electrochemistry Flashcards
(12 cards)
define standard electrode potential (Eø)
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)
how does Eø value affect whether a substance is oxidised or reduced?
more positive Eø -> the substance is more easily reduced
more negative Eø -> the substance is more easily oxidised
define standard cell potential (Ecell)?
the standard electrode potential is the potential difference between 2 half cells measureed at standard conditions (298K, 1 bar for gases, 1moldm-3 for solutions)
formula to calculate Ecell?
Ecell = Ered - Eoxi
how does standard cell potential relate to gibbs’ free energy?
△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
how can Ecell be used to predict the spontaneity of a redox reaction?
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
how does value off Ecell change with concentration of the aqueous ion?
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 off Cu2+ increases -> by LCP, POE shifts right to decrease [Cu2+] -> tendency for Cu2+ to be reduced increases -> reduction potential (Eø) becomes more positive
what are the limitaitions of using standard cell potentials to predict the spontaneity of a reaction?
- 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
- 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
differences between electrochemical cell & electrolytic cell?
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
how to calculate the quantity of charge, Q (in coulombs)?
Q = nF
where Q is the quantity of charge in coulombs, n is the number of moles of electrodes in mol, and F is the faraday constant (96500)
Q = It
where Q is the quantity fof 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)
in the electrolytic purification of copper, what happens to metal impurities with Eø values less positive and more positive than Eø(Cu2+|Cu)?
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 rerduced 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
what occurs at each electrode during anodising of Al?
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 beb anodised
cathode: Pt or graphite
