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Core 5 > Electrochemistry > Flashcards

Electrochemistry Flashcards

1
Q

What is electrochemistry?

A

Electrochemistry is the study of the relatioship between electricity and chemical change

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2
Q

What are oxidatiopn, reduction, redox and explain oxidation states using combustion of methane

A

Oxidation is loss of electrons, Reduction is the gain of electrons

Redox: chemical reaction that involves a reduction and oxidation process

CH4 + 2O2 —> CO2 + 2H2O

C is more electronegative than H so oxidation state is -4 then +4

H oxidation state is +1 then +1

O is more electronegative than carbon sp oxidation state =0 then -2

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3
Q

Which species is oxidised and which is reduced in combustion of methane?

A

C oxidation state -4 — +4 = oxidation (loss e-)

H oxidation state +1 — +1

O oxidation state 0 — -2 = reduction (gain of e-)

O2 is actimg as an oxidising agent

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4
Q

What are the half cell reactions and net reaction for combustion of hydrogen?

A

H2 oxidation: 2H2 —> 4H+ + 4e-

O2 reduction: O2 + 4H+ + 4e- —> 2H2O

Net: 2H2 + O2 —> 2H2O

reaction in forward direction is spontaneous and releasing energy

reaction in backwards direction is energy storing in the form of chemical bonds

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5
Q

What is the difference between a fuel cell and a battery?

A

A fuel cell is anm open reactive system - requires the influx of fuel and the outflux of products

Battery closed reactive system

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6
Q

How do we release energy from a fuel cell

A

Hydrogen is ionised at platinum electrode to 4H+

4e- flow through electrode then through wire to other platinum electrode.

4H+ flow through salt bridge (membrane separating two half cells permeable to H+

O2 combine with electrons and protons to form water

e- flowing through the circuit can be made to do work such as drive a motor in a car

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7
Q

How do we capture energy in a solar panel?

A

Sun releases photons

Panel converts photons absorbed to electricity

Electricity uis used to drive water splitting

Knows as an artificial leaf - chemical/ synthetic photosynthesis

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8
Q

Apllications of electrochemistry to synthesis

A

Use an electrical driving force to activate otherwise challenging chemical bond formation reactions

tertiary hydricarbon with half O2 using C5H12N and electrical system with RBC anode and Ni cathode can form tertiary alcohol

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9
Q

What is an anode, cathode, net reaction and cell potential using lithium ion fuel cell?

A

Anode - where oxidation occurs (anions attracted):

LiC6 —> Li+ + e- + C6 - oxidation

Cathode: where reduction occurs:

CoO2 + Li+ + e- —> LiCoO2 - cobalt reduction

Net reaction: LiC6 + CoO2 —> LiCoO2 +C6

Cell potential E cell = Ecathode - Eanode

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10
Q

What is the equation for Gibbs free energy?

A

ΔGcell = -nFEcell

n = no. electrons transferred

F = Faraday constant (96485 C mol-1)

Ecell = cell potential

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11
Q

What is the equation for the equilibrium constant?

A

ΔG = -RTlnK

K = exp^ΔG/RT

T = temp in Kelvin

K = equlibrium constant

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12
Q

How do you predict if an acid base reaction is spontanteous?

A

ΔGreaction = ΔG1 - ΔG2

-RTlnKrxn = -RTlnK1 + RTlnK2

lnKrxn = lnK1 - lnK2 = ln(K1/K2)

Krxn = K1/K2 = 10^-pKa1 / 10^-pKa2

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13
Q

What is the general equation used for redox reactions

A

For generic reaction where oxidied species is reversibly recuced by unspecified number of electrons we use the equation: Ox + ne- <—> Red

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14
Q

Why do we define standard electrode potentials and what are they?

A

We use standard electrode potentials to quantify the thermodynamics of reactions

They are a measure of the individual potentials of a reversible electrode under standard consitions:

Stated temp - usually 298 K

Unit activity of every ion in the reaction - 1 mol dm-3

Partial pressure of 1 bar for each gas in the reaction

Metals in their pure state

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15
Q

What do the signs of standard electrode potentials show?

What are standard elevctrode potantials defined against?

A

Very -ve = loses electrons easily

Very +ve = gains electrons easily

Defined against standartd hydrogen electrode

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16
Q

What is the equation for th standard electrode potential of a cell?

A

Ecell = Ecathode - Eanode

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17
Q

Which way do electrons spontaneously flow?

A

from the anode to the cathode

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18
Q

What are the parameters for a reaction being spontaneous?

A

DG must be negative

For this to be:

Ecathode > Eanode

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19
Q

What are the rules for writing cell diagrams?

A

Anode described first, then the cathode

Reagents described first then products

Single line drawn between two chemical species and double line represent salt bridge

Phase shows in brackets

If electrolytes are not at standard consitions concentration/pressure are specified in the brackets

Zn(s) | Zn(aq)2+ || Cu(aq)2+ | Cu(s)

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20
Q

What is a latimer diagram?

A

a summary of teh standard electrode potential data of an element

The most highluy oxidised form of the element is on the left with lower oxidation states to the right

Species are connected by arows - each arrow labelled with the standard potential for that reduction reaction

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21
Q

What is the latimer diagram for Cl and what are the oxidation states

A

ClO4- = +7

ClO3- = +3

HClO2 = +3

HClO = +1

Cl2 = 0

Cl- = -1

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22
Q

How do you calculate the standard electrode potential for the cinversion of the mist oxidised species to the most reduced ina latimer diagram?

A

Thestandard electrode potentials for each small reaction cannot be added.

Using DG = nFE the DG value for each small reaction must be determined then added

This value can be converted back to E

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23
Q

What is a comproportion reaction?

A

When two equivalents of the same elements differing in oxidation state combine to form an intermediate compound with intermediate oxidation state

ClO3- + 5Cl- + 6H+ <—> 3Cl2 + 3H2O

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24
Q

What is a Frost diagram?

A

summarises the energetics of the half-reaction in which species X with oxidation state N is conveted to its elemental form for all oxidatin states accessible to element X

ON the y axis the NE value for converting a given oxidation state to the elememental form

The oxidation state os potted on the x axis

y axis of proportional to the gibbs energy of the conversion

The gradient of a line between two species is the electrode potential for that reduction

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25
Q

How do you determine whether a species os unstable with respect to disproportionation?

A

A species that lies above the gradient between two points shows that is unstable with respect to disproportionation

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26
Q

How do you determine whether comproportionation is favourable?

A

If a the middle species in a set of three is in a ‘valley’ on the Frost diagram

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27
Q

How do you construct a Frost diagram?

A

Write down all the species in the latimer diagram

Write down the oxidation states of each

Write down the electrode potentials for each step

Multiply each electrode potential by the nukber of electrons

Add the NE values that would be required to get the species to the elemental form

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28
Q

How do you find the mopst stable oxidation state from a Frost diagram

A

Lowest point on the graph

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29
Q

What is the Nernst equation?

A

E = Eo - RT/nF lnQ

Q = reaction quotient - supposed to be acitivity of products/reactats but we simplify to concentration

The activity of a solid is 1

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30
Q

What is the equation for dE/dpH?

A

For m-proton and n-electron processes

dE/dpH = -m/n * (RT ln10) / F

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31
Q

How much does the potential of a reaction change with pH unit?

A

59 mV

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32
Q

How do we decide which pH to run a reaction at?

A

If the line of the latimer diagram is steeper at one pH this means the reduction potential is larger and therefroe the reaction is more feasible at this pH

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33
Q

What are Pourbaix diagrams?

A

plots of electrode potentials vs pH

They map out the possible stable phases of an electrochemical system and compare this to the redox reactivity

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34
Q

What is the Pourbaix diagram for water?

A

Water is only stable within a restricted potential window

At suffieciently negative pH water is susceptible to reduction:

2H+ + 2e- H2 Eo = 0V

Moving to higher pHs the electrodempotential changes with gradient -59 mV

At sufficiently high potantials water is susceptible to oxidation:

O2 (g) + 4H+ + 4e- 2H20

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35
Q

What is activation energy?

A

The energy difference between the starting materials and the transition state

If the products are at a lower energy than the reactants the reaction is usually exothermic - this gives negative delta G value

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36
Q

What is the overpotential in the oxidation process?

A

Theoretically if an applied voltage is greater than Ered this should drive oxidation

In reality the applied voltage must be greater than the overpotential for oxidation to occur

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37
Q

What s the overpotential in recution process?

A

Theoretically if an applied voltage is more negative than Ered this should drive reduction

In reality it must be more negative than the overpotential

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38
Q

How do overpotentials affect batteries?

A

A kinetic barrier to the reaction manifests itself as an Ecell that is smaller than the thermodynamic value

This qualifies as an overpotential

Ecell = Ecathode - Eanode - Eoverpotential

If there is an overpotential the energy released is lower than the theortical maximum

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39
Q

What does the theoretical Ecell look like if the anodic process has an overpotential and if the cathodic process has an overpotential?

A
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40
Q

How does an overpotential manifest itself in an energy consuming process?

A

The requirement for greater input of energy

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41
Q

What is the overpotential in photosynthesis?

A
  • 0.3V is the potential for NAD+/NADH at pH7
    0. 8V is the potential for O2/H2O at pH7

Photosynthesis has an overpotential of approx. 0.3V

Ephotosynthesis = -0.3 - (0.8 + 0.3) = -1.4V

DGphotosynthesis = -nFE = -4 x 96485 x -1.4

= 540 kJ mol-1

42
Q

In a zinc copper cell how do we determine how the concentrations of each half cell have changed at equilibrium?

A
43
Q

How do we determine the total charge that flows in a cell?

A

Total charge (C) = amount of e- (mol) x F (Cmol-1)

For the zinc copper cell2 electrons flow for every mol of Cu consumed (2 x 0.4 = 0.8 mol of electrons)

So 0.8 x 96485 = 77188 C

This gives no information about how quickly the equilibrium is reached

44
Q

What is dynamic electrochemistry?

A

monitoring the rate and energetics of an electrochemical reaction

All basic dynamic electrochemistry experiments measure electrical current (rate of flow of e-) while controlling the electrode potential Cthermodynamic driving force

45
Q

What is the most common way to make a dynmaic electrochemistry experiment?

A

The 3 electrode cell

The potential of the working electrode is measured relative to that of the reference electrode while current flows between the working electrode and the counter electrode

46
Q

What does current tell us about the working electrode?

A

Negative current = electrons moving out of the working elevctrode surface

Positive current = electrons moving into the working electrode surface

47
Q

What is faradaic current?

A

Current arising from a redox process occurring at a working electrode

Positive faradaic current indicates an oxidative process

Negative faradaic current indicates a reduction reaction is occurring

Current due to a non redox potential is described as non faradaic

48
Q

What is the reference electrode used for?

A

Standard electrode potentials are measured relative to the standard hydrgen electrode at pH0, 1 bar H2 and Pt electrpocatalyst

This is not always practical

Reference electrodes are used instead which have highly stable redox potentials vs the hydrigen electrode

reference electrode potential is measured V vs SHE so the potential of a cell measured is also defined as V vs SHE

49
Q

Why do we use a counterelectrode?

A

If current flows through the reference electrode this will perturb the ratio of oxidesed:reduced species

This would change the redox potential

50
Q

What is the supporting electrolyte?

A

The solvent system

Must have suffiecient conductivity so that the solution resistance does not convolute experiment results

In aq solution NaCl is commonly used

In organic solvents tetrabutylammonium hexafluorophosphate is a common salt to use.

51
Q

What is chronoamperometry?

A

Measure current as a function of time while constant voltage is applied

applied voltage is Eapp (V vs Ref)

Time of hold - thold

52
Q

What is voltammetry?

A

Voltage is modulated as a function of time while current is monitored - plot current vs voltage

53
Q

What is linear sweep voltammetry?

A

Linear voltage sweep is applied with either a positive (anodic) or negative (cathodic) gradient

Control the potential of the working electode (gradient of plot against time is the scan rate)

Increasing the potential increases the driving force to remove electrons

Measure current vs voltage

54
Q

What is cyclic voltammetry?

A

A forward and backwards sweep is applied with both a positive and negative gradient

Repeated voltage cycles are called scans or sweeps or CVs

Control potential of the working electrode with Einitial, Eend and Eturn (tutning point voltage)

Measure current

55
Q

What do we measure in cyclic voltammetry when there is a lack of redox activity?

A

With non-faradaic current (absence of redox active species) a baseline current response is recorded

This can be thought of as eqivalent to the solvent ad cuvetter absorbance in UV vis

This current is referred to as non-faradaic or capacitive current

It arises because of double layer rearrangements at th wokring elecrode surface-solution interface

56
Q

How does changing current affect the baseline current?

A

Steadily decreasing the voltage of the woring electrode charges the surface of the electrode with cations because of the electrostatic attraction and anions move away resulting in negative current. The rate of movement of ions is directly proportional to the rate of change of potential so a constant -ve gradient os measured

Reversing the direction of the voltage sweep reverses the direction of ion movement resulting in an equal current of opposite sign as anions move towards the electrode surace and cations move away

57
Q

How does scan rate affect ion movement?

A

Scan rate directly controls rate of movement of ions

There is a linear relationship between capacitive current and scan rate

58
Q

What is the difference between film and solution electrochemistry?

A

Film electrochemistry is when the analyste is adsorbed onto the working electrode

Solution electrochemistry is when the analyte is dissolved in the electrochemical cell solution

59
Q

What is the set up of film electrochemical experiment?

A
60
Q

What happens in the recution of CO2 with a Co porphyrin catalyst when constant voltage sufficient to reduce the Co is applied?

A

The co is reduced from Co2+ to Co+

The eletrode transfers electrons to the Co catalyst

The co catalust transfers its electrons to CO2 and H+

This converts them to CH4 and H2O

This is thought to be a spontaneous potential scale

61
Q

What does a chronoamperometry experiement tell us?

A

For the reduction of CO2 the measured current is -10mA

8 electrons are used per CH4 produced

  • 10 mA = 10 x 10-3 Cs-1
  • ve current means electrons flowing out of the electrode

We can find the maximum ate of production of CH4

62
Q

How do we know this is the maximum current?

A

We assumed 100% faradaic efficiency

assume every 8 electrons produces CH4 with no side reactions or inefficiencies

63
Q

How do we calculate the maximum yield of methane production in the reduction of CO2?

A
  • 10 mA current flows for 60 seconds
    0. 01 Cs-1 x 60 s = 0.6 C - total charge flowing out of the electrode
    0. 6 C / 96485 C (mol e-)-1 x 8 electrons (CH4)-1

= 7.7 x 10-7 mol CH4

64
Q

How do we calculate the rate of catalysis?

A

kcat = icat / n x F x mcat

kcat = rate of catalysis

icat = current of catalysis

mcat = molar amount of catalyst

65
Q

Can voltammetry be used to monitor catalytic redox chmistry?

A

Yes catalytic redox chemistry can be intergrated as a function of potential using cyclic voltammetry

The voltage of the working electrode will change

It is important to consider both faradaic and non-faradaic current

66
Q

How do we find the molar amounts of Co+ and Co2+ in the recution of CO2?

A
67
Q

What shape would the purely faradaic catakytic current have?

A

Sigmoidal

68
Q

How can purely faradaic current be thought of in terms of windows?

A

Three windows:

1 - zero faradaic current - potential of the working electrode so positivr that none of the Co2+ is reduced and catalysis does not proceed

2 - constant faradaic current - potential of the working electrode os so negative that all the cobalt of converted to Co+ so the rate of catalysis is independent of potential

3 - dramatic change in current - equilibrium ratio of MCO2+ : MCO+ is highly dependent on potential

69
Q

What does real data look like?

A

Real data is the sum of faradaic and non-faradaic current

70
Q

How do we calculate the overpotential of the catalyst?

A

Look at the m (no. protons) and n (no. electrons) values

For Co catalyst in reduction of CO2 m= 8 and n=8

When m = n E drops by 59 mV per pH unit

If the Eo value is -0.25 V vs SHE at pH0 then at pH 1

Eo = -0.25 - 0.0059 = -0.31 V

71
Q

How does the cyclic voltammogram change with a higher turnovr rate?

A

When kcat increases icat increases

The plot expands

72
Q

In what situations would an electlyte salt not be necessary?

A

If the solution includes concentrated acid the ionic strenght will be high enough

73
Q

How do we calculate the time of an experiment?

A

Voltage / scan rate

74
Q

What happens in film voltammetry of non catalytic reversible redox process?

A

electrode reversibly pushes/pulls electrons into a species adsorbed onto the surface

Copper electron transfer proteons can be adsorbed onto graphite or gold electrodes

75
Q

What would the theortical faradaic current response look like in film voltammetry on non catalytic reverible redox process?

A

Eox = Ered

These peak potentials equal the potential of the redox couple being investigated

The peak area is equal to the charge passed and provides a measure of the muber of moles of redox active material

76
Q

For film voltammetry of a reversible redox process what is the relationship between current and scan rate?

A

Linear relationship as the same amount of redox active species is on the surfact of the electrode in all experiments

77
Q

What is the total current response?

A

Totsal current response = sum of faradaic and non faradaic contributions

78
Q

In real film voltammetry of reversible redox process what effect is visible that is not predicted by theory?

A

Eox and Ered shift with scan rate

If Eox and Ered are plotted against scan rate a trumpet plot of formed

This is because the faradaic current is smaller and therefore non-faradaic contributions to the total curretn are more significant

The shift reflects kinetics of the electron transfer - rate info contained

79
Q

What is solution cyclic voltammetry used for?

A

Determining HOMO LUMO gap

Evaluating homogenous electrocatalytic processes

Observing electron transfer through molecules

80
Q

What is the set up of solution cyclic voltammetry?

A

Same as for film electrochemistry but analyte is in solution

Auxilary electrode - completes circuit

Reference electrode - defines x axis on voltammogram

Working electrode - disc of platinum / gold / glassy carbon

All electrodes must be submerged in the solution

Electrochemistry is only happening at the surface of the electrode The bulk solution remains unaffected

81
Q

Why is the solvent choice important?

A

At a certain negative anmd positive potential the solvent will be reduced and oxidised respectively

Between these two potentials is called the solvent window

The analyte must undergo redox chemistry within this window

82
Q

What is the shape of the voltammogram in solution voltammetry?

A

Duck shaped

The peak anodic potential is Epa

The peak cathodic potential is Epc

The average of these points is E1/2

The max current upon oxidation (difference between peak and baseline) is ipa

The max current upon reduction is ipc

83
Q

Why is the voltammogram duck shaped?

A

In the beginning there is no current response - the voltage is not high enough toreove and electron

The current peaks as the electrons are removed from the anion

The current decreases - fewer molecules to oxide in the diffusion sphere

Current drops to non zero baseline because molecules are diffusimg into the diffusion sphere and instantly oxidised

Current peaks in opposite ciection as electrons are added to the newly oxidised analyte

Returns to zero baseline

84
Q

What is diffusion control?

A

The solution is under diffusion control

The faradaic signal is dominated by loinear diffuciom effects

Diffusion is only significant in the direction normal to the electrode surface - reaction and transports are uniform across the surface

current is proportional to concentration of analyte and scales with squ root of scan rate

85
Q

How does scan rate affect diffusion control?

A

Faster scan rate:

shirter diffusion distance

greater concentration gradient

local diffusion rates increase

higher current response

86
Q

What is the Randles-Sevcik equation?

A
87
Q

How does the concentration at the electrode change - using Fc<—> Fc+ + e- as example?

A

high conc of Fc low conc of Fc+

Fc conc decreases and Fc+ incerases to a max at the duck’s bill

From here Fc+ decreases and Fc incerases

88
Q

What affects the redox potential of a species?

A

Electron withdrawing and donating groups

EDG - easier to oxidise, harer to reduce, stabalise cation

EWG - Harder to oxidise, easier to reduce, destabalise cation

89
Q

What are the different types of reversibility

A

Chemical reversibility

Electrochemical reversibility

90
Q

How does electrochemical reversibility change?

A

The Nernst equation tells us that for a 1 electron transfer process

DE = | Epa - Epc | = 59 mV

This should not change with scan rate

This shift in peak potential is an incrase in DE which implies slow electron transfer amd therefore poor electrochemical reversibility

When peak potentials shift at higher scan rates it shows it is less reversible at higher scan rates

91
Q

How does chemical reversibility change?

A

Reversibility decreases if the product formed after the first redox reaction undergoes a chemical reaction

In a fully reversible process | ipc/ipa | = 1

In a non reversible process the return wave could be diminished or shifted

92
Q

What mechanisms can electron transfer and chemical steps occur by?

A

E = electron transfer process

C = chemical change

A –(-e-)–> A+ —(C)—> B EC mechanisms

A —(C)—> B —(E)—> B+

93
Q

What happens in an EC mechamism?

A

electron transfer process is followed by a chemical process where the new species is redox inert

Leads to peak in forward scan and none in return scan

Sometimes chemical step is slow enough that at fast scan rates the electrochemical measurement occurs faster than the chemical reaction so the CV looks reversible

94
Q

WHat happens in a CE mechanism?

A

electron transfer step occurs after there has been a chemical reaction

Return wave is much larger than forward scan

95
Q

What are multiple redox processes?

A

Some molecules can undergo multiple redox processes

e.g Naphthalenediimide - two 1-electron reductions

Shows up as two combined ducks

96
Q

What are mixed valence complexes?

A

A discrete species in which two or more of the same redox active fragments exist in different oxidation states

Three clases

Class I: valence trapped - no electronic comunication - no e- transfer

Class II: moderately coupled - some electronic comunication - barrier to electron transfer

Class III: fully delocalised - complete electronic communication - electrons spread over both centres

97
Q

What happens in a CV of a mixed valence molecule?

A

First the molecule is neutral

First 1 electron transfer occurs

At this pooint a mixed valence species is generated - one centre is oxidised and the other is not

After the second oxidation there is no longer a mixed values species

98
Q

How can we study mixed valence species?

A

Comparing DE1/2 values - can evaluate the stability of MV state

Can infer the extent of electron transfer

99
Q

How can the mixed valence species be seen to exist?

A

In equilibrium with the neutral starting material and over-oxidised species

Represented as comproprtionation constant - Kc

The higher the value of kc the more stable the MV ion is with repect to conversion to the neutral or doubly oxidised species

Greater stability is inferred to be due to greater electonic coupling

100
Q

How can we calculate Kc?

A

Variation of the Nernst equation:

Kc = e^(DE1/2 n1n2F/RT)

Core 5 (6 decks)

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