Ch 20: Electrochemistry
Problem Set #3, 10c-f, 12, 14, 22, 32, 46, 49, 65, 68; Recommended: 9, 11, 19,
33, 47, 80
20.1: Oxidation-reduction (redox) reactions: reaction with a change
• At least one element’s oxidation number will __________ and one will
__________
• Consists of
oxidation: __________ of e-; oxidation number __________
reduction: __________ of e-; oxidation number __________
Oxidizing agent or
: gains e-, causing another
substance to
Reducing agent or
: loses e-, causing another
substance to
Ex: Determine half-reactions: Cr2O72- (aq) + Cl- (aq) → Cr3+ (aq) + Cl2 (g)
20.2: Balancing Redox Reactions:
“Method of Half-Reactions” Steps:
1. Assign
to all species; based on this, split the halfreactions of reduction and oxidation.
2. For each, balance all elements except
3. Balance oxygen by adding
to the opposite side.
4. Balance hydrogen by adding
to the opposite side.
5. Balance charges by adding
to side with overall positive charge.
6. Multiply each half-reaction by an integer so there are an
of e- in each.
7. Add the half reactions; cancel any species; check final balance.
Ex: Balance the following reaction, which takes place in an acidic solution.
Cr2O72- (aq) +
Cl1- (aq) → Cr3+ (aq) + Cl2 (g)
Reduction:
Oxidation:
Overall:
1
• Since occurs in acidic solution,
• If reaction occurs in basic solution, balance using same method but
Ex: MnO41- (aq) + Br1- (aq) → MnO2 (s) +BrO31- (aq) in basic solution.
20.3: Voltaic (or Galvanic) Cells: device that utilizes energy released in a
__________ electrochemical reaction by directing __________ along an
external pathway, rather than directly between
Electrodes: metals connected by
Anode: half-cell where
Cathode: half-cell where
Ex:
occurs
occurs
Zn (s) + Cu2+ (aq) → Zn2+ (aq) + Cu (s)
Oxidation half-cell:
Reduction half-cell:
* Notation:
2
20.4: Electromotive force, emf or E: potential difference between cathode and
anode that causes a flow of e-.
– The result of the difference in
produced at
each
• Also called
• Units =
• Standard conditions
• Eºcell is unique to each set of
Standard reduction potentials: assigned to each reduction reaction
Eºcell =
• All Eºred are referenced to
Ex: Zn (s) + Cu2+ (aq) → Zn2+ (aq) + Cu (s)
Anode:
• This is oxidation, but Eºred is recorded as
Cathode:
• Positive voltages =
• Changing the stoichiometric coefficients of a half-cell
_______________ the value for the Eo, since potential is a measure
of
• However, having more moles means the reaction will
Using the Standard Reduction Potential table
• When Eºred is very negative, reduction is
Li1+ (aq) + e- → Li (s)
Eºred =
Li (s) _____________ easily, so it is the strongest _____________
agent; _____________ on the Standard Reduction Potential chart
signifies the _____________.
• When Eºred is very positive, reduction is
F2 (g) + 2 e- → 2 F1Eºred =
F2 _____________ easily, so it is the strongest _____________
agent; _____________ on the chart signifies the _____________.
• Predicting metal “activity series”: most reactive metals are near
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20.5: Spontaneity of Redox Reactions
Gibbs free energy and emf:
R=
N=
Faraday’s constant, F =
•
= 1 mole of e-, which has a charge of
• Units =
• As Eº increases, ∆Gº decreases (becomes more likely to be
20.6: Effect of Concentration on Ecell
The Nernst Equation:
• At equilibrium, ∆G =
∆Go
Negative
0
Positive
K
Eocell
Reaction outcome
20.8-9: Electrolytic cells:
• Electrical energy is used to cause a _____________ reaction to occur
• Current (I): a measure of the
• Units =
4
Ex: Na+ (l) + Cl- (l) → Na (l) + Cl2 (g)
Oxidation half-cell:
Reduction half-cell:
• In a voltaic cell: anode is
• In an electrolytic cell: anode is
cathode is
cathode is
20.9: Quantitative Electrolysis
Ex: A current of 0.452 A is passed through an electrolytic cell containing
molten NaCl for 1.5 hours.
• Write the electrode reactions.
Oxidation:
Reduction:
• Calculate the mass products formed at each electrodes.
Strategy: calculate the total charge of e- transferred, and use F to
convert to moles.
5
Electrolysis of Aqueous Mixtures
• If the voltage on an aqueous mixture of CuCl2 and ZnCl2 in an
electrolytic cell is slowly increased, what products will form at each
electrode?
o What exists in solution that could be reduced?
Eºred =
Eºred =
Eºred =
, it is the most probable
o Since Eºred ( ) is the most
reaction, and therefore will occur most easily (at the
):
o What exists in solution that could be oxidized?
Eºox =
Eºox =
, it is the most probable
o Since Eºox ( ) is the most
reaction, and therefore will occur most easily (at the
):
Electroplating
ƒ The
is a silver bar, and the
is an iron spoon.
Anode:
Cathode:
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