![Nils Walter: Chem 260
Carrying the concept further
Reduction of Cu2+: Cu2+(aq) + 2e- →
→
→
→ Cu(s)
In general: redox couple Ox/Red, half-reaction Ox + ν
ν
ν
νe- →
→
→
→ Red
Any reaction can be expressed in redox half-reactions:
Expansion of gas: H2(g, pi) →
→
→
→ H2(g, pf)
2 H+(aq) + 2e- →
→
→
→ H2(g, pi)
2 H+(aq) + 2e- →
→
→
→ H2(g, pf)
Dissolution of a sparingly soluble salt: AgCl(s) →
→
→
→ Ag+(aq) + Cl-(aq)
Ag+(aq) + e- →
→
→
→ Ag(s)
AgCl(s) + e- →
→
→
→ Ag(s) + Cl-(aq)
Reaction quotients: ]
[ −
≈
= − Cl
a
Q Cl
]
[
1
1
+
≈
=
+ Ag
a
Q
Ag](https://image.slidesharecdn.com/lecture37-220908065345-cb828001/85/lecture37-pdf-3-320.jpg)
lecture37.pdf
- 1. Nils Walter: Chem 260 Electrochemical cells = electronic conductor + surrounding electrolyte electrode compartment If two different electrolytes are used: Electrolytic cell: electrochemical cell in which a non-spontaneous reaction is driven by an external source of current Galvanic cell: electrochemical cell in which electricity is produced as a result of a spontaneous reaction (e.g., batteries, fuel cells, electric fish!)
- 2. Nils Walter: Chem 260 Reactions at electrodes: Half-reactions Redox reactions: Reactions in which electrons are transferred from one species to another +II -II 0 -II 0 +IV reduced oxidized Any redox reactions can be expressed as the difference between two reduction half-reactions in which e- are taken up CuS(s) + O2(g) → → → → Cu(s) + SO2(g) E.g., Reduction of Cu2+: Cu2+(aq) + 2e- → → → → Cu(s) Reduction of Zn2+: Zn2+(aq) + 2e- → → → → Zn(s) Cu2+(aq) + Zn(s) → → → → Cu(s) + Zn2+(aq) More complex: MnO4 -(aq) + 8H+ + 5e- → → → → Mn2+(aq) + 4H2O(l) Half-reactions are only a formal way of writing a redox reaction Difference:
- 3. Nils Walter: Chem 260 Carrying the concept further Reduction of Cu2+: Cu2+(aq) + 2e- → → → → Cu(s) In general: redox couple Ox/Red, half-reaction Ox + ν ν ν νe- → → → → Red Any reaction can be expressed in redox half-reactions: Expansion of gas: H2(g, pi) → → → → H2(g, pf) 2 H+(aq) + 2e- → → → → H2(g, pi) 2 H+(aq) + 2e- → → → → H2(g, pf) Dissolution of a sparingly soluble salt: AgCl(s) → → → → Ag+(aq) + Cl-(aq) Ag+(aq) + e- → → → → Ag(s) AgCl(s) + e- → → → → Ag(s) + Cl-(aq) Reaction quotients: ] [ − ≈ = − Cl a Q Cl ] [ 1 1 + ≈ = + Ag a Q Ag
- 4. Nils Walter: Chem 260 Reactions at electrodes Galvanic cell: Electrolytic cell: Red1 → → → → Ox1 + ν ν ν νe- Ox2 + ν ν ν νe- → → → → Red2 Half-reactions Red1 → → → → Ox1 + ν ν ν νe- Ox2 + ν ν ν νe- → → → → Red2
- 5. Nils Walter: Chem 260 Insoluble-salt electrode: metal (e.g., Pt) solution (e.g., H+) 2H+ + 2e- H2(g) 2 2 ) ( + = H a H p Q Types of electrodes I Pt(s)| || | H2(g)| || | H+(aq) Gas electrode: Gas (e.g., H2) metal (e.g., Ag) solution (e.g., Cl-) AgCl(s) + e- Ag(s) + Cl-(aq) − = Cl a Q Ag(s)| || | AgCl(s)| || | Cl-(aq) Porous, insoluble salt (e.g., AgCl)
- 6. Nils Walter: Chem 260 Fe3+ + e- Fe2+ + + = = 3 2 Re Fe Fe Ox d a a a a Q Types of electrodes and how to put them together in a galvanic cell Pt(s)| || | Fe2+(aq),Fe3+(aq) + + = 2 2 Cu Zn a a Q Zn(s)| || | ZnSO4(aq) | || | | || | CuSO4(aq)| || | Cu(s) Redox electrode: Daniell cell: Right: Cu2+(aq) + 2e- → → → → Cu(s) Left: Zn2+(aq) + 2e- → → → → Zn(s) Cu2+(aq) + Zn(s) Cu(s) + Zn2+(aq)
- 7. Nils Walter: Chem 260 Cell reaction and potential Cell reaction: Difference of electrode half-reactions (Reduction at Cathode - Oxidation at Anode) Cathode (Right): Cu2+(aq) + 2e- → → → → Cu(s) Anode (Left): Zn2+(aq) + 2e- → → → → Zn(s) Overall (R-L): Cu2+(aq) + Zn(s) Cu(s) + Zn2+(aq) e- disappear this way the reaction becomes spontaneous } Cell potential E: Potential difference between the electrodes Maximum electrical work done in a galvanic cell: w’ = - ν ν ν νF × × × × E = ∆ ∆ ∆ ∆rG Ox + ν ν ν νe- → → → → Red Þ Þ Þ Þ ν ν ν νNA e- transferred per mole of reaction Þ Þ Þ Þ ν ν ν νNA × × × × (-e) = -ν ν ν νF charge transferred Faraday constant = eNA = 96,485 Cmol-1