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MATE40005 > Electrochemistry > Flashcards

Electrochemistry Flashcards

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1
Q

Define Electrochemistry

A

Branch of physical chemistry that studies the relationship between electricity and chemical change
Electrical flow can be the cause or the outcome of a chemical reation

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2
Q

Corrosion

A

Huge problem, public safety concerns
Tackling enable extension of component lifetime, better use of natural resources, tin concentration in aluminium components and copper concentration in steels for nuclear applications

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3
Q

Batteries

A

First fuel cell was developed in 1838 by William Grove
Difference between a battery and a fuel cell is that a battery is an enclosed system
Fuel cell is continous flow of reagents

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4
Q

What are electrolysers?

A

Fuel Cells in reverse
provide a way of converting electricity into fuels and valuable chemical such as H2 or potentially NH3

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5
Q

Electrodeposition and Electrochemical Refining

A

Electrolysis was used for extraction of metals - allows several elements such as potassium, sodium, barium, calcium etc.
Used in extracting in Aluminium
Electrodeposition allows plating and coating for corrosion protection as well as nanopatterning

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6
Q

Redox Reaction

A

Reaction that involves a transfer of electrons
OIL RIG

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7
Q

Spectator Ions

A

Present in the system but do not actively participate in the reaction
Do not always have to be specified in the equation

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8
Q

Oxidation number

A

Describes the degree of oxidation of an atom in a molecular compound
Number of electrons each atom will give or recieve

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9
Q

Rules for oxidation numbers

A

Atoms in elemental state = 0
Oxidation number of monoatomic ion is the formal charge of ion Na+ = +1
Hydrogen normally has +1, unless metallic hydride -1
Halogens -1
Oxygen has an oxidation number -2, unless peroxide then -1
superoxide ion o2- = -1/2
Metallic Ions have positive oxidation numbers which correlates to the group they are in

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10
Q

what is the chemical potential?

A

the rate of change in gibbs free energy of a thermodynamic system, respect to the change in number of atoms/molecules/ions of species within the system

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11
Q

what is an equation that relates the chemical potential and volume and concentration?

A

mui = mu i 0 + RTln[i]

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12
Q

Chemical Equilibrium

A

Point in which reactants and products are present in concentrations which have no further driving force to change over time
deltarG = 0

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13
Q

How does an electrostatic charge build up?

A

If you move charged ions or electrons in a solution
This will be the driving force for the reverse reaction
Slows down reaction
Affects the equilibrium concentration

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14
Q

what is the electrochemical equilibrium a combination of?

A

electrostatic equilibrium + chemical equilibrium

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15
Q

what is the electrochemical potential the sum of?

A

chemical potential and the electrostatic potential

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16
Q

electrostatic potential energy

A

E = charge number x electron charge x number of moles x avagadro x electrostatic potential

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17
Q

electrochemical potential

A

mu i = mu i + charge i x faraday x electrostatic potential

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18
Q

Nernst Equation

A

∆𝜙 =∆𝜙0−𝑅𝑇/𝐹 x 𝑙𝑛 [𝐹𝑒2+]/[𝐹𝑒3+]

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19
Q

Types of electrodes

A

Metallic Cations in solution with an intert electrode
Metallic cation and a solid metal redox equation
Metal/insoluble salt electrodes
Platinised platinum used for an electrode

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20
Q

1atm

A

1.01 x 10^5Pa

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21
Q

sample a has a Cu sample with ZnSO4 solution
sample b has a Zn sample with CuSO4 solution
in a nothing happens, in b Cu granules start depositing and Zn is reduced

A

Zn is a stronger reducing agest, loses electrons than Cu
Cu is a stronger oxidising agent, gains electrons and forms copper granules

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22
Q

how to build a battery

A

two half cell reactions
external connection using a conductive wire for electron transfer
internal connection has to be a saline bridge allows diffusion of ion and closes the circuit
reductition cathode, rhs

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23
Q

standard cell potential under standard conditions

A

t = 298K
gaseous species 1atm, 1.01 x 10^5 Pa
ionic species 1moldm-3
electrodes are a solid conductor

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24
Q

standard potential of cell

A

ecell = eRHS - eLHS

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25
electrode potential
unit of measure of the EMF of a cell made up of two electrodes
26
Aqueous Corrosion
Iron is oxidised to Fe 3+ Oxygen is reduced O2 + 2H2O + 4e- -----> 4OH-
27
what do the potentials indicate?
positive potential indicates a spontaneous formal cell reaction negative potential energy is consumed in order for reaction to happen more negative, more reducing
28
what does a voltmeter measure?
The potential at any point in time by temporarily blocking electron flow
29
Reaction Quotient
bB + cC ↔ dD + eE Q = activity of D^d x activity of E^e / activity of C^c x activity of B^b
30
general equation to describe ecell even not at equilibrium
Ecell = Ecell 0 - RT/nF lnQ independent of the number of electrons transferred
31
Work Done
delta rG = -nFEcell spontaneous reactions positive emf bigger the PD, the higher the driving force for the rection
32
cell diagrams
reduction on the right left is oxidation double line phase interface single line is better
33
rate of a reaction
speed at which the reaction occurs measure the speed which it reaches equilibrium
34
current a reaction kinetics
i = nFA(kox[Red]0 - kred[Ox]0)
35
Potentiostat
Focus and Perform measurements on a single interface Set up so you can measure the potential between the working electrode and the reference electrode All current is flowing between the working electrode and the secondary electrode
36
why do you need a background electrolyte?
minimise ionic migration to interface the electrolyte minimise the iR drop region of interfacial potential drop keep activity coefficients constant
37
activated complex theory
[ox]=[red] no current is flowing free energy of products = free energy of reagents free energy activation barrier for this to happen arrhenius equation = k0 = Zexp (-delta reductions G / RT)
38
butler volmer equation how current varies with potential
39
tafel analysis
simplification of equation for highly positive or high negative volumes
40
debye length
distance between a central ion to its nearest counter ion in a capacitor distance between double layer
41
capacitor
device that stores electrostatic energy C = E0 A/L supercapacitors C = E A/d
42
total capacitance
C = C1 x C2 / C1 + C2
43
supercapacitor
much higher capacitance, hgih surface area at the electrode surface voltage of supercapacitor since it is wet is much less behave differently compared to Li-ion battery charge is stored via double layer capacitance hgiher rate of charging/discharging longer life cycle of capacitor
44
li-ion battery
li-ion battery stores charge in bulk of eletrode via intercalation higher energy density charge/discharge rate slow, diffusion slow, shorter life cycle
45
helmholtz model
sheet of ionic charge that runs in solvated ions ions arrange themselves along surface of electrode hel back by hydration sphers electrolyet and the charge distribution is static activity of charged species immediately changes to the bulk
46
guoy chapman model
dynamic thermal motion of ions around the double layers diffuse double layer, ions of the opposite charge cluster together closer to the electrode surface and ions of same charge are repelled higher concentration of ions gives a thinner double layer potential at the electrode surface now varies inversely with potential bulk electrolyte because of the diffuse double layer underempahsises the strucutre of the double layer and disregards the OHP
47
stern model
outer helmholtz plane near the electrode followed by the diffuse double outer layer potential at the electrode surface varies linearly at first then inversely when crossing over to the diffuse double layer doesn't account for adsorbed species on the electrode surface
48
grahame model
built on stern model to account for solvated cations and adsorption inclusion of an inner helmholtz plane, adsorbed species accounts for the charge that is required to form adsorbed species pseudocapacitance
49
energy stored in capacitor
e = 1/2 CE^2
50
PEM electrolysers
splits water into O2 and H2 through electrolysis electricity require to drive reaction as it is not thermodynamically spontaneous 1.23 V for water electrolysis
51
PEM fuel cell
reverse of the electrolyser, reacting the H2 fuel to convert it back into electricity could be used as a high energy power source for automotive cars
52
Pt catalyst
electrochemical reaction on fuel cell Pt density required to catalyse the oxygen reduction compared to anode, due to large overpotential Pt is scarce problem evolved
53
Efficiency of PEM Fuel Cells
E operating = Ecell - iR - nORR - nHOR - ntransort ORR refers to oxygen reduction reaction at cathode HRR refers to hydrogen oxidation reaction at the anode of the PEM fuel cell
54
efficiency of PEM electrolyser
E operating = Ecell + iR + nORR + nHOR + ntransort ORR refers to oxygen reduction reaction at cathode HRR refers to hydrogen oxidation reaction at the anode of the PEM fuel cell
55
exchange current density j0
i0/A where a is area catalysts with high j0 is very active, reaction is highly reversible catalyst with low j0 is inactive, reaction is highly irreversible
56
ideal reaction
high j0 is ideal and results in a more efficient system
57
processes at electrochemical interface
mass transfer of species between bylk solution and the electrode surface electron transfer at the electrode - electrolyte interface chemical reactions, either preceding or following electron transfer surface processes such as adsorption, desorption and electrodeposition - dissolution
58
hydrogen evolution process
A proton in the solution combines with an electron from the Pt catalyst surface to form atomic hydrogen which adsorbs onto a vacant active site on the surface 2. To get the molecular hydrogen, there are 2 possible routes that the reaction can take: a. Recombination route: 2 adjacent hydrogen atoms react to form gaseous hydrogen molecules, leaving behind a vacant site; OR b. Adsorbed hydrogen atom reacts with another proton in the solution and an electron from the Pt electrode to form gaseous hydrogen molecules
59
when does mass transport become a critical factor?
there is limited solubility of electroactive species extremely dilute solution are used for electrochemical processes limited ability to provide agitation in narrow gaps
60
mass transport
oxidised species in bulk reaches the surface region close to electrode electrode is adsobred onto surface electron transfer occurs at the electrode and the reduced species is formed reduction goes from the electrode surface to teh bulk
61
3 forms of mass transport which cna affect a reaction
diffusion - arises from local uneven concentrations of species convection - generated in solution moving with a velocity migration - movement of species in the presence of a potential gradient
62
Nernst - Planck Equation
diffusion migration concvention
63
difference between diffusion and migration
diffusion occurs in the presence of a concentration gradient and affects all species charged particles feel the effect of an electric potential and migrate
64
overpotentials
concentration - flatter with increasing current density until the limiting current is achieved and the curve plateaus ohmic - curve increases linearly with current density kinetic - curve increases exponentially with current density
65
rotating disk electrode
working electrode that is used in a standard three electrode setup for measuring reaction rates steady state between convection flux and diffusion flux
66
corrosion protection
organic coating prevents direct contact of water and o2 on metal surface passive oxide layer dense layer prevents water and o2 from getting through to metal surface mild steel corrodes as iron does not form a dense oxide layer stainless steel doesnt corrode as chromium forms a dense oxide layer
67

MATE40005 (4 decks)

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