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CHEM > unit 3 aos 2 - electrolysis > Flashcards

unit 3 aos 2 - electrolysis Flashcards

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1
Q

what is a spontaneous reaction?

A

a reaction which occurs by itself without the input of energy

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2
Q

what needs to be inputted to force a non-spontaneous reaction?

A

electrical energy

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3
Q

electrolysis

A

the input of electrical energy is to force a non - spontaneous reaction

  • electrons are forcefully moved in the opposite drection
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4
Q

what are cells in electrolysis called?

A

electrolytic cells

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5
Q

GALVANIC vs ELECTROLYTIC : type of redox reaction

A

G: spontaneous redox reaction
(strong oxident/reductant pair)

E: non - spontaneous redox reaction
(weak oxident/reductant pair)

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6
Q

GALVANIC vs ELECTROLYTIC: energy conversions

A

G: chemical TO (produce) electrical

E: (consumes) electrical TO chemical

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7
Q

ELECTROLYTIC set up

A
  • one shared electrolyte ( bc not a spontaneous reaction = reactants will not react with eachother if placed togeather)
  • no salt bridge
  • both electrodes in the shared electrolyte
  • cathode is ALWAYS inert
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8
Q

how do you know which side is positive and negative in a cell?

A

TALLER line = positive

SHORTER line = negative

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9
Q

what reaction occurs at each electrode?

A

SAME as galvaniic:
- anode: oxidation
- cathode: reduction

POLARITY just CHANGE

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10
Q

ion movement

A

cathions TO cathode
anions TO anode

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11
Q

why is a salt bridge not required?

A

bc the movement of electrolytes maintain electric neutrality

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12
Q

what do we do to ensure that the electrolytic reaction IS ACCURATLY predicted

A

write out all speices present
ALSO CONSIDER WATER

strongest oxident: top LEFT
strongest reductant: bottom right

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13
Q

what observations could be made in electrolytic cells?

A
  • change in mass
  • change in pH
  • bubles forming
  • colour changes (more/less intensly..)

as a last resort = volume

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14
Q

cathode reactivity in electrolysis

A
  • strongest reductant MUST BE PRESENT and the ANODE
  • metals at the cathode are considered UNREACTIVE thus are inert
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15
Q

what is the formula to calculate the EMF of a cell

A

EMF = oxident - reductant

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16
Q

what voltage is reqired for electrolysis

A

A voltage that is GREATER THAN the difference in EMF calculated

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17
Q

what conditions are electrochemical series constructed at?

A

SLC and 1.0M

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18
Q

when would cloride ions react more preferentially than water?

A

ON ECS cl is just above water SO

at concentrations greater than around 4.0M, the cloride ions would become a stronger reductant and react in preference to water

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19
Q

what about sodium ions?

A

on ECS Na is a few species below water SO

even at high concentrations, sodium ions would not react in preference to water

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20
Q

MOLTEN definition

A

the liquid form of solid extremly hazardous substance at HIGH temperatures, which is normally in solid form at SLC conditions

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21
Q

is water present in molten conditions?

A

NO, so do not consider water when finding the strongest oxident/reductant pair

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22
Q

how do the states change in molten conditions?

A

(aq) becomes (l) pr (s)
- any is accpeted if question does not specify.

  • BUT DOUBLE CHECK question to be sure
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23
Q

purpose of electrolysis

A

Produces products unlikely to exist in nature.

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24
Q

purpose of molten cells???

A
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25
molten electrolyte purpose
to react with species weaker than water
26
what material is typically used at anode?
graphite because it is inert and cheap platinum is also inert BUT more expensive than graphite
27
what material is typically used at cathode?
iron, because the cathode is unreactive in these electrolytic cells and even cheaper than graphite
28
what are the benifits to lowering the overall melting points of the electrolytic cells?
- it requires less energy to maintain molten conditions - thus would be cheaper
29
WHY are other electrolytes added?
- to lower the melting points by forming temporary ionic bonds with the exisiting electroltyte. which weakens the ionic bond between existing electrolyte - the advantages: less energy required + cheaper
30
what is the PURPOSE of having a barrier within electrolytic cells?
- to seperate products and so prevent them from direct contanct and spontaneous redox reactions - ALSO allows the flow of ions
31
what is the PURPOSE of constantly removing products?
- reduce the chance of products comming into contact with one another and reacting back - to prevent buildup of products as they may interfere with the reaction
32
what is the PURPOSE of an enclosed container?
- to prevent oxygen gas from spontaneously reacting with products
33
what is the problem with hydrogen gas?
- it is colourless - it is odorless - it is tasteless THEREFOR it is dificult to detect if hydrogen gas leaks.
34
why is Hydrogen gas dangerous?
it is flammable
35
what safety precautions can be taken to help mitigate (reduce) the risk of hydrgen gas?
1) ignition protection - do this by keeping away from ignition sources 2) removal of combustible vapours - do this by keeping in well-ventilated areas 3) storgage conditions - store in cool and dry areas 4) physical protection - wear PPE
36
how is hydrogen gas stored? and why?
Stored in gas tanks as a liquid - bc more can be stored in the same volume - bc it is easier to transport
37
how is hydrogen gas turned into liquid
context: boiling point of hydrogen gas is very low (-250ish) - HIGH PRESSURE (about 70000kPa) is used to lower its temp and make it a liquid
38
how is hydrogen gas produced?
1) electrolysis of water 2) steam reforming
39
STEAM REFORMING
- is very NON - RENEWABLE - its products are CO(g) or CO2(g) note: methane is currently obtained from fossil fuels but work is done to obtain it from biogas
40
Polymer Electrolyte Membrane (PEM) Electrolyser
- solar/wind energy is used (renewable) - electrolyte is a solid - electrodes PICCY - reaction is the electrolysis of ACIDIC water DISADVANTAGE: expensive
41
anode + cathod half- equations of PEM
cathode: 2H^+ (ag) + 2e^- -> H2(g) anode: 2H2O (l) -> O2(g) + 4H^+ (aq) + 4e^- OVERALL REACTION: 2H20(l) -> 2H2(g) + O2 (g)
42
Polymer Electrolyte Membrane (PEM) Electrolyser - ENERGY CONVERSIONS
solar -> electric -> chemical
43
Artificial Photosynthesis
- reaction equations are the same as PEM - high energy efficiency - solar energy is inputted instread of electrical energy - sunlight is shone on to the left electrode, causing the water to oxidise - solar -> chemical
44
what are protons?
H+ ions esentially
45
alternative name of Artificial Photosynthesis
"water oxidation and proton reduction catalyst systen"
46
SIMILARITIES between PEM electrolyser & Artificial Photosynthesis Cell
- reaction: the electrolosys of acidic water - both use solar (but PEM could instead use wind) - UN SD goals: 7, 13, 6 - Green prinicple priniciple: catalysis - disadvantages: expensive & water supply
47
DIFFERENCES between PEM electrolyser & Artificial Photosynthesis Cell
electrolyte: - PEM: solid - APC: aqueous energy converstions: - PEM: solar to electrical to chemical - APC: solar to chemical
48
PRIMARY cells
CANNOT be rechaged energy conversions: chemical TO elctrical
49
SECONDARY CELLS
CAN be recharged energy conversions: chemical TO elctrical & vice versa
50
DISCHARGE
spontaneous + galvanic - chemical TO electrical "when phone is being used" - electrons NATURALLY flow (->)
51
RECHARGE
non-spontaneous + electrolytic - electrical TO chemical "when phone is being charged" - electrons FORCED TO flow (<-)
52
discharge VS recharge
- the POLARITIES of the electrodes do not swap - the LOCATION of the cathode & anode swap and the POLARITY of the electrodes stay the same
53
electrolyte movement
still the same: anions TO anode cations TO cathode
54
what is required for a cell to be rechargable
- the reactants/products must remain in contact with the electrodes - also reactants/products MUST not participate in side reactions
55
WHAT are side reactions in primary cells
- they are an unintended chemical reaction which occures alongside the main reaction
56
WHAT state of matter will remain in contact with the electrode?
solid
57
Lead-Acid Accumulator (context)
- most common secondary cell that is tested by VCAA - found in car batteries - accumulator: another term for a secondary cell
58
Battery life of a secondary cell
- is decreaed by the reactants/products falling off the electrode and losing contact
59
What causes products to fall off the electrodes?
- physical stress to the battery: causes R/P to fall off the electrode - overheating: increases the likelihood of side reactions - overcharging the battery: does both of the above
60
REDOX flow battery
- a rechargable cell with a continuous supply of reactants
61
connected cells
where there are a mixture galvanic/ electrolytic / both cells connected to eachother - our job to figure out what they are on the connected cells (generally)
62
GALVANIC cells (terminology cues)
look for: - galvanic - spontaneous
63
GALVANIC cells (external circuit)
attached to a LOAD
64
ELECTROLYTIC cells (terminology cues)
look for: - electrolytic - non - spontaneous
65
ELECTROLYTIC cells (external circuit)
attached to a power source
66
electroplating
Plating a metal onto another metal via electrolysis by transferring the metal from one electrode to the other - it is also cheap and easy to set up
67
uses for electroplaating
- prevent rusting/corrosion - improve appearance - gain some properties of the coated metal for instance: + copper metal = good conductor + zinc metal = corrosion - resistence + nickel metal = coins (wear resista ce)
68
Electroplating process
THE anode metal oxidises into its ion form and moves to the cathode and reacts
69
electroplating process
OBJECT TO BE ELECTROPLATED: - @ cathode - attached to the negative terminal (RC-) METAL USED TO ELECTROPLATE - @ anode - attacjed to the positive terminal (AO+)
70
in electrolating is there a change in concentration
NO bc. the same amount of metal used at the cathode to electroplate is being prodced at the anode
71
Voltage inputted during electroplating
usually VERY LOW volatage is inputted
72
what if high voltage is imputted in electroplating
- the reactivity around the electrodes increases - there is MORE likelihood of substances reacting - if like over 4.0V, IT may cause other species other than the strongest oxidents and reductants to react
73
NEXT order reactions
assume current oxident/reductant runs out, move to the next strongest oxident/reductant
74
what is the endgame scenario?
electrolysis of water = water is used up = electrodes not touching the electrolyte - thus the e;ectrolytic cell breaks and no longer operates
75
what is electric charge?
the physical property of matter that causes matter to experience a force when placed in an electromagnetic feild simple def: how many positive or negative substances are there? denoted: Q si units: C
76
electrical flow
the flow of charged particles denoted: I si units: A
77
formula that links electrical charge and flow?
Q = IT
78
faradays first law of electrochemistry
Q is DIRECTY proportional to mass - eg. as MORE electrical charge passes throught he cell, the amount of substance that react/are produced also INCREASES
79
faradays constant
the quantity of charge carried by one more of electrons - 96500 C / mol
80
FORMULA linking electric charge and moles of electrons
n(e^-) = Q / F
81
Farafay's second law of electrochemistry
TO consider the stoichiometric ratio between n(e^-) : n(metals)
82
STEPS to derive a charge of a ion
1) find n(metal) by n(metal) = m/Mr 2) find n(e^-) by Q/F 3) find stoichometric ration by divinding the smallest number present
83
comparison of mass depositied
????
84

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