Electrochemistry Flashcards
galvanic cell
electrochemical cell
source of electrical energy, spontaneous, two redox couples
cell potential =
difference in potential of two couples
E(cell) = E(red) - E(ox)
corrosion metal reductant/oxidant?
metal = reductant
O2 is the oxidant
which is the oxidant in electrochemical cells?
the more POSITIVE E˚V
redox couple define
oxidant/reductant
eg. Fe3+/Fe2+
LHS is CAPABLE of acting as an oxidant, or reductant if RHS, regardless of E˚ value
electrode potential indicates…
tendency of the oxidant of the redox couple to accept e- and the tendency of the reductant to donate
E˚(oxidant/reductant) is relative to standard H electrode, E˚(H+/H2) = 0.00V
determine if a reaction is spontaneous from E˚ values
E˚(couple containing oxidant) is more positive than E˚(couple containing reductant), positive E(cell)
Standard cell potential applies….
applies to all cells where all substances are present in their STANDARD STATE (1 molL-1 for solutes, 100kPa for gases)
E˚
Note: under non-standard conditions, E can be +ve (spontaneous) while E˚ is -ve (non spontaneous)
Nernst equation
E = E˚ - (RT)/zF lnQ
RT/F = o.0257V at 298K
z= #e- transferred from reductant to oxidant
Q=reaction quotient
how does changing concentrations impact the direction of change of cell potential?
use Nernst eq
increase conc product –> more -ve E
increase conc reactant –> more +ve E
from the reaction quotient Q
when the system is at equilibrium, what is the equation for cell potential?
at equilibrium, no difference in potential between electrodes, so E=0 and Q=K
E˚ = RT/zF lnK
given E, calculate K
use Nernst to calculate E˚
use E˚ to calculate K (at equilibrium)
what is a concentration cell?
a reversible galvanic cell where both electrodes use the same couple
eg. metal and metal ion
potential difference due to differences in concentrations of metal ions in the two halves
electrode w/ greater conc metal ions = consumes
define electrolysis
using electricity to decompose unreactive compounds to their elements
electrolysis -ve electrode
e- accepted, reduction, cathode
connected to -ve terminal of power source
cations to the cathode
electrolysis +ve electrode
e- donated, oxidation, anode
anions to the anode
electrolysis of aq soln rules
at the -ve cathode, H2 is produced UNLESS there is a better e- acceptor in sln.
eg. Zn2+, Ag+, Cu2+
at anode, O2 produced unless a better e- donor is present
eg. halide ions, (must be conc Cl-)
which anion is NOT reactive at either electrode in a cell?
SO4^2-
eg. CuSO4 sln
Cu produced, O2 produced
3 steps of the refinement of bauxite
Bayer process
- bauxite treated w/ alkali, amphoteric Al2O3 dissolves (forming Al(OH)4 - ion)
- Al(OH)3 ppt from sln by cooling
- Al(OH)3 is heated to give alumina (Al2O3)
process Al2O3 to get aluminium
Al2O3 add cryolite (Na3AlF6) and CaF2 to lower melting point.
electrolysis: Al3+ + 3e- –> Al
at cathode
at anode, O2 produced, reacts w/ C electrodes to produce CO2
Al more dense, sinks, siphoned out
overall eqn for Al2O3 to Al
2Al2O3 + 3C –> 4Al(l) + 3CO2(g)
what is red mud
toxic waste from the aluminium production process - red due to iron oxides, strongly alkaline due to residual NaOH
oxidants reactivity series
halogens and O2
highest reactivity to lowest:
F2, Cl2, O2, Br2, I2 (Ag+, Cu2+, Zn2+, H in H2O, Mg2+, Na+)
reductants reactivity series
metals and H2
highest reactivity to lowest: Na, Mg, H2, Zn, Cu, Ag (I-, Br-, O in H2O, Cl-(dil), F-)
EXCEPT conc Cl- has greater reactivity than O in H2O
