electrochemistry Flashcards
(99 cards)
1
Q
Cathodic vs anodic processes
A
cathodic = e- transfer from the electrode (reduction of species in solution)
anodic = e- transfer to the electrode (oxidation of the species in solution)
2
Q
Galvanic cell
A
galvanic = galvin is always right = e- go left to right
anode=neg (oxidation), cathode=pos (reduction), discharges, positive Ecell (spontaneous), negative G
3
Q
change in G for galvanic vs electrolytic
A
galvanic = negative, electrolytic=positive
4
Q
what is pos vs neg reduction potential?
A
pos = wants to be reduced (be in metallic form)
neg= wants to be oxidised (be in ionic form)
5
Q
Ecell of a Daniell cell
A
1.1V
6
Q
Cell notation key points
A
anode on left, salt bridge, cathode on right, solid metals on outside, metal| ion|| ion| metal
7
Q
A in water at 298K
A
0.5
8
Q
oxidation/reduction at anode/cathode?
A
oxidation at anode,
OILRIG
reduction at cathode
remember as OIL and anode is neg
9
Q
in eq which side do the e- go on?
A
the side with the ion only, as +e-
10
Q
Gas|inert type electrode
A
an inert metal in contact with a gas and solution of its ions (standard hydrogen electrode SHE)
11
Q
electrodes of first kind
A
metal electrodes in direct eqm with the cation derived from the metal
e.g. Cu and Cu2+
12
Q
electrodes of second kind, e.g.?
A
3 phases: metal covered by its sparingly soluble salt immersed in electrolyte of the same anion as in the salt, e.g. calomel Hg(l)|Hg2Cl2 (s) | KCl (aq)
13
Q
most stable electrode type best to worst
A
gas = most stable,
second kind,
Least stable = first kind
14
Q
ionophore?
A
neutral carrier for ions, cannot diffuse out of a membrane so attach selectively to analytes, give a phase-boundary potential
15
Q
Which often is the larger value cathode or anode?
A
Cathode potential
16
Q
What is Y in the equation?
A
Activity coefficient
17
Q
Inverse of log10
A
10^x
18
Q
Inverse of ln
A
e^x
19
Q
What does E0 equal in a concentration cell?
A
0
20
Q
Equation for Q
A
[Cathode ion]^x / [anode ion]^x
x = the balancing number if applicable from the half equations
21
Q
What is μ?
A
Electrical mobility
22
Q
Fully solvated ion pair
A
Ions apart and have bigger hydrodynamic radius
Lower charge
23
Q
Solvent-shared ion pair
A
Oscillating charge
Ions hydration spheres are just touching but ions aren’t touchign
24
Q
Contact ion pair
A
Ions radii are touching and results in a smaller hydrodynamic radius
25
How does concentration affect number of ion pairs
Higher conc = higher proportion of paired ions
26
Which ion pairs contribute to conductivity?
Only fully solvated
27
What is ionic atmosphere relaxation
When the negative charge lags behind the positive ion centre, under applied electric field, making the ion asymmetric
Even at room temp
Front of the ion provides resistance to movement
28
What does A represent in the mole conductivity equation?
The electrophoretic effect = the movement of ions under electric field
29
What is the result of higher voltage in electrophoresis?
Higher voltage = faster separation = less diffusion
30
Electrophoresis at pH below the isoelectronic point
Protein = positive and moves to cathode
31
Electrophoresis at pH above the isoelecrtonic point
Protein = negative = moves to anode
32
Specific energy
How much is available
33
Specific power
Speed of energy delivery
34
Theoretical specific capacity
Amount of charge per kg of reactants
35
Practical capacity
Real amount of charge available - always lower than theoretical specific capacity
36
equation for batteries compared to electrolysis?
batteries = A -> B + e- (oxidation)
electrolysis = A + e- -> B (reduction)
37
For oxidation does the oxidation number go up or down?
up
38
For reduction does the oxidation number go up or down?
down
39
what equations are for a Daniell cell? CHECK
Zn -> Zn^2+ + SO4^2- (anode)
Cu^2+ + SO4^2- + 2e- -> Cu(s) (cathode)
40
how do you convert from standard calomel electrode (SCE) to std h2 electrode (SHE)?
ev of SHE = ev of SCE + 0.242
41
what is really important to remember for the nernst eq?
the MINUS in front of the R or n (depending on if using the faraday const or not)
42
what is the Gibbs Donnan effect?
the movement of ions to balance charge but not necessarily concentration, as a generated potential acts as an electrostatic barrier so there is competition between the conc grad and electrostatic effect
43
what is specific energy vs specific power?
specific energy = how much energy is available
specific power = speed of energy delivery
44
properties of water
high dielectric constant
high surface tension
high thermal capacity
permanent dipole moment
45
what does Hsol < 0 mean?
releases heat when dissolving (hot pack)
46
what does Hsol > 0 mean?
cools down when dissolving (cold pack)
47
what does Hsol > 0 mean?
cools down when dissolving (cold pack)
48
how does charge shielding affect coulomb potential?
non-shielded = the potential follows coulomb's law
shielded = potential falls steeply due to the dipolar solvent molecules
49
effects of the ionic atmosphere on Y
spherical haze of anions - dense solvent reduces motion
solvation (hydration) shell - stabilises ions w/ the dipoles, reduces effective charge
coulomb potential electrostatic field - solvent permittivity reduces its long-range value
conc of anions and cations - strongly influences ionic atmosphere by changing the local conc of the charges
ionic charge - can change the ionic strength so impacts Y
50
how will Ecell > 0 affect deltaG
deltaG < 0
51
what is std cell potential?
2.303
52
what does charge separation at the electrode result in? what are the charge carriers?
the generation of a potential difference - e- are the charge carriers in metals, in solutions ions are the charge carriers instead
53
what is the Helmholtz double layer for a negatively charged electrode?
a compact layer of polarised solvent molecules and metal cations, then a diffuse mobile layer of screening negatively charged counterions
potential decreases linearly in the compact layer
54
what is the Helmholtz double layer for a positively charged electrode?
negative ions move towards the electrode to form a double layer, and positive ions will be repelled and form a screening layer at some distance, where the electrode potential drops to zero
55
what is fugacity
the activity of the gases
56
electrodes of the third kind?
cationic electrodes which are responsive to the conc of a cation other than the cation of the electrode metal (redox electrodes)
57
what are redox electrodes?
when a noble inert metal is used when a solution of two oxidation states of the same species are present (e.g. Fe(II) and Fe (III))
58
porous silicate glass membranes?
exchange their cation for another cation in the solution
gives a difference in activity of the ion on the two sides of the membrane
good for single charged small cations (H+, Na+, Ag+, Li+, NH4+)
59
solid state membranes?
made of a single crystal, and sensitive to a single ion
can have interferences though, as will be sensitive to similar ions (e.g. Br = Cl as well etc)
60
polymer membranes?
ionophore in a porous polymer
aka ion-exchange resin electrodes
61
what is the moving boundary method used for?
when there's a big difference in conductivity (ion mobility) and density between the two solutions - gives a sharp phase boundary
62
alternatives to the moving boundary method of measurement?
u-tube electrophoresis - side-products dont mix
capillary electrophoresis - higher SA:vol = more efficient
gel electrophoresis - immobilised on agar gel gives better separation
63
how do proteins move by electrophoresis?
move in a pH gradient until they reach their isoelectric point, at which point they have a charge of zero and won't move further in the electric field
64
what is the difference between electrophoresis and chromatography?
chromatography flows parabolic concave due to friction of the eluent phase with the sides of the column = LOW RESOLUTION
electrophoresis has no side drag so the flow is flat with HIGH RESOLUTION
65
what are the conditions needed for the gibbs-donnan effect?
two chemical potentials of the same
66
what is donnan dialysis?
exchanging multivalent cations for several monovalent cations - anions are the impermeants (bc tend to be bigger)
consumes less power than normal dialysis and can be used continuously
67
why is there never eqm w/ charged particles?
bc the membrane itself has a potential
68
what is discharge?
losing charge = losing e-
69
using the std reduction potentials, which species prefer to be oxidised and which reduced?
higher potentials = prefer to be oxidised (ion)
lower potentials = prefer to be reduced (metal)
70
How is the mass of a substance produced at an electrode significant?
It is proportional to the number of moles of electrons transferred at that electrode
71
cathode vs anode?
AN OX RED CAT
Anode = where oxidation takes place
Cathode = where reduction takes place
72
Anodic water chemical equation
2H2O -> O2 + 4H+ + 4e-
Forms ions and electrons
73
Cathodic water chemical equation
2H2O + 2e- -> H2 + 2OH-
Electrons used and ions produced
74
Electrolytic cell
electroylytic = lying = e- right to left,
anode=pos (oxidation), cathode=neg (reduction),
recharges, Ecell=neg (non-spontaneous), G = positive
Non-spontaneous but spontaneous in the opposite direction
75
What does being a stronger oxidant mean?
Stronger oxidants have a greater tendency to form cations than others
76
How do the reduction and oxidation potentials change across the galvanic series?
Left to right the reduction potential increases as the elements are more thermodynamically stable in reduced (metallic) form
Right to left the oxidation potential increases as the elements are not thermodynamically stable in the oxidised (ionic) form
77
The galvanic series (rough)
Left = most reactive & very reactive in their metallic state (Li, K, Na, Mg)
Right = least reactive & unreactive in their metallic state (Ag, Pd, Pt, Au)
78
How does the galvanic series work?
Those on the left are able to reduce those on their right
79
Daniell cell
See revision cards
80
What potentials do we use in the electrochemical series?
REDUCTION potentials
81
How do ion-dipole forces affect the hydration sphere?
Smaller ion = greater energies involved = high charge density = tight, well-formed hydration sphere
82
How does solvation enthalpy change with ionic radius?
Increasing ionic radius (higher disorder) = less negative (increasing) solvation enthalpy
More negative solvation enthalpy, smaller ionic radius = tight-packed hydration sphere, lower disorder
Less negative solvation enthalpy, bigger ionic radius = weakly oriented water molecules, higher disorder
83
What are the assumptions of Debye-Huckel theory?
- No ion-solvent interactions
- ions are approximated as point charges
- the electrolyte is fully dissociated in M+ and X- = implies that external forces between ions are weaker than thermal forces in the solution
* this is valid only at extreme dilutions *
84
How does a non-ideal dilute solution compare to an ideal solution?
The non-ideal dilute solution is more stable as it has lower gibbs free energy than the ideal solution
85
How does solvent affect the final activity of an ion?
- concentration- dependent charge shielding effects
- reduction of coulomb interaction range in a solvent with high permittivity
- hindered ion mobility in dense solvents
86
How does the solvation (hydration) shell effect coulomb potential?
Reduces the effective charge - the charge is less effective at attracting opposite charges and repelling the same charge
Also stabilised ions with their dipoles
Hydration shell also minimises deltaG (free energy)
87
What is an ionic atmosphere?
A spherical haze of ions
88
How does solvent affect the ionic atmosphere?
A dense solvent reduces motion and has a strong effect on effective time averaging - viscous solvent will have a smaller time average spherical haze
Solvent permittivity reduces the long-range value of coulomb potential
89
Define the faraday constant
It equals the charge in coulombs of 1 mole of electrons
90
What is a concentration cell?
When two half cells are the same and differ only in concentration
91
What is a Helmholtz double layer?
A charged electrode will repel charges of the same sign, while attracting the counterions. A double layer of opposite polarity will therefore form.
The layer is held together by electrostatic attraction and so is very compact and dense
92
For SHE how do you calculate the activity?
Activity of a gas is called fugacity, the fugacity is equal to the pressure (standard = 1atm), hence E = 0V
93
Calomel electrode notation
Hg (l) | Hg2Cl2 (s)| KCl (aq.)
94
in a conc cell what is E std
0
95
does a high or low coulomb potential show a good solvent?
low bc shows that the ions are only interacting weakly so can't form a crystal
96
what is Y (the activity) for ideal solutions?
1
97
what is the sum of all transport number in a given electrolytic solution equal to?
1
98
what do you need to make sure to remember to include in half eq?
WATER if O ions are incl
99
for a concentration cell how do you do the half eq?
only w/ the metal ions and electrons as only the conc of solution is changing
