Electrochemistry

Question 1

galvanic cell

Answer 1

electrochemical cell

source of electrical energy, spontaneous, two redox couples

Question 2

cell potential =

Answer 2

difference in potential of two couples

E(cell) = E(red) - E(ox)

Question 3

corrosion metal reductant/oxidant?

Answer 3

metal = reductant

O2 is the oxidant

Question 4

which is the oxidant in electrochemical cells?

Answer 4

the more POSITIVE E˚V

Question 5

redox couple define

Answer 5

oxidant/reductant

eg. Fe3+/Fe2+

LHS is CAPABLE of acting as an oxidant, or reductant if RHS, regardless of E˚ value

Question 6

electrode potential indicates…

Answer 6

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

Question 7

determine if a reaction is spontaneous from E˚ values

Answer 7

E˚(couple containing oxidant) is more positive than E˚(couple containing reductant), positive E(cell)

Question 8

Standard cell potential applies….

Answer 8

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)

Question 9

Nernst equation

Answer 9

E = E˚ - (RT)/zF lnQ

RT/F = o.0257V at 298K
z= #e- transferred from reductant to oxidant
Q=reaction quotient

Question 10

how does changing concentrations impact the direction of change of cell potential?

Answer 10

use Nernst eq

increase conc product –> more -ve E
increase conc reactant –> more +ve E

from the reaction quotient Q

Question 11

when the system is at equilibrium, what is the equation for cell potential?

Answer 11

at equilibrium, no difference in potential between electrodes, so E=0 and Q=K

E˚ = RT/zF lnK

Question 12

given E, calculate K

Answer 12

use Nernst to calculate E˚

use E˚ to calculate K (at equilibrium)

Question 13

what is a concentration cell?

Answer 13

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

Question 14

define electrolysis

Answer 14

using electricity to decompose unreactive compounds to their elements

Question 15

electrolysis -ve electrode

Answer 15

e- accepted, reduction, cathode

connected to -ve terminal of power source

cations to the cathode

Question 16

electrolysis +ve electrode

Answer 16

e- donated, oxidation, anode

anions to the anode

Question 17

electrolysis of aq soln rules

Answer 17

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-)

Question 18

which anion is NOT reactive at either electrode in a cell?

Answer 18

SO4^2-

eg. CuSO4 sln
Cu produced, O2 produced

Question 19

3 steps of the refinement of bauxite

Answer 19

Bayer process

1. bauxite treated w/ alkali, amphoteric Al2O3 dissolves (forming Al(OH)4 - ion)
2. Al(OH)3 ppt from sln by cooling
3. Al(OH)3 is heated to give alumina (Al2O3)

Question 20

process Al2O3 to get aluminium

Answer 20

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

Question 21

overall eqn for Al2O3 to Al

Answer 21

2Al2O3 + 3C –> 4Al(l) + 3CO2(g)

Question 22

what is red mud

Answer 22

toxic waste from the aluminium production process - red due to iron oxides, strongly alkaline due to residual NaOH

Question 23

oxidants reactivity series

Answer 23

halogens and O2

highest reactivity to lowest:
F2, Cl2, O2, Br2, I2 (Ag+, Cu2+, Zn2+, H in H2O, Mg2+, Na+)

Question 24

reductants reactivity series

Answer 24

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

Question 25

conductivity of molecular aqueous solutions

Answer 25

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

Question 26

what is produced in the electrolysis of concentrated NaCl?

full eqn

Answer 26

anode (oxidation): 2Cl- –> Cl2(g) + 2e-

cathode (reduction): 2H2O + 2e- –> H2(g) + 2OH-
product NaOH

2NaCl(aq) + 2H2O –> Cl2(g) + H2(g) + NaOH(aq)

Question 27

reaction of NaOH and Cl

Answer 27

makes bleach, NaOCl

occurs when products of the electrolysis of concentrated NaCl are allowed to mix

Cl2 + 2NaOH –> NaCl + NaOCl + H2O

Question 28

electroplating process

Answer 28

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+)

Question 29

Faraday’s law of electrolysis

Answer 29

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-)

Question 30

charge formula

Answer 30

Q = It

charge = current (Cs-1) x time (t)

Question 31

calculate the number of moles of e- associated with a particular charge

Answer 31

Q = n(e-) x F

number of e- in 5000C:
n(e-) = 5000C / 96500 Cmol-1 = 0.0518 mol

Question 32

find amount of substance based on amount of e-

Answer 32

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

Question 33

define reversible galvanic cell

Answer 33

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

Question 34

define electrolytic cell

Answer 34

battery in external circuit having potential difference greater than the opposite in sign to the reversible potential

non spontaneous