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Electrochemistry

Interconversion of electrical

& chemical potential energy

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Electrochemistry

Always involves redox reactions.

1. Energy released by spontaneous reaction is converted to electrical energy (voltaic or galvanic cell)

OR2. Electricity drives nonspontaneous

reaction (“electrolytic cell”)

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Redox Reactions

Mg + 2HCl MgCl2 + H2

For example:

ox # 0 +1 +2 0

• Mg is oxidized (loses e-)• H+ is reduced (gains e-)• Cl- is a spectator

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Redox Reactions

For you to review:

Assigning oxidation numbersActivity series

more active metal oxidationmore active nonmetal reduction

Balancing redox reactions (attached)

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Activity SeriesWill this reaction occur spontaneously?

Mg + Co2+ Mg2+ + Co

Yes!

Mg is more active than Co.

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Redox Balancing: Practice

Cu + HNO3 Cu2+ + NO + H2O

CN- + MnO4- CNO- + MnO2

acid

base

‘Odd’ atoms

O with H2O

H with H+

Charge with e-

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Galvanic (Voltaic) Cell

net: Zn + Cu+2 Zn+2 + Cu

Spontaneous “half reactions” are separated in a galvanic cell(to use the flow of electrons).

Zn + CuSO4 ZnSO4 + Cu

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Voltaic Cell

Zn + Cu+2 Zn+2 + Cu

Oxidation at Zn electrode (anode)

Zn Zn+2 + 2e-

Reduction at Cu electrode (cathode)

Cu+2 + 2e- Cu

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(-) (+)

Voltaic CellOxidation

Zn Zn+2 + 2e-Reduction

Cu+2 + 2e- Cu

Voltaic Cell videohttp://www.mhhe.com/physsci/chemistry/essentialchemistry/flash/galvan5.swf

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Electrochemical Cells

Electric current (e-) flows through the wire due to difference in electrical potential, measure by

cell voltage or emf.

Non-reacting cations and anions flow through the salt bridge to maintain electrical neutrality.

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Standard Electrode Potentials

It is impossible to measure the emf(voltage) of a single half cell, so one is arbitrarily set to equal zero.

2H+(1M) + 2e- H2 (1 atm)

Standard hydrogen electrode (SHE)

EoSHE = 0 volt (definition)

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S.H.E.

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Standard Reduction Potential

All other half-cell potentials are measured relative to SHE, and are assigned Eo values if:

solutes are 1M gases are 1.0 atm temperature is 25oC

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Standard Reduction Potential

Eo for: Zn(s) Zn+2(aq) + 2e-

How do you tell it is oxidation?

0.76V

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Standard Reduction Potential

Under these conditions, Zn is found to be the anode (oxidation).

And the voltmeter reads: Eo

cell = 0.76 V

Zn(s) Zn+2(aq) + 2e-

Standard Reduction Potential

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Eocell = Eo

cathode + Eoanode

Eocathode = reduction potential of

reduction half-cell (cathode)

Eoanode = reduction potential of

oxidation half-cell (anode)(switch the sign when

putting into the equation)

switch sign for oxidation

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Standard Reduction Potential

Eocell = Eo

cathode + Eoanode

0.76 V = 0.0 V + EoZn+2/Zn

= EoH+/H2 + Eo

Zn+2/Zn

(reduction) (oxidation)

EoZn+2/Zn = 0.76 V (oxidation)

so reduction potential is -0.76V

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Standard Reduction Potential

See problem set for the standard reduction potentials for many half reactions.

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Some “Rules”

Eo’s are all written as reductions.

Zn+2 + 2e- Zn

(switch Eo sign if oxidation)

Eo’s are listed for standard conditions.

1M, 1 atm., 25oC

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Some “Rules” cont.

A redox reaction is thermodynamically favored in the forward direction if Ecell is positive.

Zn + Cu+2 Zn+2 + Cu

Eocell = +1.10 V

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Some “Rules” cont.

The more positive Eo is, the greater tendency for reduction.

F2 + 2e - 2F-

Greatest tendency to go in forward direction.

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Some “Rules” cont.

Half cells are reversible: any electrode can act as anode (oxidation) or cathode (reduction)

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Some “Rules” cont.

The “Diagonal Rule”

Draw a diagonal from Lio to F2.Under standard conditions, a reaction is spontaneous (+Ecell) if the reactants can be connected by this diagonal.

Don’t forget water at 1.23V & -.83V!

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Diagonal Rule

Diagonal connects reactants of spont. redoxreaction

Runs as reduction

Runs as oxidation

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Some “Rules” cont.

Changing the stoichiometric coefficients of a half-cell reaction, does not change Eo.

I2 + 2e- 2I- Eo = 0.53 V

2I2 + 4e- 4I- Eo = 0.53 V

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Some “Rules” cont.

Eo is a thermodynamic function like H, G & S. Reversing the direction of a reaction changes the sign of Eo.

I2 + 2e- 2I- Eo = 0.53 V

2I- I2 + 2e- Eo = - 0.53 V

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Some “Rules” cont.

These rules apply whether the reactants are separated in a galvanic cell or all mixed together.

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Redox Prediction

Write the net ionic reaction that takes place when Br2 is added to a solution of NaCl and NaI.

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You have:Br2(l) Na+(aq)Cl-(aq)I-(aq)H2O

Look for the longest spontaneous diagonal. It identifies the reactants.

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Redox Prediction

A Cr electrode in 1M Cr(NO3)3 is connected to a silver electrode in 1M AgNO3. What is the anode and cathode?Write the net ionic reaction and calculate Eo

cell.

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You have:CrCr3+

AgAg+

H2O

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Eocell DGo Keq

DGo, the amount of energy

available to do work, is given by:

DGo = -n F Eocell

n = number of moles of e-

F = Farady constant

= 96,500 J/V mol e-

= 96,500 C/mol e-

For spontaneous reaction:(+) (-) (>1)

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Eocell DGo Keq

DGo = -n F Eocell

Also, since

DGo = -RT lnK

then

Eocell = RT

nFlnK

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Eo

DGo KDGo = -RT lnK

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Eocell DGo Keq

DGo K Eo Rxn. at Std. State

- > 1 + Spontaneous

0 = 1 0 At equilibrium

+ < 1 - Non-spontaneous

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Show Your Stuff

1.Calculate Keq for the following reaction at 25oC:

Sn(s) + 2Cu+2 Sn+2 + 2Cu+

2.Calculate DGo for the following

reaction at 25oC:

2Au(s) + 3Ca+2 2Au+3 + 3Ca(s)

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Effect of Concentration on Eo

If concentrations are not standard (1.0M), use LeChatelier’s principle to see the effect on voltage:

e.g. If a reactant concentration is greater than 1.0M, reaction will go in forward direction to a greater extent, so cell voltage will increase.

Effect of Concentration on E

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Co+2(aq) + Fe(s) Co(s) + Fe+2(aq)

Calculate Eocell when: [Co+2] = [Fe+2] = 1.0M

Does Ecell increase or decrease when:

1. [Co+2] = 0.5M2. [Fe+2] = 1.5M3. More Co(s) is added

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Batteries

Portable electrochemical cell

used as a source for electricity.

Dry cellCar battery (Pb)Fuel cell

Read !!!

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Electrolysis

The opposite of a galvanic cell. Find the shortest opposite diagonal.

Use of electrical energy to cause a non-spontaneous chemical reaction to occur. This process is carried out in an electrolytic cell.

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Electrolysis: NaCl(l)

Anode 2Cl- Cl2 + 2e-

Cathode 2Na+ + 2e- 2NaOverall 2Na+ + 2Cl- 2Na + Cl2

Since Eo = -4 V, at least 4V must be supplied to cause the reaction to occur.

Electrolysis: NaCl(aq)

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What are the two half-reactions and the voltage required for this electrolysis?

Use the table of reduction potentials to find shortest nonspontaneous diagonal (electrolytic cell), not the longest spontaneous diagonal (galvanic cell).

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You have:• Na+

• Cl-

• H2O

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Electrolysis: H2O

What are the two half reactions for the electrolysis of acidifiedwater to produce hydrogen gas and oxygen gas?

Find shortest nonspontaneous diagonal.

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You have:• H+

• H2O

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Electrolysis: H2O

Anode 2H2O O2 + 4H+ +4e-

Cathode 4H+ + 4e- 2H2Overall 2H2O 2H2 + O2

Eo = -1.23 V

H2SO4 is added as electrolyte & source of H+.

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Hall Process

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Electrolysis of KI(aq)

Lab

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Quantitative Electrolysis stoichiometry with electrons!

andcurrent = charge/timeampere = coulomb/sec.

1A = 1 C/s

1 F = charge of 1 mol e-

= 96,500 C/mol e-

Electrolysis demohttp://media.pearsoncmg.com/bc/bc_0media_chem/chem_sim/electrolysis_fc1_gm_11-26-12/main.html

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Quantitative Electrolysis

In an electrolytic cell, Ca+2 is converted to Ca at the cathode. How much Ca is produced by 0.452 A (current) in 1.50 hours?

Ca+2 + 2e- Ca

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Quantitative Electrolysis0.452 A = 0.452 C/s

0.452 C1 s

x1.5 hr x 3600shr

x 1 mol e-

96,500 Cx

1 mol Ca2 mol e-

x 40.08 g Ca1 mol Ca

= 0.507 g Ca

Ca+2 + 2e- Ca

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Time to Rock ‘n Roll

In the electrolytic reduction of acidified water, what volume of hydrogen gas at STP is produced using a current of 1.26 A for 7.44 hours?

2H+ + 2e- H2