathode and makes it impure

26
Q

What happens to anode when refining metals?

A

Decreases in mass/size

27
Q

What happens to cathode when refining metals?

A

Increases in mass/size

28
Q

What happens at bottom of electrolye when refining metals

A

Anode sludge forms:

the impurities from anode fall because they’re less reactive

29
Q

Why does copper have to be purified?

A
  • impurity increases electrical resistance in copper wires
  • increase in resistance = increase in temperature
  • can cause fire
30
Q

Why does the concentration of electrolyte stay the same when refining metals

A

For each metal atom oxidised to metal ion at anode,

only one metal ion is reduced to metal atom at cathode.

pH and colour of electrolyte stays the same.

31
Q

Electroplating definiton

A

The process where the surface of a metal is coated with a layer of metal using electricity

32
Q

In electroplating, what is at the anode?

A

the METAL you coat with

33
Q

In electroplating, what is at the cathode?

A

the OBJECT you coat

34
Q

In electroplating, what is the electrolyte?

A

aqueous
same metal name
nitrate (/soluble salt)

35
Q

Why is metal electrolyte same as electrode

A

so different metal in electrolyte does not elecroplate the object at cathode

36
Q

Reasons for electroplating:

A
  • Corrosion resistance
  • Look more expensive/improve appearance
37
Q

Aluminium ore?

A

Bauxite

38
Q

What is the type of electrolyte in extraction of aluminium?

A

Molten

39
Q

Alumina:

A

Aluminium oxide

40
Q

Why can’t an aqueous solution of bauxite be used?

A

Hydrogen will discharge at cathode instead of aluminium

41
Q

Whydo we need to melt alumunium oxide to molten?

A

aluminium oxide cannot conduct electricity as solid since its an ionic compound

melting point = 2050C

lots of energy required which is expensive

42
Q

Why is molten cryolite used?

A
  • Lowers the working temperature of cell from 2050C to about 900C
  • Improves electrical conductivity of electrolyte.
    (Aluminium oxide is a poor conductur of electricity, molten cryolite good conductor)
  • Saving energy and less expensive
43
Q

Why must Anode be replaced frequently in bauxite reaction?

A

Anodes are made of graphite (carbon) which react with oxygen produced at anode at high temp to produce CO2

CO2 escapes and decreases anode in mass/size

44
Q

Bauxite:
Anode reaction:

A

2O²- → O₂ + 4e-

Oxygen gas formed

45
Q

Bauxite:
Cathode reaction:

A

Al³+ +3e- → Al

46
Q

Reactions at anode:

A

Oxidation
X → Y + Z

47
Q

Reactions at cathode

A

Reduction
X + Y → Z

48
Q
A
49
Q

Hydrogen-oxygen fuel cell equation:

A

2H₂ + O₂ –> 2H₂O

50
Q

Advantages of Hydrogen-oxygen fuel cell:

A
  • Water is the only product
  • Produce MORE energy per gram of fuel burnt than other fuels
  • Very effecient
51
Q

Disadvantages of Hydrogen-oxygen fuel cell:

A
  • Storage of hydrogen (explosive) in pressurised fuel tanks (expensive)
  • Manufacture of hydrogen requires fossil fuels (air pollution)
  • Fuel cells do not work well at low temperatutes (more energy more expensive)
52
Q

Bauxite extraction process

A

Molten bauxite mixed with molten cryollite

to lower operating temperature from 2050C to 900C and to improve conductivity of cryollite.

Graphite inert electrodes are used

53
Q

What states are the halogen deposts at anode

(Based on their room temp state)

A

Chlorine = pale yellow-green gas
Bromine = red - brown liquid
Iodine = grey- black solid

54
Q

Electron flow in external circuit

A

Positive to negative
(Anode to Cathode)

55
Q

Electrolysis process simplified

A

Anode —> Electrons—> External circuit—-> Cathode

Anode: Anion attracted to anode, loses extra electrons to circuit, forming to atoms

Cathode: Cation attracted to cathode, gains the electrons from circuit, forming to atoms

56
Q

Why would electrolyte go from coloruled to colourless

A

Since the TRANSITION METAL cation is removed (thats what makes it coloured)

chemistry (8 decks)

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