How to balance ionic equations in Alkaline Medium?
(I) Standard Electrode Potential
What information can we obtain from SEP?
Sign – oxidation or reduction favoured
Magnitude – the extent the reaction is favoured
How to measure SEP?
- connecting the redox couple (half cell) to SHE.
- learn to draw the set-up + cell notation
- remember to include standard conditions!
| Galvanic Cells | Electrolytic Cells |
| - calculate E°cell à predict feasibility of redox rxn - predict change in E° at non std conditions using LCP | - predict species discharged/ pdts at each electrode - calculate amt of product using Q = It = nF |
(II) Electrochemical/ Galvanic Cell
1. Identify possible oxidation/ reduction reactions
2. Find electrode potentials for these half equations
3. Calculate Ecell = Ered – Eox (> 0: feasible; < 0: not feasible)
| Ecell | |
| + | feasible |
| – | not feasible |
(III) Electrolytic Cell
Identify which species are discharged:
1. Identify all ions in solution (including H2O)
2. Cations attracted to cathode;
Anions attracted to anode
3. Write electrode potential for all the reduction reactions
| E° | |
| Cathode | more + or less – is discharged |
| Anode | more – or less + is discharged |
Note:
For redox active electrodes (e.g. Cu, Fe), need to compare the E° values for the metals together the ions at the anode.
Calculate amount of products discharged
Q = I t = n F
I: current (A) n: moles of electrons!
t: time (s) F: Faradays constant (96500 C/mol)
1. Determine moles of electrons transferred
2. From half equation, determine moles of pdt from molar ratio
(IV) Others
- Electrons always flow from A à C
- Anode à Oxidation
- Cathode à Reduction
- For a rechargeable battery:
Charging à Galvanic
Discharging à Electrolytic
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