lecture 41 electrochemistry v n.
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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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review
Review

Galvanic Cells:

Reaction is spontaneous

Eocell > 0

The “product” is an electrical current

Some can be reversed (recharged)

electrolytic cells

Electrolytic Cells

Reaction is not spontaneous

Eocell < 0

Used for producing a chemical product

electrolysis of water

Electrolysis of Water

We want:

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

i.e. non-spontaneous!

electrolysis of water2

Electrolysis of Water

Note reversal of polarity!

battery

-

+

H2O(l) +

salt

anode

cathode

2 H2O + 2 e-®

H2(g) + 2 OH-

2 H2O ®

O2(g) + 4 H+ + 4 e-

electrowinning of metals
Electrowinning of Metals

-

+

CuCl2(aq)

anode

cathode

at the cathode cu 2 cu s or h 2 o h 2 g
at the cathode:

Cu+2® Cu(s) ??

or

H2O® H2(g) ??

rule of thumb

Rule of thumb…

The cathode reaction with the most positive reduction potential will occur preferentially

cu 2 aq 2 e cu s e o 0 15 v 2 h 2 o l 2 e h 2 g 2 oh e o 0 83 v
Cu+2(aq) + 2 e-Ž Cu(s) Eo = +0.15 V

2 H2O(l) + 2 e-Ž H2(g) + 2 OH- Eo = - 0.83 V

slide11

at the anode:

Cl-® Cl2(g) ??

or

H2O® O2(g) ??

same rule of thumb

Same rule of thumb…

The anode reaction with the most positive oxidation potential will occur preferentially

same rule of thumb1

Same rule of thumb…

2 Cl-(aq) Ž Cl2(g) + 2 e- Eo = -1.36 V

2 H2O(l)Ž O2(g) + 4 H++ 4 e- Eo = - 1.23 V

Thus, we predict water gets oxidized at the anode.

overpotential

Overpotential

Erequired > Eocell for electrolysis

In this case, Cl- is oxidized

electrowinning of metals1
Electrowinning of Metals

-

+

CuSO4(aq)

anode

cathode

2 Cl- ®

Cl2(g) + 2 e-

Cu+2(aq) + 2 e-®

Cu(s)

overall reaction

Overall Reaction:

Cu+2(aq) + 2 Cl-(aq) Ž Cu(s) + Cl2(g)

Eocell = -1.21 V

electrorefining of metals

Electrorefining of Metals

-

+

Impure Cu

Pure Cu

Cu

making fluorine f 2 g

Making Fluorine, F2(g)

Electrolysis of HF · 2 KF(l)

oxidation: 2 F-® F2(g) + 2 e-

reduction: 2 H+ + 2 e-® H2(g)

making chlorine cl 2 g

Making Chlorine, Cl2(g)

Electrolysis of brine (NaCl(aq))

oxidation:

2 Cl-(aq)® Cl2(g) + 2 e-

reduction:

2 H2O(l) + 2 e-® H2(g) + 2 OH-(aq)

overall

Overall,

2 NaCl(aq) + 2 H2O(l) ® Cl2(g) + H2(g)

+ 2 NaOH(aq)

cleaners,

paper, etc

disinfection,

plastics (PVC)

manufacture of aluminum

Manufacture of Aluminum

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)

manufacture of aluminum1

Manufacture of Aluminum

Bauxite = impure Al2O3(s)

m.p. = 2045oC

Solution?

1. Lower the melting point

2. Perform electrolysis

manufacture of aluminum2

Manufacture of Aluminum

Al2O3(s) + AlF3(s) + 3 NaF(s)

Na3AlF6, cryolite

m.p. » 900oC

manufacture of aluminum3

Manufacture of Aluminum

reduction: AlFxOy3-x-2y + n e- ® Al(l) + ?

oxidation:AlFxOy3-x-2y® O2(g) + n e- + ?

oxidizes C

electrodes!

sinks!