Electrode Potentials and Cells + Commercial Applications

Question 1

IUPAC convention for writing half-equations for electrode reactions.

Answer 1

tips for writing the half equations

• half equations in the exam are represented by the forward reaction being REDUCTION eg
  Zn2+ + 2e- –> Zn and Cu2+ + 2e- –> Cu
  (forward reactions in reduction)
• the most negative electrode must be flipped so that the equation is showing OXIDATION.
  eg Zn is the most negative electrode so:
  Zn –> Zn2+ + 2e-

therefore the combined equations would be:
Cu2+ + Zn –> Cu + Zn2+

Question 2

how do you know which electrodes are oxidising or reducing agents?

Answer 2

the most positive electrode will be OXIDISING AGENTS, therefore they are more easily reduced (able to gain electrons)

the most negative electrode will be REDUCING AGENTS, therefore they are more easily oxidised (lose electrons)

Question 3

What is the Standard Hydrogen Electrode? (SHE)

Answer 3

it is used a reference to measure standard electrode potentials.

1. temperature = 298K
2. Pressure = 100kPa
3. Conc of ions = 1.0 mol dm-3 of HCl
4. Pt electrode to carry the flow of ions.

Question 4

How to calculate the standard cell potential?

Answer 4

Most positive - most negative

Question 5

How to write the cell notation?

Answer 5

Reduced Form|Oxidised Form||Oxidised Form|Reduced Form

*if they are in the same state, then there is no need for a single vertical line
*need to add Pt(s) if there is no solid to carry the flow of charge
*double vertical line = salt bridge

eg Zn(s)|Zn2+(aq)||Cu2+(aq)|Cu(s)

eg Mg(s)|Mg2+||Fe3+(aq), Fe2+(aq) |Pt(s)

Question 6

What are the electrodes for Batteries?

Answer 6

Positive electrode = Li+ + CoO2 + e- –> Li+[CoO2]-
Negative electrode = Li –> Li+ + e

Overall equation on discharge = Li + CoO2 –> Li+[CoO2]-

Overall equation on recharge = Li+[CoO2]- –> Li + CoO2
*rechargeable batteries work by plugging them to a supply current which FORCES electrons to flow in the OPPOSITE way.