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Chemistry > Electrolysis Expanded > Flashcards

Electrolysis Expanded Flashcards

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1
Q

What is electrolysis

A

the process of electricity splitting an ionic compound into its component metal and non metal

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2
Q

the compound MUST contain ions therefore

A

only ionic compounds can be used

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3
Q

the compound’s ions MUST be free to move therefore

A

the compound must be MOLTEN (l) or in a SOLUTION (aq)

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4
Q

the anode is

A

POSITIVE

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5
Q

the cathode is

A

NEGATIVE

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6
Q

positively charged ions

A

are always attracted to the CATHODE as opposite charges attract

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7
Q

negatively charged ions

A

are attracted to the ANODE as opposite charges attract

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8
Q

Passing an electric current through ionic substances that are molten or in a solution does what?

A

breaks them down into elements, process called electrolysis

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9
Q

Equations for electrolysis of CuCl2

A

at cathode: Cu[2+] (aq) + 2e- –> Cu (s) therefore electrons are gained i.e. reduction

at anode: 2Cl[-] (aq) –> Cl2 (g) + 2e- therefore electrons are lost i.e. oxidation

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10
Q

In electrolysis, a liquid ionic substance such as lead bromide is needed to conduct electricity and allow current to flow around the electrolytic cell, what is there name given to these conductive substances

A

electrolyte

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11
Q

how can you test whether a particular substance can be used as an electrolyte

A

make a complete circuit with a battery and a bulb and if the bulb lights up then it is a conductor therefore suitable as an electrolyte

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12
Q

what flows around the circuit in electrolysis

A

electrons

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13
Q

always answer the substance names for electrolysis with what

A

the name and ions i.e. bromide IONS

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14
Q

how to explain that a substance can conduct electricity

A

as it has free delocalised electrons that electricity can pass through

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15
Q

how can ionic structures only work in electrolysis if they are molten

A

as they have a lattice structure without delocalised electrons therefore the ions are not able to move freely and carry charge

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16
Q

what if a substance cannot be easily dissolved in water

A

heat it until it reaches its melting point so it will become molten

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17
Q

a NEGATIVE reaction in electrolysis is a

A

reduction reaction

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18
Q

a POSITIVE reaction in electrolysis is a

A

oxidation reaction

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19
Q

if a circuit is in use via electrolysis of molten compounds the circuit will stop working if

A

the heat source is removed and the molten substance solidifies as electrons will no longer be carried around the circuit

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20
Q

usually at the negative terminal connected to the power source in electrolysis i.e. the cathode but soon after a metallic build up is found underneath it and on the anode

A

a gas is formed usually of the non metal substance

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21
Q

the circuit goes from

A

negative and around to positive

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22
Q

when electrons are picked up from the cathode by the positive metal they are replaced with

A

the electric pump of the circuit which is filled with more electrons

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23
Q

when electrons are taken away from the anode by the negative non metal they are pumped away by

A

the electric pump of the circuit which will take the the extra electrons via the appliance by the power source

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24
Q

electrons flow in the external circuit because of the … changes to the ions arriving at the electrodes and we say that the ions are … at the electrodes

A

chemical

discharged

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25
Q

what does discharging an ion mean

A

it means that it loses its charge, either giving up its electrons or receiving electrons from it

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26
Q

positive ions are known as

A

cations because they are attracted to the cathode

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27
Q

negative ions are known as

A

anions because they are attracted to the anode

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28
Q

some substances cannot be electrolysed molten

A

as they dont melt, but just break up or their melting point is too high for labs

29
Q

when electrolysing something in an aqueous solution what is produced at the cathode

A

hydrogen, this comes from the water in the solution as it is a very weak electrolyte and only ionises slightly to give hydrogen ions and hydroxide ions

30
Q

what is the equation for electrolysing water

A

H2O H[+] (aq) + OH[-] (aq)

31
Q

why do monatomic and simple molecular substances not conduct electricity

A

as they have no free electrons and do not form ions

32
Q

explain why giant covalent substances apart from graphite do not conduct electricity

A

as all the electrons on the outer shell are bonded to the atom

33
Q

Rules of electrolysis of solutions at the anode

A

if it is a carbonate nitrate or sulphate, then the ions stay in the solution and oxygen is given off, but if it is a chloride bromide or iodide then the OH[-] stays in the solution and the halogen is given off

34
Q

Rules of electrolysis of solutions at the cathode

A
  • if the metal ion present is LESS reactive than hydrogen then the METAL is formed
  • if the metal ion present is MORE reactive than hydrogen then HYDROGEN is formed
35
Q

if the compound contains oxygen then what will be produced at the anode

A

oxygen

36
Q

if the substance is aqueous then either hydrogen or hydroxide will be left in the pot but if it is molten then

A

nothing is left in the pot

37
Q

how can salt be extracted from underground deposits

A

by solution mining, and pumping very hot water under pressure down into the salt deposit, when the salt dissolves into it, the solution is pumped back to the surface

38
Q

what can concentrated salt be electrolysed to produce

A

three useful chemicals: sodium hydroxide, chlorine, hydrogen

39
Q

explain the left side of the diaphragm cell

A

concentrated sodium chloride goes in through the top half of that side, passes by a titanium anode, chlorine goes out through the top

40
Q

explain the right side of the diaphragm cell

A

sodium hydroxide solution contaminated with sodium chloride goes down the bottom half of that side, passes by a steel cathode, hydrogen goes out through the top

41
Q

explain the whole diaphragm cell

A
  • designed to keep the products apart
  • if the chlorine make contact with sodium hydroxide, bleach is made
  • if chlorine makes contact with hydrogen, the mixture made would violently explode on exposure to heat/sunlight giving hydrogen chloride
42
Q

in a diaphragm cell what happens at the titanium anode

A

chloride ions are discharged to produce chlorine gas

equation: 2Cl[-] (aq) –> Cl2 (g) + 2e[-]

43
Q

in a diaphragm cell what happens at the steel cathode

A

it is too difficult to discharge sodium ions, so hydrogen ions from the water are discharged instead to produce hydrogen gas
equation: 2H[+] (aq) + 2e[-] –> H2 (g)

44
Q

what happens to replace the discharged hydrogen

A

more water keeps splitting but hydroxide ions are also formed after each split, thus having a build up of sodium and hydroxide ions in the right compartment

45
Q

with residue build up in the right compartment of a diaphragm cell from sodium and hydroxide what happens

A

sodium hydroxide solution is formed, which is then contaminated with unchanged sodium chloride, the solution is concentrated via evaporation, but most sodium chloride crystallises into salt, this can be recycled by redissolving it into water

46
Q

uses of sodium hydroxide

A
  • the purification of bauxite to make aluminium oxide for production of aluminium
  • to help make paper; helps break wood down into pulp
  • to help make soap; reacts with animal and vegetable fats and oils to make compounds
  • to help make bleach; it is formed when reacted with chlorine in the cold
47
Q

uses of chlorine

A
  • sterilising water to make it safe to drink
  • making hydrochloric acid by a controlled reaction with hydrogen
  • making bleach
48
Q

the ions in the solution of the electrolysis of Brine (chloralkali industry)

A

sodium ions and chloride ions from NaCl:
NaCl –> Na+ + Cl-
hydrogen ions and hydroxide ions from water:
H2O –> H+ + OH-

49
Q

the positive ions in the electrolysis of brine: Na+ and H+ go to what

A

the cathode, here the ion of the least reactive element is discharged so the reaction is:
2H+ + 2e- –> H2(g)

50
Q

the negative ions in the electrolysis of brine: Cl- and OH- go to what

A

the anode, here the ion of the least reactive element is discharge so the reaction is:
2Cl- –> Cl2(g) + 2e-

51
Q

what are the products of electrolysing sodium chloride solution

A
  • hydrogen
  • chlorine
  • sodium hydroxide left in the solution
  • remember CatHy’s AnCl
52
Q

uses of hydrogen

A
  • making margarine
  • hydrochloric acid
  • as a non-polluting fuel
53
Q

Faraday’s first law

A

The amount of substance produced by electrolyse is proportional to the quantity of electricity used

54
Q

Faraday’s second law

A

For a given quantity of electricity the quantity of substance produced is proportional to its mass of the substance

55
Q

Quantity of electricity =

A

charge

56
Q

Charge in Coulombs equation (C)

A

= I x t
Current in Amps (I)
Time in seconds (s)

57
Q

what does one faraday (F) mean

A

the charge of one mole of electrons

58
Q

what is one faraday (F) equivalent to

A

96, 000 Coulombs

59
Q

number of moles of electrons equation

A

Q = nF
charge in Coulombs (C) = number of moles of electrons x Faraday constant (96,000Cmol-1) never use in actual Faradays

OR

n= Q/F

60
Q

What mass of aluminium will be deposited if a current of 1.5A is passed through aluminium oxide (dissolved in molten cryolite) for 40 minutes? [Data: Al=27 RAM; 1 Faraday = 96,000 C/mol]

A 
1) Find the total charge passed
Charge passes = 1.5A x 40 x 60s = 3600C
2) Convert this to moles of electrons 
number of moles of electrons = 3600C/96000 C/mol
3) Write out cathode equation and deduce molar ration of electrons to substance, and therefore moles to substance.
Equation is: Al3+ + 3e- --> Al
Molar Ratio 3:1
moles: 0.0375 ---> 0.0375/3
4. Convert moles of product into mass of product 
mass = moles x RAM
RAM Al = 27
Mass = 0.0125 x 27 = 0.3389
61
Q

Diatomic bonds: molecules with 2 bonds

A

H, N, O, F, Cl, Br, I

62
Q

What volume of hydrogen gas would be produced from the electrolysis of some hydrochloric acid by a current of 0.5A for 2.5 hours

A 
Q = 0.5 x 9000
Q = 4500 C
n = 4500/96000 = 0.046875
2H+ + 2e- --> H2
= 2:1
0.046875/2 = 0.0234375
ANS x 24 = 0.5625 dm(cb)
63
Q

what is contact process

A

the industrial method of producing sulphuric acid

64
Q

3 stages of contact process

A

1) making sulphur dioxide
2) making sulphur sulphur trioxide
3) making the sulphuric acid

65
Q

Stage 1 Method 1 of the contact process with balanced equation

A

raw materials of sulphur and air:

S(s) + O2(g) –> SO2(g)

66
Q

Stage 1 Method 2 of the contact process with balanced equation

A

raw materials of sulphide ores and air:

4FeS2(s) + 11O2(g) –> 2Fe2)3(s) +8SO2(g)

67
Q

Stage 2 of the contact process and balanced equation:
the reaction is … this means that at the same time as the forward reaction producing products there is also a … reaction which produces the …

A 
reversible
backward
reactants
2SO2(g) + O2(g)  2SO3(g)
triangle H = -196kJ mol-1 = exothermic
68
Q

conditions and explanation for the contact process:
Catalyst
Temperature
Pressure

A

Catalyst:
Vanadium(V) oxide –> has no effect on the percentage conversion, speeds up reaction. without it, the reaction would be extremely slow
Temperature:
450 degrees C –> forward reaction exothermic, higher conversions if at a lower temperature, but low temperature = slow reaction –> still 99.5% conversion rate
Pressure:
1-2atm –> 3 gas molecules left side, only 2 on right side, high pressures would increase product but not economically worthwhile

69
Q

Stage 3 of the contact process and balanced equation

A

sulphur trioxide is not reacted with water to produce sulphuric acid as it produces an uncontrollable fog of concentrated sulphuric acid
sulphur trioxide is reacted with sulphuric acid to form fuming sulphuric
this is then reacted with water to produce sulphuric acid
1) H2SO4(l) + SO3(g) –> H2S2O2(l)
2) H2S2O2(l) + H2O (l) –> 2H2SO4(l) (converted into twice as much sulphuric acid

Chemistry (11 decks)

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