Putting redox reactions to work. Electrons are transferred Lose Electrons Oxidation Gain Electrons Reduction.

Putting redox reactions to work

Electrons are transferred

Lose Electrons Oxidation

Gain Electrons Reduction

Made of two half-cells

Based upon two half-reactions

Electrons travel between the two half-cells

Also called voltaic cells

Convert chemical energy into electrical energy

Spontaneous

Convert electrical energy into chemical energy

Non-spontaneous

Write the reaction for solid magnesium placed in a copper (II) sulfate solution.

Mg (s) + CuSO4 (aq) MgSO4 (aq) + Cu (s)

Balance the reaction using the half-reaction method

Mg (s) + CuSO4 (aq) MgSO4 (aq) + Cu (s) 0 +2+6 -2 +2+6 -2 0

Balance the reaction using the half-reaction method

Mg (s) + CuSO4 (aq) MgSO4 (aq) + Cu (s) 0 +2+6 -2 +2+6 -2 0

Mg (s) Mg2+ (aq) + 2e-

Cu2+ (aq) + 2e- Cu (s)

Balance the reaction using the half-reaction method

Mg (s) + CuSO4 (aq) MgSO4 (aq) + Cu (s) 0 +2+6 -2 +2+6 -2 0

1(Mg (s) Mg2+ (aq) + 2e-)

1(Cu2+ (aq) + 2e- Cu (s))

Mg (s)+CuSO4 (aq)+2e- MgSO4 (aq)+Cu (s)+2e-

Anode

Cathode

Salt Bridge

Flow of electrons

Potential (either half-cell or cell)◦ Pull on the electrons◦ Electromotive force (emf)◦ Volt (V)◦ Joule/Coulomb (J/C)

Voltmeter◦ Analog◦ Digital

Potentiometer Positive potential…spontaneous Negative potential…nonspontaneous

Standard Reduction Potentials Chart◦ Only reduction reactions◦ Must look up the reverse of the oxidation and flip

the sign of the potential

Add standard half-cell potentials to get standard cell potential

1(Mg (s) Mg2+ (aq) + 2e-) E˚ox= +2.37 V

1(Cu2+ (aq) + 2e- Cu (s)) E˚red= +0.342 V

Mg (s)+CuSO4 (aq)+2e- MgSO4 (aq)+Cu (s)+2e-

E˚cell = +2.71 V

Oxidation||Reduction

X(s)|X+(aq)||Y+(aq)|Y(s)

Mg(s)|Mg2+(aq)||Cu2+(aq)|Cu(s)

1(Mg (s) Mg2+ (aq) + 2e-)

1(Cu2+ (aq) + 2e- Cu (s))

Mg (s)+CuSO4 (aq)+2e- MgSO4 (aq)+Cu (s)+2e-

Cu(s)|Cu2+(aq)||Ag1+(aq)|Ag(s)

1(Cu (s) Cu2+ (aq) + 2e-) E˚ox= -0.342V

2(Ag1+ (aq) + 1e- Ag (s)) E˚red= +0.800V

Cu(s)+2Ag1+(aq)+2e- Cu2+(aq)+2Ag (s)+2e-

E˚cell= +0.458V

Series of electrochemical cells connected to each other

Completes the circuit

Dry cell◦ Flashlight battery◦ Watch battery

Wet Cell◦ Car battery

Carbon-Zinc Battery◦ Zinc casing…anode◦ Carbon rod…cathode◦ MnO2 is actually reduced

◦ Alkaline battery…has KOH rather than NH4Cl

Carbon-Zinc Battery◦ Zn(s) Zn2+(aq) + 2e-

◦ 2NH41+(aq) + 2MnO2(s) + 2e- Mn2O3(s) + 2NH3(g) + H2O(l)

Lead-Acid Storage Battery Pb(s) + PbO2(s) + H2SO4(aq) PbSO4(s) +

H2O(l) Spontaneous & nonspontaneous

Not 1M

Require additional calculations

Can manipulate potential to a particular V

Nernst Equation◦ Ecell = E˚cell – {(0.0592/n)(logQ)}

n Q

A 0.500M solution of copper (II) sulfate is reacted with magnesium metal. A 0.750M solution of magnesium sulfate is one of the products. What is the cell potential?◦ Write two half reactions◦ Write balanced equation◦ Determine n◦ Determine E˚cell

◦ Use Nernst to solve for Ecell

Mg(s)|Mg2+(aq)||Cu2+(aq)|Cu(s)

1(Mg (s) Mg2+ (aq) + 2e-) E˚ox= +2.37 V

1(Cu2+ (aq) + 2e- Cu (s)) E˚red= +0.342 V

Mg (s)+CuSO4 (aq)+2e- MgSO4 (aq)+Cu (s)+2e-

E˚cell = +2.71 V

Ecell = 2.71 V – {(0.0592/2)(log([0.75]/[0.5]))}

Ecell = 2.71 V – {(0.0296)(0.176)}

Ecell = 2.71 V – 0.00521

Ecell = 2.70 V

Cu(s)|Cu2+(0.0100M)||Ag1+(0.0250M)|Ag(s)