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
conductivity of molecular aqueous solutions
may have no, low or high conductivity, depending on the EXTENT to which they react with water to give ions:
eg. H2SO4 + H2O –> H3O+ + HSO4-
strong conductor
what is produced in the electrolysis of concentrated NaCl?
full eqn
anode (oxidation): 2Cl- –> Cl2(g) + 2e-
cathode (reduction): 2H2O + 2e- –> H2(g) + 2OH-
product NaOH
2NaCl(aq) + 2H2O –> Cl2(g) + H2(g) + NaOH(aq)
reaction of NaOH and Cl
makes bleach, NaOCl
occurs when products of the electrolysis of concentrated NaCl are allowed to mix
Cl2 + 2NaOH –> NaCl + NaOCl + H2O
electroplating process
electricity reduces metal ions to form a Coating on the CATHODE
the cation of the metal to be electroplated MUST be the best electron acceptor in soln (above H in H2O, so Zn2+, Cu2+ or Ag+)
Faraday’s law of electrolysis
Q = znF
Q is charge (Coulombs)
z is #e- in electrode reaction producing the product
n is amount (mols)
F is Faraday constant, 96500 e- mol-1 (the charge on 1 mol of e-)
charge formula
Q = It
charge = current (Cs-1) x time (t)
calculate the number of moles of e- associated with a particular charge
Q = n(e-) x F
number of e- in 5000C:
n(e-) = 5000C / 96500 Cmol-1 = 0.0518 mol
find amount of substance based on amount of e-
n(substance) = n(e-) / z
z is number of e- involved in half eqn, or the coefficient
eg. Cu to Cu2+ involves 2e-
n(substance) = n(e-)/2
define reversible galvanic cell
an electrochemical cell with resistance in the external circuit that just stops e- flow in the cell
although there is a potential difference (reversible potential, E) between electrodes, NO reaction occurs
define electrolytic cell
battery in external circuit having potential difference greater than the opposite in sign to the reversible potential
non spontaneous
