electrochemistry

Question 1

define standard electrode potential (Eø)

Answer 1

the standard electrode potential of a half cell is the potential difference between the half-cell and a standard hydrogen electrode at standard conditions (298K, 1 bar for gases and 1 moldm-3 for solutions)

Question 2

how does Eø value affect whether a substance is oxidised or reduced?

Answer 2

more positive Eø -> the substance is more easily reduced
more negative Eø -> the substance is more easily oxidised

Question 3

define standard cell potential (Ecell)?

Answer 3

the standard electrode potential is the potential difference between 2 half cells measured at standard conditions (298K, 1 bar for gases, 1moldm-3 for solutions)

Question 4

formula to calculate Ecell?

Answer 4

Ecell = Ered - Eoxi

Question 5

how does standard cell potential relate to gibbs’ free energy?

Answer 5

△G = -nFEcell

note that △G = △H - T△S

n = number of moles of electrons transferred per mole of the cell reaction
F = faraday’s constant, 96500

Question 6

how can Ecell be used to predict the spontaneity of a redox reaction?

Answer 6

positive Ecell = negative △G: spontaneous at standard conditions
negative Ecell = positive △G: non-spontaneous under standard conditions
Ecell = 0: the reaction is at equilibrium, i.e no net reaction in either direction

Question 7

how does value of Ecell change with concentration of the aqueous ion?

Answer 7

Cu2+ (aq) + 2e- ⇌ Cu (s)
Eø at 298K & 1moldm-3 = +0.34V
Eø at 298K & 0.1moldm-3 = +0.31V

conc of Cu2+ decreases -> by LCP, POE shifts left to increase [Cu2+] -> tendency for Cu2+ to be reduced decreases -> reduction potential (Eø) becomes less positive

similarly: conc of Cu2+ increases -> by LCP, POE shifts right to decrease [Cu2+] -> tendency for Cu2+ to be reduced increases -> reduction potential (Eø) becomes more positive

Question 8

limitations of using Ecell to predict spontaneity of reaction?

Answer 8

1. non-standard conditions. Ecell values only apply at standard conditions -> changes in concentration, pressure & temp may affect the values of electrode potentials -> may also affect cell potential calculated, and hence the prediction of spontaneity
2. high activation energy. Eø values cannot predict the rate of a redox reaction; it only can show whether a reaction is thermodynamically feasible (spontaneous) or not. if activation energy is very high, although it is thermodynamically feasible, the reaction rate may be (effectively) zero

Question 9

differences between electrochemical cell & electrolytic cell?

Answer 9

spontaneity of reaction

• electrochemical: reaction is spontaneous
• electrolytic: reaction is non-spontaneous

conversion of energy

• electrochemical: chemical energy to electrical energy
• electrolytic: electrical energy to chemical energy

polarity of anode

• electrochemical: negative
• electrolytic: positive

polarity of cathode

• electrochemical: positive
• electrolytic: negative

polarity of anode & cathode may be different for electrolytic & electrochemical cell, BUT oxidation still occurs at the anode and reduction still occurs at the cathode

Question 10

how to calculate the quantity of charge, Q (in coulombs)?

Answer 10

Q = nF
where Q is the quantity of charge in coulombs, n is the number of moles of electrons in mol, and F is the faraday constant (96500)

Q = It
where Q is the quantity of charge in coulombs, I is the current in amperes (A) per second, and t is the time in seconds

F = Le, where L is the avogadro constant (6.02 x 10²³) and e is the charge on one electron (1.60 x 10⁻¹⁹)

Question 11

in the electrolytic purification of copper, what happens to metal impurities with Eø values less positive and more positive than Eø(Cu2+|Cu)?

Answer 11

for metal impurities with Eø values less positive than Eø(Cu2+|Cu), they are more readily oxidised than Cu, so they are oxidised at the anode and enter the electrolyte as metal ions.
these metal ions have Eø values less positive than Eø(Cu2+|Cu), so they are less easily reduced than Cu2+ ions -> they are not reduced and remain in the electrolyte

for metal impurities with Eø values more positive than Eø(Cu2+|Cu), they will not be oxidised at the anode, and will stay as solid metal, eventually dropping off as anode sludge

Question 12

what occurs at each electrode during anodising of Al?

Answer 12

anodising of Al is a surface treatment that increases the thickness of the corrosion resistant Al2O3 (s) layer on the surface of an aluminium object by electrolysis

anode: 2Al (s) + 3H2O (l) -> Al2O3(s) + 6H+ (aq) + 6e-
cathode: 2H+ + 2e- -> H2 (g)

electrolyte: dilute H2SO4
anode: aluminium object to be anodised
cathode: Pt or graphite

Question 13

predict the products of electrolysis of concentrated NaCl

Answer 13

anode:
Cl2 + 2e- ⇌ 2Cl- — Eø = +1.36V (1)
O2 + 4H+ + 4e- ⇌ 2H2O — Eø = +1.23V

• high Cl- conc causes POE of (1) to shift left -> Eø(Cl2|Cl-) decreases and becomes less positive than Eø(O2|H2O) -> Cl- is preferentially oxidised at the anode
• (under dilute aqueous conditions), Eø(O2|H2) is less positive than Eø(Cl2|Cl-) -> H2O has a higher tendency to be oxidised than Cl-, so H2O is preferentially oxidised to O2

cathode:
Na+ + e- ⇌ Na — Eø = -2.71V
2H2O + 2e- ⇌ H2 + 2OH- — Eø = -0.83V

• the difference between Eø(Na+|Na) and Eø(H2O|H2) is too large, so the effects of concentration is not enough to cause Na+ to be reduced
• since Eø(H2O|H2) is more positive than Eø(Na+|Na), H2O is preferentially reduced