Lecture 41 - Electrochemistry V. Review. Galvanic Cells: Reaction is spontaneous E o cell > 0 The “product” is an electrical current Some can be reversed (recharged). Electrolytic Cells. Reaction is not spontaneous E o cell < 0 Used for producing a chemical product.
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Reaction is spontaneous
Eocell > 0
The “product” is an electrical current
Some can be reversed (recharged)
Reaction is not spontaneous
Eocell < 0
Used for producing a chemical product
2 H2O(l) 2 H2(g) + O2(g)
2 H2O(l) O2(g) + 4 H+(aq) + 4 e- Eo = -1.23 V
2 H2O(l) + 2 e- H2(g) + 2 OH-(aq) Eo = -0.83 V
Eocell = -2.06 V
Note reversal of polarity!
2 H2O + 2 e-®
H2(g) + 2 OH-
2 H2O ®
O2(g) + 4 H+ + 4 e-
Cu+2® Cu(s) ??
H2O® H2(g) ??
The cathode reaction with the most positive reduction potential will occur preferentially
2 H2O(l) + 2 e- H2(g) + 2 OH- Eo = - 0.83 V
Cl-® Cl2(g) ??
H2O® O2(g) ??
The anode reaction with the most positive oxidation potential will occur preferentially
2 Cl-(aq) Cl2(g) + 2 e- Eo = -1.36 V
2 H2O(l) O2(g) + 4 H++ 4 e- Eo = - 1.23 V
Thus, we predict water gets oxidized at the anode.
Erequired > Eocell for electrolysis
In this case, Cl- is oxidized
2 Cl- ®
Cl2(g) + 2 e-
Cu+2(aq) + 2 e-®
Cu+2(aq) + 2 Cl-(aq) Cu(s) + Cl2(g)
Eocell = -1.21 V
Electrolysis of HF · 2 KF(l)
oxidation: 2 F-® F2(g) + 2 e-
reduction: 2 H+ + 2 e-® H2(g)
Electrolysis of brine (NaCl(aq))
2 Cl-(aq)® Cl2(g) + 2 e-
2 H2O(l) + 2 e-® H2(g) + 2 OH-(aq)
2 NaCl(aq) + 2 H2O(l) ® Cl2(g) + H2(g)
+ 2 NaOH(aq)
Why not just electrowin Al+3(aq)?
Al+3(aq) + 3 e- ® Al(s) Eo = -1.66 V
2 H2O(l) + 2 e- ® H2(g) + 2 OH-(aq)Eo = -0.83 V
(reduction of water is favored)
Bauxite = impure Al2O3(s)
m.p. = 2045oC
1. Lower the melting point
2. Perform electrolysis
Al2O3(s) + AlF3(s) + 3 NaF(s)
m.p. » 900oC
reduction: AlFxOy3-x-2y + n e- ® Al(l) + ?
oxidation:AlFxOy3-x-2y® O2(g) + n e- + ?