Chapter 17 Assigned Problems:

16 (a, d), 17, 21, 25 (a), 27, 35, 37, 47, 49, 55, 57, 61, 63, 65, 67, 71 (a-b), 73, 79, 83

Chapter 17 – Electrochemistry - Chemistry of electro n flow in redox reactions

Galvanic Cell – device that produces electricity by means of a spontaneous redox reaction. Reactants must be separated from each other.Anode – electrode where oxidation occurs.

Cathode – electrode where reduction occurs.

Salt Bridge – allows migration of ions into solutions, where ions are non-reactive. Used to maintain electrical neutrality of solution.

When will this battery quit working?Cell Diagram notation: (path from anode to cathode through salt bridge)

Zn(s) | Zn+2(aq, 1M) | KCl (sat’d) | Cu+2

(aq, 1M) | Cu (s) where | = phase boundary)

Cu(s) + Ag+(aq) � Cu+2

(aq) + Ag (s)

Voltage of a battery – EMF (electromotive force) (E ) - Cell Potential – Cell Voltage

EMF depends on: 1) the nature of the reactants2) Concentrations of the reactants3) Temperature (car batteries in winter?)

To determine the voltage of an electrochemical reac tion:

Arbitrarily set the voltage of a particular reactio n to zero.

Standard Hydrogen Electrode (SHE)

H2 ↔ 2H+ + 2e- Eo = 0 volts

Eo = standard state (1 M HCl, 25 oC, 1 atm H2)

Use a SHE with other substances to measure their re lative EMF (voltage)

Zn(s) → Zn+2(aq) + 2e- Eo = ?

2H+(aq) + 2e- → H2(g) Eo = 0 volts

Zn(s) + 2H+(aq) → H2(g) + Zn+2

(aq) Eocell = 0.76 volts, as measured on a voltmeter

Anode reaction?Cathode reaction?

Zn(s) → Zn+2(aq) + 2e- Eo = .76 volts

Cu+2(aq) + 2e- → Cu(s) Eo = ?

Zn(s) + Cu+2(aq) → Zn+2

(aq) + Cu(s) Eocel

l = 1.10 volts

Anode reaction?Cathode reaction?

A reaction is spontaneous if E ocell is positive. The more positive the voltage, the

more spontaneous the reaction.Rules for calculating cell voltage:1. Eo applies to forward reaction. Change the voltage si gn if you reverse the

reaction.2. At standard state, any species on the left side o f the chart will spontaneously

react with a species on the right, below it. (This is the format of the AP Exam)

3. Changing the stoichiometriccoeffecients does not change the value of E o.

Problem: Arrange the following in increasing stren gth as an oxidizing agent.Sn+2, Al+3, Ag+ and MnO 4

-

Problem: Predict what will happen if Br 2(l) is added to a solution containingCl-(aq) and I -

(aq)

*

*

*

Problem: A galvanic cell consists of a Mg electrod e in 1M Mg(NO 3)2 and an Ag electrode immersed in 1M AgNO 3. Determine the balanced equation for the reaction that will take place, and calculate th e EMF of this cell. What reaction will occur at the cathode?

**

Problem: Predict the voltage of the following react ion. Note which reaction is the oxidation vs. the reduction reactio n.

Zn(s) | 1M Zn+2(aq) and Al (s) | 1M Al +3

(aq)

Homework Problems: 16 a&d, 25a, 27, 35, 37, 47, 49

Spontaneity of redox reactions or how E o relates to ∆G and K

∆Go = -n F Eocell where R = 8.31 J/K x mole

n = total moles of electrons involved in redox reac tion

F = 1 Faraday = 96,500 Coulombs/mole electrons= 96,500 J/volt x mole

Eocell = R T ln K

nF

If standard temp and ln converted to base 10

Eocell = .0591 volts log K

n

Problem: Calculate K for the following reaction at 25oCSn(s) + 2Cu+2

(aq) → Sn+2(aq) + 2Cu+

(aq)

Eocell = .0591 volts log K

n

Problem: Calculate ∆Go for Au (s) + 3Ca+2(aq) → 2Au+3

(aq) + 3Ca(s)

∆Go = -n F Eocell

The effect of concentration on a cells EMF

Nernst Equation

E = Eo – 2.303 R T Log Q n F

at 25oC, R T and F are constants

E = Eo - .0591 volts Log Qn

Problem: Predict the voltage of : Co (s) + Fe+2(aq) ↔ Co+2

(aq) + Fe(s) at 25 oC

When [Co +2] = .15 M[Fe+2] = .68 M

At what concentration will this reaction become spo ntaneous?

The standard EMF for the following reaction is +.76 volts. Zn(s) + 2H+

(aq) → Zn+2(aq) + H2(g)

Calculate the [H +] when [Zn +2] = 1.0 M and P H2= 1.0 atm and the voltage

of the cell is measured at .54 volts.

Concentration Cell – By using a cell with half react ions that are the same chemicals, but different concentrations, a cell can be made.

Problem: Which half reaction will be oxidized, and which reduced to make this spontaneous?

The reaction will be Fe +2 + 2e- → Fe(s) Eo = -.44 volts

Fe(s) → Fe+2 + 2e- Eo = +.44 volts

.1 M Fe+2

.01 M Fe+2

Calculate the voltage of this cellHomework: 55, 57, 61, 63, 65, 67, 71 a & b, 73,

Lead Acid Storage Battery

Pb(s) + SO4-2

(aq) → PbSO4(s) + 2e- ( Anode)

PbO2(s) + 4H+ + SO4-2 + 2e- → PbSO4(s) + 2H2O(l) ( Cathode)

Pb(s) + PbO2(s) + 4H+ + 2SO4-2(aq) → 2PbSO4(s) + 2H2O(l)

At normal temperature, each of the 6 cells produces about 2 volts, hooked up in series, makes about 12 volts.

When the reaction is reversed, the reactants are replenished, and the battery is recharged.

Dry Cell (Leclanche) Battery

Anode: Zn → Zn+2 + 2e-

Cathode: 2NH 4+ + 2MnO2 + 2e- → Mn2O3 + 2NH3 + H2O

Alkaline Battery:

Anode: Zn + 2OH - → ZnO + H2O + 2e-

Cathode: 2MnO 2 + H2O + 2e- → Mn2O3 + 2OH-

Why does a battery die?E = Eo - .0591 volts Log Q

n

Fuel Cell : Device that collects the energy of a co mbustion type of reaction and converts the chemical energy to elect ricity rather than heat.

2H2 + 4OH- → 4 H2O + 4e- Anode Rxn

4e- + O2 + 2H2O → 4OH- Cathode Rxn

2H2 + O2 → 2H2O produces 1.23 volts

Corrosion – the deterioration of metal by electroche mical processes.Iron rustsSilver tarnishesCopper turns to patina

The Fe+2 reacts with O 2 to form FeO and ultimately, Fe 2O3, or rust.

Aluminum corrodes more quickly with oxygen than iro n, and forms a waterproof,tough Al 2O3 layer, which seals the inner aluminum from corrodin g…

To Prevent Corrosion:

1. Paint - seals out reducing substances, keeps meta ls from oxidizing

2. Alloy – Iron mixed with Cr and Ni = Stainless Stee l. Cr reacts w/ O 2 in the air to form a tough, impenetrable chrome oxide laye r.

3. Galvanize – Coat iron with a layer of Zinc, a more reactive metal.

Zn → Zn+2 + 2e- Eo = .76 volt

Fe+2 + 2e- → Fe Eo = -.44 volt4. Cathodic Protection

Electrolysis – a chemical reaction is caused by forc ing an electrical current through a set of reactants that normally wo uld not react. (Opposite of a Galvanic reaction)

Downs Cell – used to produce Na (s) and Cl 2(g)

Cathode Reaction:

2Na+(l) + 2e- → 2Na(l)

Anode Reaction:

2Cl-(l) → Cl2(g) + 2e-

Spontaneous reaction of Na(s) + Cl2(g)

Electrolysis of water:

Cathode reaction:

4H+(aq) + 4e- → 2H2(g)

Anode reaction:

2H2O(l) → O2(g) + 4H+ + 4e-

Production of Aluminum from Bauxite (Aluminum oxide ) Hall Process

Overall reaction is: 2Al 2O3 + 3C → 4Al + 3CO2

Electroplating:

Gold plated jewelry and silver plated jewelry

Electroplating and its Stoichiometry

The unit for measuring the amount of charge is the Coulomb

1 mole of electrons has a charge of 96,500 Coulombs

1 Amp = 1 Coulomb of chargesecond

Problem: In an electrolytic cell, 10.0 amps is pas sed for 30. minutes through a Cu+2 solution. How many grams of Cu are plated?

Problem: How long (minutes) must a current of 5.00 amps be applied to Ag + to produce 10.5 grams of silver metal?

Homework: 17, 21, 79, 83