U3AOS1: options for energy production - electrochemistry Flashcards
1
Q
define oxidation
A
the loss of electrons
2
Q
where does oxidation occur
A
anode
3
Q
how can you identify oxidation?
A
-loss of electrons
-increase in oxid number
-gain oxygen
-loss of hydrogen
4
Q
define reduction
A
the gain of electrons
5
Q
where does reduction occur
A
cathode
6
Q
how can you identify reduction?
A
-gain of electrons
-decrease in oxid number
-lose oxygen
-gain of hydrogen
7
Q
what happens to the reductant?
A
-lose electrons bc undergoes oxidation
-causes another substance to be reduced (the oxidant)
8
Q
what happens to the oxidant?
A
-gain electrons bc undergoes reduction
-causes another substance to be oxidised (the reductant)
9
Q
KOHES in basic conditions
A
do normal KOHES, then add OH- ions to each side to balance out H+ ions and form water molecules
10
Q
what is a direct redox reaction?
A
when reactants are directly in contact with each other, so a spontaneous reaction has occurred without providing an external energy source - generally exothermic, release heat energy
11
Q
what is an indirect redox reaction?
A
when the reactants are not in contact and are in separate containers, connected by a wire which facilitates the reaction - releases electrical energy and a little bit of heat energy
12
Q
does a direct or indirect reaction produce a voltage?
A
indirect
13
Q
potential impacts and observations of redox reaction
A
-reaction may not be observed bc too slow
-another product may form at an electrode
-different voltage produced
14
Q
what are redox reaction predictions impacted by?
A
-changes in temperature
-higher/lower concentrations than 1.0M
-conditions and experimental design
15
Q
define galvanic cell
A
an electrochemical cell where chemical energy is converted into electrical energy via sponatneous reactions
16
Q
function of salt bridge
A
-allows the flow of ions between two half cells
-maintains electrical neutrality/even distribution of charge
-completes the circuit
17
Q
function of electrolyte
A
the source of ions, can allow for flow of electrical charge within the cell
18
Q
how to calculate electric potential
A
E(cathode) - E(anode)
19
Q
define fuels cells
A
a type of galvanic cells that converts chemical energy from a fuel to electrical energy
20
Q
what happens to the fuel and oxygen in fuel cell?
A
fuel is oxidised at anode
oxygen is reduced at cathode
21
Q
electrolyte in fuel cell
A
-can be solid or liquid
-allows the movement of charged ions, and reactant species must have access to electrolyte
22
Q
electrodes in fuel cell
A
porous electrodes so:
-greater surface area for reactions to occur
-may contain a catalyst
-allow reacting gases/ions to access electrolyte
23
Q
difference between fuel cells and other galvanic cells
A
constant supply of reactants vs finite quantity of fuel
porous electrodes vs non-porous electrodes
24
Q
advantages of fuel cells
A
-higher energy conversion efficiency bc less intermediate, energy conversions
-low chemical pollution
-quiet operation
-low running costs
-unlimited electrical energy produced w constant fuel supply
25
disadvantages of fuel cells
-manufacturing processes + materials are expensive
-need reliable and continual supply of fuel
-transportation and storage of fuel = difficult
-need to maintain at high temp
-expensive electrodes bc catalysts = exp
26
energy efficiency in fuel cells vs combustion
-coal fired power stations have many intermediate energy conversions so 30%
-fuel cells directly convert chemical -> electrical so 60% efficiency
27
why are fuel cells expensive in some aspects?
-porous electrodes and catalysts = exp
-fuel cell has to be run at high temps
-pure hydrogen is difficult to obtain
28
how is hydrogen used in a fuel cell renewable and carbon neutral?
-can be produced from electrolysis of water
-electricity used for this can be renewable eg solar/hydro powered
-no CO2 produced
29
how is hydrogen fuel cell non-renewable?
-hydrogen gas can be sourced from fossil fuels, releasing CO2 or CO in reactions = non renewable resources
-CO2 released from these reactions, and in transport or storage of fuel
30
safety and storage of hydrogen fuel
-highly flammable and ignited easily
-flame = almost invisible, colourless and odourless gas
-lightness t/f would dissipate into air
-high energy content so release high energy
-can be stored in metal hydrides, as a liquid but at very low temps, and in high pressure tanks
31
define primary cells
-a type of galvanic cell, non-rechargeable electrochemical cell where chemical reaction generates electrical energy
-amount of energy supplied is limited to amount of reactants available
32
why are primary cells non-rechargeable?
products do not remain in contact w electrodes
as a result, reaction can reach equilibrium and all reacts may be consumed
33
features of a dry cell:
-elongated anode to ^SA:V ratio
-electrolytic paste = balance charge, source of ions for reactants
-porous separator: prevent a spontaneous reaction from occurring b/w unwanted products, allows certain ions to pass through and prevent accumulation of charge (act as a salt bridge)
34
why is a paste better than an aqueous solution in a primary, dry cell?
-more concentrated so ^ ROR
-more practical
-aq solutions can leak
-water can react w products
35
features of an alkaline button cell:
-electrolyte can be liquid as water is produced
-greater life span than dry cells
-anode does not corrode easily bc alkaline = better as acids corrode metals easily
36
factors impacting primary cell selection
-initial and operating costs
-size and shape
-mass
-memory effect
-voltage provided eg. aq sols can do no more 2V
-discharge curve
-shelf life
-current-ease of disposal and env factors
37
define secondary cell
type of galvanic cell that is rechargeable and converts chemical energy to electrical energy
38
what is needed to recharge a secondary cell?
-need a higher voltage to recharge than is produced in discharge
-the pos terminal of power supply needs to be connected to pos terminal of cell and vice versa
-products in contact w electrodes
39
why are secondary cells rechargeable?
-in reversible form
-products of discharge reaction (SPECIFIC) stay in contact w electrodes
40
discharge of secondary cells
-acts as a galvanic cell
-produces electrical energy from chemical energy
-anode is negative, oxidation occurs
-cathode is positive, reduction occurs
-electrons flow from anode -> cathode
41
recharge of secondary cells
-acts as a electrolytic cell
-produces chemical energy from electrical energy
-anode is positive, OIL
-cathode is negative, RIG
-electrons still flow from anode to cathode -> must be pushed so e- can be consumed at cathode
42
define battery life
number of discharge/recharge cycles before a battery becomes unusable and essentially dies
43
how does temp affect battery life?
HIGH TEMP - ROR of side reactions also increases, compromising battery's functionality. while ROR of discharge also increases, overall battery life decreases
LOW TEMP - cell reaction rate also decreases of discharge, less electricity generated. expands battery life but slow rate
44
factors affecting battery life
-temp
-reactants and products = detached from electrodes
-impurities in cell react w products and reactants
-build of products = polarisation, accum of solids at electrodes so less effectively recharged
-corrosion of internal components
-leakage of electrolyte sol. decrease contact b/w reactants
45
what is self discharge
-loss of the electrical capacity of a battery due to the deterioration of battery components
-also caused by side reactions even when batter X used
-to decrease self discharge, should be stored at low temp
46
what is electrolysis
when electricity is converted into chemical energy via non-spontaneous reactions
47
uses of electrolysis
-convert ions into metals eg electroplating
-production of reactive metals
-electrorefining
-recharging secondary cells
48
anode is ___ in electrolysis
positive
49
cathode is ___ in electrolysis
negative
50
why is there a separator in electrolysis
-so products do not react with each other, otherwise a spontaneous reaction will occur
51
what reacts at the electrodes in electrolysis
strongest oxidising agent is reduced at cathode
stronger reducing agent is oxidised at anode
can be water
52
temp in molten/aq electrolysis
high temp for molten -> liquid states
room temp for aq sols -> normal, ECS states
53
why must the reactants be separated in galvanic cells?
-indirect reaction must occur to produce a voltage. if a direct reaction occurred, then only thermal energy will be produced
-sometimes there is a screen eg. to prevent gases to react w metals eg. H2 gas
54
what is needed for electrolysis of water
an electrolyte eg. H2SO4 and KNO3 = added in low concentrations
55
what happens in electroplating
cathode is where object to be plated is connected so metal ions can be reduced onto it
anode is source of ions = piece of metal
electrolyte = also has source of metal ions
56
why are temps reduced eg. aluminium prod
-on an industrial scale = operation costs should be kept as low as possible to reduce amount of energy used
-also way to reduce emissions of process
-difficult to maintain high temps
57
faraday's second law of electrolysis
the amount in mol of any substance discharged at an electrode during electrolysis is the reciprocal of the charge on the ion
58
faraday's first law of electrolysis
the amount of any substance discharged at an electrode during electrolysis is directly proportional to the quantity of electric charge passed through the cell
59
what are the substances called that reduce melting point of molten compound in electrolysis?
fluxes eg cryolite
60
main uses of electrolysis
-producing chemical substances eg. electrolysis of water, electroplating, electrorefining
-for rechargeable cells
61
why might the experimental voltage produced in a galvanic cell be lower than theoretical voltage?
the external circuit/wiring provides resistance = frictional force, so thermal energy is produced in this resistance. voltage produced is lower
62
features of a salt bridge
-prevents an accumulation of charge by maintaining an even distribution of charge
-completes internal circuit
-cations move to cathode, anions move to anode
63
features of salt bridge compound
-soluble
-low reactivity
-ionic compound, so has cations and anions
64
feature of electrolyte in galvanic cell
-provides a source of ions
-conductive of electricity
-has to be soluble, so if a contaminate enters, a precipitate will not form
65
limitations of electrochem series
-only applies to reactions in aq. sols
-can only apply to SLC
-does not provide indication of reaction rate
-does not tell us if a precipitate is formed
66
if there are two possible cathode reactions, how do we know which one is the stronger oxidant?
you would have to put them both together in separate half cells and see which one undergoes oxidation and reduction. can use voltmeter to measure voltage produced. BUT we do know they are stronger than SHE
67
features needed in electrolytic cell
-electrolyte, acts as a source of ions reacting at electrodes and completes inner circuit
-two conductive electrodes
-an external source of electrons to push them from anode to cathode
68
why must the minimum voltage required be of that which is produced in galvanic cell?
so the resistance in electrical circuit can be overcome. the non-spontaneous reaction can occur
69
why must the separator in electrolysis be semi permeable?
-first, need separator to separate products that can spontaneously react
-second, must be semipermeable so complete circuit and allow movement of ions
70
exception of chlorine reacting in electrolysis in aq. solutions
in dilute solutions, changing conc of chloride electrolyte may reduce its standard electrode potential so Cl- is a stronger reductant than water. if conc = BIGGER THAN 2M, may be oxidised. so produce chlorine gas instead of oxygen
71
in electroplating, is the concentration of the metal changing?
no. molar ratios of reduction and oxidation equations are the same, so concentration remains constant. the metal ions used must be a stronger oxidant than water
72
why may the mass lost and gained in electroplating not be equal?
bc the reduced metal may not adhere to the cathode, so instead it flakes away
73
faraday's law formulas
Q = I t (where I=current in amps and t=time in seconds)
n(e-) = Q (coloumbs) / F (96500)
74
why may an alcohol eg methanol have a lower expected mass/volume than theoretical?
-alcohols tens to have lower boiling point, so could evaporate away
-side reactions occurring that may consume the methanol produced
-reactants escape eg. CO2
75
why can sulphate and nitrate ions be ignored in electrolysis?
-both sulfur and nitrogen have high ON in ions
-more likely to be reduced at cathode
-BUT, cations so neg charge. cathode in electrolysis i s negative, they will repel from each other
-the ions will not undergo reduction
76
KOHES in alkaline conditions
do KOHES normally
to cancel out H+ ions, add same number of OH- ions to BOTH SIDES
they will form water molecules
77
differences between galvanic vs electrolysis
neg anode vs pos anode
pos cathode vs neg cathode
spontaneous vs non-spontaneous
separate half cells vs one cell
chemical -> electrical vs electrical -> chemical
salt bridge vs no salt bridge
no power supply vs power supply
78
similarities btw galvanic and electrolysis
oxidation still occurs at anode
reduction still occurs at cathode
79
what is brine
concentrated NaCl - can cause Cl- to be a stronger reductant than water, reduce standard electrode potential, and be preferentially oxidised to water
80
safety considerations with Cl2 g, H2 g, NaOH
-chlorine gas is toxic
-hydrogen gas is higher flammable
-NaOH is caustic
81
safety measures
-use fume cupboard
-keep gases away from naked flame, ignition sources
-wear lab coat, safety goggles, neutralise any spills w water
82
what is a lead acid battery?
-most widely used secondary cell for cars and trucks
-6 separate cells connected together
-sulfuric acid = electrolyte
-total potential is 12V
83
why is a lead acid battery rechargeable?
-alternator that supplies electrical energy produces 14V
-products = PbSO4 forms a solid on surface of electrodes
84
how can you improve battery life?
-store at consistent temps and avoid high temps
-increase amount of reactants present
-reduce surface area of anode and cathode so longer lasting
85
what are the discharge reactions in a lithium-ion cell?
anode: Li(s) -> Li+ + e
cathode: CoO2(s) + Li+ + e- -> LiCoO2(l)
86
what are some hazards of a cell being in the body eg as a pacemaker?
-battery may overheat
-content of device/electrolyte may leak ou
-toxic to body
87
why are electrodes on fuel cells expensive?
-porous
-may contain catalyst
-may be from high quality metals
88
how can we increase the amount of mass deposited on the cathode in electrolysis?
-increase the time of electrolysis
-increasing conc, SA and temp = increase ROR, not mass
89
how may H2(g) be produced renewably?
-from electrolysis of water
-2H2O(l) -> 2H2(g) + O2(g)
-use of solar/hydro energy to produce electricity required for electrolysis
90
state the functions of the electrolyte in a fuel cell?
-allows movement of ions between cathode and anode
-conductive of electricity
-balance a build up/accumulation of charge in the cell
91
give some safety considerations for H2 gas
-highly flammable, low flashpoint so should minimise ignition/spark sources
-colourless/odorless so cannot be detected easily, H2 detectors would help identify leakages
-needs to be stored under high pressure, so should be careful abt sustaining pressure
92
-what are an example of inert electrodes?
-graphite
-platinum
-DONT PUT CARBON
93
why is the experimental voltage produced different to theoretical voltage?
-not set up at SLC
-other side reaction occurring eg in lemon
94
can molten compounds have water?
NO
LIQUID STATE
95
what voltage is required to recharge a secondary cell>
GREATER than the voltage produced from the discharge reaction
96
eventually a galvanic cell may cease producing electrical energy. why?
the cell reaction will be at equilibrium
97
how is the aluminium air cell similar to a fuel cell?
has a constant/continuous supply of oxygen as reactant
98
how is the aluminium air cell similar to a primary cell?
when all of the aluminium has reacted, cell will stop functioning and no longer produce electrical energy
99
what happens in precipitates attach to electrodes?
bc cell reactions occurs on SURFACES of electrodes, then anything sticking to and blocking the electrodes prevents reactants from reacting w surface of electrodes. cannot access them. the cell cannot function
100
what can you not forget in aq. solutions?
WATER
101
what would be reduced first in reaction w copper - water, or magnesium?
WATER
stronger oxidant than Mg2+ ions, so preferentially reduced at the cathode and bubbles of oxygen gas will form
102
function of porous separator in lead acid battery
-allow movement of ions b/w electrodes
-prevent electrodes from making contact
103
how is a secondary cell different to a fuel cell?
-more convenient for on-off usage bc not need continuous supply of reactants
-can be electrically recharged vs continuous reactants needed
-cheaper than fuel cells
-more suitable for today's electronic devices
104
molten HF is not used in electrolysis to produce fluorine because
HF is a molecule
105
aqueous HF is not used in electrolysis to produce fluorine because
oxygen would be produced
106
when two sets of half cells are connected, which one acts electrolytic?
-need to consider potential difference (voltage)
-cell that provides the greatest voltage will generate electrolytic function in the other cell
107
in electrorefining, what must you consider?
-only apply voltage in which reaction of metal you WANT can be oxidised ie. not provide enough voltage for stronger reductants to be oxidised
-must run for certain amount of time, otherwise next oxidant will reduce onto cathode, ruining aesthetics.
108
what may not be true regarding a galvanic cell?
-that the maximum voltage delivered by the cell is. eg 1.56 V
-this could the max voltage delivered ONLY AT SLC, so we don't know what max is if conditions change
109
determine the environmental impacts of the electrolysis of water
-depends on source of electrical energy
-if renewable eg. hydro/solar, then minimal impact
-if non-renewable eg. coal/gas, then CO2 from combustion likely to have negative impact on greenhouse gas emissions
110
difference between load and power source
LOAD = putting energy out of a cell eg a voltmeter
POWER SOURCE = supplying cell with electrical energy
111
state the function of the electrolyte in a fuel cell
-carry current between electrodes by movement of cations to cathode and anions to the anode
-complete the circuit by allowing flow of charged particles through the cell
112
porous electrodes are often used in acidic fuel cells bc
they are more efficient that solid electrodes at moving charges and reactants
113
propose a method to determine how quickly a solution of h2o2 decomposes
-use a known amount of solution
-measure volume of o2 produced over time
-use a gas syringe apparatus to measure volume
-or can measure change in ph using ph probe
114
what is the applied voltage required for an electrolysis cell to operate?
has to be >___
use of exactly __ V will not generate any reaction due to overpotential needed
