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CHEM352 > electrochemistry > Flashcards

electrochemistry Flashcards

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)

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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

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3
Q

change in G for galvanic vs electrolytic

A

galvanic = negative, electrolytic=positive

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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)

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5
Q

Ecell of a Daniell cell

A

1.1V

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6
Q

Cell notation key points

A

anode on left, salt bridge, cathode on right, solid metals on outside, metal| ion|| ion| metal

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7
Q

A in water at 298K

A

0.5

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8
Q

oxidation/reduction at anode/cathode?

A

oxidation at anode,
OILRIG
reduction at cathode

remember as OIL and anode is neg

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9
Q

in eq which side do the e- go on?

A

the side with the ion only, as +e-

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10
Q

Gas|inert type electrode

A

an inert metal in contact with a gas and solution of its ions (standard hydrogen electrode SHE)

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11
Q

electrodes of first kind

A

metal electrodes in direct eqm with the cation derived from the metal
e.g. Cu and Cu2+

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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)

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13
Q

most stable electrode type best to worst

A

gas = most stable,
second kind,
Least stable = first kind

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14
Q

ionophore?

A

neutral carrier for ions, cannot diffuse out of a membrane so attach selectively to analytes, give a phase-boundary potential

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15
Q

Which often is the larger value cathode or anode?

A

Cathode potential

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16
Q

What is Y in the equation?

A

Activity coefficient

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17
Q

Inverse of log10

A

10^x

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18
Q

Inverse of ln

A

e^x

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19
Q

What does E0 equal in a concentration cell?

A

0

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20
Q

Equation for Q

A

[Cathode ion]^x / [anode ion]^x

x = the balancing number if applicable from the half equations

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21
Q

What is μ?

A

Electrical mobility

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22
Q

Fully solvated ion pair

A

Ions apart and have bigger hydrodynamic radius
Lower charge

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23
Q

Solvent-shared ion pair

A

Oscillating charge

Ions hydration spheres are just touching but ions aren’t touchign

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24
Q

Contact ion pair

A

Ions radii are touching and results in a smaller hydrodynamic radius

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25
Q

How does concentration affect number of ion pairs

A

Higher conc = higher proportion of paired ions

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26
Q

Which ion pairs contribute to conductivity?

A

Only fully solvated

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27
Q

What is ionic atmosphere relaxation

A

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

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28
Q

What does A represent in the mole conductivity equation?

A

The electrophoretic effect = the movement of ions under electric field

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29
Q

What is the result of higher voltage in electrophoresis?

A

Higher voltage = faster separation = less diffusion

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30
Q

Electrophoresis at pH below the isoelectronic point

A

Protein = positive and moves to cathode

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31
Q

Electrophoresis at pH above the isoelecrtonic point

A

Protein = negative = moves to anode

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32
Q

Specific energy

A

How much is available

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33
Q

Specific power

A

Speed of energy delivery

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34
Q

Theoretical specific capacity

A

Amount of charge per kg of reactants

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35
Q

Practical capacity

A

Real amount of charge available - always lower than theoretical specific capacity

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36
Q

equation for batteries compared to electrolysis?

A

batteries = A -> B + e- (oxidation)

electrolysis = A + e- -> B (reduction)

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37
Q

For oxidation does the oxidation number go up or down?

A

up

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38
Q

For reduction does the oxidation number go up or down?

A

down

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39
Q

what equations are for a Daniell cell? CHECK

A

Zn -> Zn^2+ + SO4^2- (anode)
Cu^2+ + SO4^2- + 2e- -> Cu(s) (cathode)

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40
Q

how do you convert from standard calomel electrode (SCE) to std h2 electrode (SHE)?

A

ev of SHE = ev of SCE + 0.242

41
Q

what is really important to remember for the nernst eq?

A

the MINUS in front of the R or n (depending on if using the faraday const or not)

42
Q

what is the Gibbs Donnan effect?

A

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
Q

what is specific energy vs specific power?

A

specific energy = how much energy is available
specific power = speed of energy delivery

44
Q

properties of water

A

high dielectric constant
high surface tension
high thermal capacity
permanent dipole moment

45
Q

what does Hsol < 0 mean?

A

releases heat when dissolving (hot pack)

46
Q

what does Hsol > 0 mean?

A

cools down when dissolving (cold pack)

47
Q

what does Hsol > 0 mean?

A

cools down when dissolving (cold pack)

48
Q

how does charge shielding affect coulomb potential?

A

non-shielded = the potential follows coulomb’s law

shielded = potential falls steeply due to the dipolar solvent molecules

49
Q

effects of the ionic atmosphere on Y

A

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
Q

how will Ecell > 0 affect deltaG

A

deltaG < 0

51
Q

what is std cell potential?

A

2.303

52
Q

what does charge separation at the electrode result in? what are the charge carriers?

A

the generation of a potential difference - e- are the charge carriers in metals, in solutions ions are the charge carriers instead

53
Q

what is the Helmholtz double layer for a negatively charged electrode?

A

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
Q

what is the Helmholtz double layer for a positively charged electrode?

A

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
Q

what is fugacity

A

the activity of the gases

56
Q

electrodes of the third kind?

A

cationic electrodes which are responsive to the conc of a cation other than the cation of the electrode metal (redox electrodes)

57
Q

what are redox electrodes?

A

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
Q

porous silicate glass membranes?

A

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
Q

solid state membranes?

A

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
Q

polymer membranes?

A

ionophore in a porous polymer

aka ion-exchange resin electrodes

61
Q

what is the moving boundary method used for?

A

when there’s a big difference in conductivity (ion mobility) and density between the two solutions - gives a sharp phase boundary

62
Q

alternatives to the moving boundary method of measurement?

A

u-tube electrophoresis - side-products dont mix
capillary electrophoresis - higher SA:vol = more efficient
gel electrophoresis - immobilised on agar gel gives better separation

63
Q

how do proteins move by electrophoresis?

A

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
Q

what is the difference between electrophoresis and chromatography?

A

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
Q

what are the conditions needed for the gibbs-donnan effect?

A

two chemical potentials of the same

66
Q

what is donnan dialysis?

A

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
Q

why is there never eqm w/ charged particles?

A

bc the membrane itself has a potential

68
Q

what is discharge?

A

losing charge = losing e-

69
Q

using the std reduction potentials, which species prefer to be oxidised and which reduced?

A

higher potentials = prefer to be oxidised (ion)
lower potentials = prefer to be reduced (metal)

70
Q

How is the mass of a substance produced at an electrode significant?

A

It is proportional to the number of moles of electrons transferred at that electrode

71
Q

cathode vs anode?

A

AN OX RED CAT

Anode = where oxidation takes place

Cathode = where reduction takes place

72
Q

Anodic water chemical equation

A

2H2O -> O2 + 4H+ + 4e-

Forms ions and electrons

73
Q

Cathodic water chemical equation

A

2H2O + 2e- -> H2 + 2OH-

Electrons used and ions produced

74
Q

Electrolytic cell

A

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
Q

What does being a stronger oxidant mean?

A

Stronger oxidants have a greater tendency to form cations than others

76
Q

How do the reduction and oxidation potentials change across the galvanic series?

A

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
Q

The galvanic series (rough)

A

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
Q

How does the galvanic series work?

A

Those on the left are able to reduce those on their right

79
Q

Daniell cell

A

See revision cards

80
Q

What potentials do we use in the electrochemical series?

A

REDUCTION potentials

81
Q

How do ion-dipole forces affect the hydration sphere?

A

Smaller ion = greater energies involved = high charge density = tight, well-formed hydration sphere

82
Q

How does solvation enthalpy change with ionic radius?

A

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
Q

What are the assumptions of Debye-Huckel theory?

A
  • 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
Q

How does a non-ideal dilute solution compare to an ideal solution?

A

The non-ideal dilute solution is more stable as it has lower gibbs free energy than the ideal solution

85
Q

How does solvent affect the final activity of an ion?

A
  • concentration- dependent charge shielding effects
  • reduction of coulomb interaction range in a solvent with high permittivity
  • hindered ion mobility in dense solvents
86
Q

How does the solvation (hydration) shell effect coulomb potential?

A

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
Q

What is an ionic atmosphere?

A

A spherical haze of ions

88
Q

How does solvent affect the ionic atmosphere?

A

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
Q

Define the faraday constant

A

It equals the charge in coulombs of 1 mole of electrons

90
Q

What is a concentration cell?

A

When two half cells are the same and differ only in concentration

91
Q

What is a Helmholtz double layer?

A

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
Q

For SHE how do you calculate the activity?

A

Activity of a gas is called fugacity, the fugacity is equal to the pressure (standard = 1atm), hence E = 0V

93
Q

Calomel electrode notation

A

Hg (l) | Hg2Cl2 (s)| KCl (aq.)

94
Q

in a conc cell what is E std

A

0

95
Q

does a high or low coulomb potential show a good solvent?

A

low bc shows that the ions are only interacting weakly so can’t form a crystal

96
Q

what is Y (the activity) for ideal solutions?

A

1

97
Q

what is the sum of all transport number in a given electrolytic solution equal to?

A

1

98
Q

what do you need to make sure to remember to include in half eq?

A

WATER if O ions are incl

99
Q

for a concentration cell how do you do the half eq?

A

only w/ the metal ions and electrons as only the conc of solution is changing

CHEM352 (4 decks)

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