Standard Electrode Potential (SEP)

 

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How to draw the set-up for measuring SEP?

  1. Learn to draw the 3 types of half cells: (1) ion-metal (2) ion-ion (3) ion-gas.
  2. Determine the type of half cell the qns is asking for.
  3. Connect the half cell to SHE.

 

3 types of half cells:

 

Example: Draw the setup for measuring E⁰Fe3+/Fe2+

 

Fe3+/Fe2+ is an ion-ion half cell

H+/H2 (SHE) is an ion-gas half cell

Note:

1. Ensure all parts are labeled e.g. salt bridge.

2. Ensure standard conditions for all reagents i.e.

- 1 atm, 298 K for gases

- 1 mol dm–3 for solutions

 

How to draw a cell diagram?

Cell diagram is a simplified representation of a galvanic cell.

  1. Anode on the left; Cathode on the right; Salt bridge (represented as ||) in the middle i.e. Anode || Cathode
  2. Electrodes at extreme ends.
  3. Anode half cell (oxidation) write species in increasing oxidation no.; Cathode half cell (reduction) write species in decreasing oxidation no.
  4. Use ‘,’ to separate species in the same phase; Use ‘|’ to separate species in different phases.

 

Example: Draw the cell diagram for the cell represented above.

 

1. Since E⁰Fe3+/Fe2+ = +0.77 V (more + than SHE), it must be the cathode.

(anode) SHE || Fe3+/Fe2+ (cathode)

 

2. Electrodes at extreme ends

Pt     SHE || Fe3+/Fe2+     Pt

 

3. Anode (increasing ON); Cathode (decreasing ON)

Pt     H2     H+ || Fe3+     Fe2+     Pt

 

4. Same phase (,); Different phase (|)

Pt (s) | H2 (g) | H+ (aq) || Fe3+ (aq) , Fe2+ (aq) | Pt (s)