putting redox reactions to work. electrons are transferred lose electrons oxidation gain...

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)