Electrochemistry

Electrochemistry

Electrode Potentials

Electrochemistry

1. What are electrode potentials?

Electrochemistry

1. What are electrode potentials?

• Electric potential arising from the separation of charges in the equilibrium in a redox half equation.

• It is measured with respect to S.H.E.

Examples:

Zn2+ (aq) + 2e– Zn ⇌ (s)

Fe3+ (aq) + e– Fe⇌ 2+ (aq)

2H+ (aq) + e– H⇌ 2 (g)

Cu2+ (aq) + 2e– Cu (s)⇌

Electrochemistry

2. What can we know from electrode potentials?

Electrochemistry

2. What can we know from electrode potentials?

1. [Sign]

+ forward reaction (reduction) favoured

– backward reaction (oxidation) favoured

2. [Magnitude]

– the extent the reaction is favoured

Electrochemistry

3. How can we measure standard electrode potentials?

Electrochemistry

3. How can we measure standard electrode potentials?

- connect the half cell that you wish to study to the SHE

Note:• You need to be know how to draw the cell setup.• Things to note when drawing half cell: (1) salt bridge, (2)

voltmeter and (3) ensure all gases, solutions are at standard conditions.

Electrochemistry

4. How can we use standard electrode potentials?

Electrochemistry

4. How can we use standard electrode potentials?

- to predict whether a redox reaction is feasible when 2 species are mixed.

Examples:

I2 + Cr2+ feasible?

V3+ + Cu2+ feasible?

Electrochemistry

4. How can we use standard electrode potentials?

Strategy

1. Identify possible oxidation/ reduction reactions

2. Find electrode potentials for these half equations

3. Calculate Ecell = Ered – Eox

Ecell

+ feasible

– not feasible

Electrochemistry

Galvanic Cells

Electrochemistry

5. How to distinguish anode and cathode?

Galvanic Cell

Electrochemistry

5. How to distinguish anode and cathode?

Positive remove electrons Reduction Cathode

Negative produce electrons Oxidation Anode

Electrons flow from AC

Galvanic Cell

Electrochemistry

6. How to draw cell diagram?

Electrochemistry

6. How to draw cell diagram?

Example

Electrochemistry

6. How to draw cell diagram?

1. Anode || Cathode

2. Electrodes at the extreme ends.

3. Anode: Fe2+ Fe3+

Cathode: H+ H2

4. If redox species of the same phase, separate by comma; different phase by |

Pt (s) | Anode || Cathode | Pt (s)

Pt (s) | Fe2+ Fe3+ || H+ H2 | Pt (s)

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

Electrochemistry

Electrolytic Cells

Electrochemistry

7. How to distinguish anode and cathode?

Electrolytic Cell

Electrochemistry

7. How to distinguish anode and cathode?

Positive attract anions oxidized Anode

Negative attract cations reduced Cathode

Electrons flow from AC

Electrolytic Cell

Electrochemistry

8. How to predict what species is discharged?

1. Identify all ions in solution (including H2O)

2. Cations attracted to cathode;

Anions attracted to anode

3. Find electrode potentials for these half equations

E°

Cathode more positive or less negative is discharged

Anode more negative or less positive is discharged

Electrochemistry

Calculations (Strategy)

Question will surely involve 1 or more of the formulas:

Q = I t = n F

I: current (A) n: moles of electrons!

t: time (s) F: Faradays constant (96500 C/mol)

F = Le L: Avogadro’s constant

e: 1.6 x 10–19 C