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3/4 Chemistry. > Electrochemistry > Flashcards

Electrochemistry Flashcards

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1
Q

oxidation occurs at the

A

anode

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2
Q

reduction occurs at the

A

cathode

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3
Q

define oxidation

A

-loss of electrons
-increase in oxidation number
-gain of oxygen
-loss of hydrogen

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4
Q

define reduction

A

-gain of electrons
-decrease in oxidation number
-loss of oxygen
-gain if hydrogen

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5
Q

reductant AKA

A

reducing agent

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6
Q

oxidant AKA

A

oxidising agent

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7
Q

oxidant undergoes

A

reduction

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8
Q

reductant undergoes

A

oxidation

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9
Q

How does H oxidation state change

A

+1 in compounds with non metals eg HCl
-1 in compounds with metal hydrides eg NaH

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10
Q

what must the sum of oxidation numbers be

A

in a neutral compound oxidation numbers should sum to zero

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11
Q

in a polyatomic ion the oxidation number must

A

sum to the charge of the ion

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12
Q

How do oxidation numbers work in compounds without oxygen or hydrogen

A

the more electronegative element has a negative oxidation number, equivalent to what the charge of its ion would be

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13
Q

oxygen usually has an oxidation number of

A

-2

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14
Q

Fluorines oxidation number is always

A

-1

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15
Q

when does oxygen have a different oxidation number

A

-in peroxides it becomes -1 eg. H2O2
-when bonded to fluorine it becomes +2

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16
Q

KOHES stands for

A

balance Key element
balance Oxygen using water
balance Hydrogen by adding H+
balance charges by adding Electrons
add States

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17
Q

equation for Cr2O72- into Cr3+

A

Cr2O72- + 14H+ + 6e- –> 2Cr3+ + 7H2O

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18
Q

equation for Mn04- into Mn2+

A

MnO4- + 8H+ + 5e- –> Mn2+ + 4H2O

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19
Q

Whats a direct redox reaction

A

reactants are directly mixed in a single beaker and a spontaneous reaction occurs which typically releases thermal energy

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20
Q

Whats an indirect redox reaction

A

reactants are in two separate beakers and an indirect redox reaction occurs which typically produces electrical energy

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21
Q

How do you explain whether a reaction will occur in terms of reductants

A

(reactant species) is a WEKAER REDUCTANT than the CONJUGATE REDUCTANT of the (oxidant species), which is (stronger reductant) hence reaction will/ will not occur

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22
Q

How do you explain whether a reaction will occur in terms of oxidants

A

(oxidant species) is a WEKAER OXIDANT than the CONJUGATE OXIDANT of the (reductant species), which is (stronger oxidant) hence reaction will/ will not occur

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23
Q

Limitations of predicting using ECS

A

changes in temperature –> reaction may not be observed as the rate of reaction is too slow

Higher/lower concentration than 1M–> another product may form at an electrode OR if concentration too low the reaction may to be observed

Different conditions/experimental design –> different voltage produced

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24
Q

When can a weaker oxidant or reductant preferentially react

A

-It is possible when the weaker oxidant/ reductant has concentration greater than 1.0M eg 4-5M
-it is possible when the stronger oxidant/reductant has concentration less than 1M eg 0.01 M

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25
Q

What are SLC

A

25 degrees celsius , 100kPa and 1M solutions

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26
Q

define galvanic cell

A

electrochemical cells in which chemical energy from spontaneous redox reactions are converted into electrical energy

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27
Q

define anode

A

negatively charged in galvanic cell where oxidation occurs

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28
Q

define cathode

A

positively charged in galvanic cels where reduction occurs

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29
Q

define salt bridge

A

structure which allows for transfer of ions between two half cells to maintain electrical neutrality of cell and complete the circuit

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30
Q

define electrolyte

A

source of ions which are used to allow for flow of electric charge within a cell

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31
Q

define electrode

A

electrical conductor that is either unreactive or involved in reaction

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32
Q

define standard half cell

A

cells consisting of both members of conjugate redox pair and an electrode at SLC

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33
Q

define standard electrode potential

A

potential of the half reaction (reduction), given in volts, relative to standard H electrode

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34
Q

energy conversion is galvanic cells

A

chemical –> electrical

35
Q

Cations of salt bridge move towards

A

cathode

36
Q

anions of salt bridge move towards

A

anode

37
Q

electrons flow from

A

anode to cathode

38
Q

how to calculate electric potential

A

E(top in ECS) - E(bottom in ECS)

39
Q

Fuel cell vs galvanic cell

A

-fuel cell requires constant supply of reactants and electricity is continuously generated whereas in a galvanic cell a fixed amount of fuel is used to generate a finite amount of electricity
-fuel cells use porous electrodes whereas galvanic cells use inert or contain reactants/products
-fuel cells operate at higher temperatures
-fuel cells separate half cells in same vessels whereas galvanic cells have half cells in seperate vessels
-Both convert chemical into electrical energy

40
Q

in fuel cells , fuel reacts at ___
and ___ reacts at cathode

A

anode
oxygen

41
Q

function of electrolyte in fuel cell

A

-allows for movement of charged ions/source of ions which are needed for reaction to proceed
-transports H+ or OH- in acidic and alkaline fuel cells respectively

42
Q

function of electrodes in fuel cells

A

-site of oxidation /reduction
-porous, increase SA and rate of reaction
-may contain catalysts which lower activation energy and provide alternative reaction pathway
-pores allow gases to access electrode

43
Q

how to convert KOHES into alkaline conditions

A

add OH- to both sides, OH- and H+ cancel out to form water

44
Q

list 3 advantages of fuel cells

A

-high energy efficiency
-fuel flexibility
-quiet operation
-no need to be recharged
-low maintenance due to lack of moving parts
-low running costs

45
Q

disadvantages of fuel cells

A

-infrastructure and manufacturing is expensive
-need reliable and continual supply of fuels
-difficult to store, transport and distribute H2

46
Q

compare fuel cell and combustion engine

A

-fuel cells are much more energy efficient than combustion engines because combustion engines require many intermediate steps whereas fuel cells convert chemical directly into electrical energy.
-fuel cells can have overall less impacts on environment (because more efficient)
-fuel cells more expensive because H very expensive to store and obtain
-fuel cells quieter

47
Q

intermediate steps for combustion engines

A

chemical to thermal to mechanical to electrical

48
Q

Safety problems with hydrogen gas

A

-highly flammable
-colourless, tasteless and odourless so difficult to detect
-must be compressed and stored at very low temperatures so expensive

49
Q

When can fuel cells be beneficial to the environment

A

if the fuel supplied eg hydrogen gas is obtained via renewable resources then there is a negligible change to CO2 levels

50
Q

rechargeable batteries aka

A

secondary cells

51
Q

primary cells are

A

used until the supply of reactants run out (not rechargeable)

52
Q

Define memory effect for batteries

A

some cells have decreased capacity to be fully charged. This occurs when rechargeable batteries are not fully discharged before recharging

53
Q

When do mass of cells matter

A

in portable devices lightness is prioritised whereas vehicles can often accommodate heavier and bulkier batteries

54
Q

what two reactions do rechargeable cells undergo

A

discharge and recharge

55
Q

when a secondary cell discharges it coverts ___ into ___ and acts as a ___ cell

A

chemical
electrical
galvanic

56
Q

when a secondary cell recharges it coverts ___ into ___ and acts as a ___ cell

A

electrical
chemical
electrolytic

57
Q

What are the conditions for a secondary cell to be recharged

A

-the products of the discharge reaction must stay in contact with the electrodes
-a voltage higher than the voltage supplied during discharge is required

58
Q

during recharge the positive terminal of the power supply connects to the ___ an the negative terminal of the power supply connects to the ___

A

positive electrode
negative electrode

59
Q

polarity of electrode during recharge

A

anode is positive
cathode is negative

60
Q

what occurs at anode and cathode in recharge

A

oxidation at anode and reduction at cathode

61
Q

what dictates the polarity of electrode in recharge

A

electrons are pushed to the cathode and pulled away from the anode by the power supply therefore reduction at cathode and oxidation at anode

62
Q

define battery life

A

number of charge/discharge cycles before a battery becomes unusable

63
Q

what factors effect battery life

A

-high temperatures will deteriorate the cell at a greater rate ,increase the rates of side reactions and compromise the battery’s functionality (parts)
-low temperatures decrease the electric potential
-fluctuations in temperature can also impact battery life
-reactants and products may fail to adhere to electrodes
-reactants and products may be converted to inactive forms
-other chemicals may interfere with internal mechanism of batteries
-internal components may corrode
-impurities could react with reactants or products
-electrolyte may leak decreasing contact between electrodes and electrolyte
-memory effect

64
Q

how to increase battery life of rechargeable cells

A

store batteries at lower temperatures eg 10-15 degrees celsius

65
Q

how to increase the battery life of primary cells

A

-same principles as secondary cells but could also increase amount of reactants

66
Q

polarities of electrodes in electrolysis

A

positive anode and negative cathode

67
Q

what type of reactions occur in electrolysis

A

non spontaneous redox reactions

68
Q

functions of electrolysis

A

-electroplating
-produce reactive metals
-electrorefining
-recharge of secondary cells
-electrolysis of water to produce oxygen and hydrogen gas

69
Q

describe electron movement in electrolytic cells

A

electrons are pushed to the cathode and pulled away from the anode by the power supply therefore reduction at cathode and oxidation at anode

70
Q

liquid/ molten vs aqueous

A

aqueous contains WATER

71
Q

energy conversion in electrolytic cells

A

electrical to chemical

72
Q

in electroplating what is where

A

-object to be plated at cathode
-anode is the metal which will be plate on cathode eg silver metal

73
Q

what ions should always comprise the electrolyte

A

highly soluble ones
eg NO3- or SO42-

74
Q

in electrolysis of water what happens where

A

at cathode water is reduced to four hydrogen gas
at anode water is oxidised to from oxygen gas

75
Q

why is Aluminium dissolved in cryolite

A

lowers melting point of alumina from 2000 to 1000 degrees celsius approx.

76
Q

cryolite formula

A

Na3AlF6

77
Q

what is faradays first law

A

amount of any substance discharged at an electrode during electrolysis is directly proportional to quantity of electrical charge passed through the cell (linear relationship)

78
Q

the greater the charge of the ion the ___ mass deposited on cathode

A

less
because of molar ratios eg 1 mole of electrons will produce 1/2 mole of Mg

79
Q

what is Faradays second law

A

amount (in mol) of any substance discharged at an electrode during electrolysis is the reciprocal of the charge of ion

80
Q

at which electrode is the impure metal during electrorefining (eg.blister copper)

A

at the anode

81
Q

onto which electrode does the pure metal collect in electrorefining

A

the cathode

82
Q

explain the events occuring at the anode during electrorefining

A

the impure metal(eg. blister copper) is placed at the anode where the strongest reductants are oxidised(metals such as Pb and Ni).

the reductants weaker than the metal being refined(eg. Ag, Au and Pt) are not oxidised and hence fall to the bottom of the anode forming a muddy mixture known as anodic sludge.

83
Q

features of daniell(galvanic) cell which allows electrcity production

A

The two half-cells are separated so that electrons can flow through the connecting wire.
 The two half-cells are connected via a salt bridge to complete the circuit and allow electrons to
flow through the connecting wire.

3/4 Chemistry. (11 decks)

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