Electrochemistry Putting redox reactions to work Redox review

Electrochemistry Putting redox reactions to work

Redox review Electrons are transferred Lose Electrons Oxidation Gain Electrons Reduction

Electrochemical Cells Made of two half-cells Based upon two half-reactions Electrons travel between the two half-cells

Galvanic Cells Also called voltaic cells Convert chemical energy into electrical energy Spontaneous

Electrolytic Cells Convert electrical energy into chemical energy Non-spontaneous

Making a Galvanic Cell Write the reaction for solid magnesium placed in a copper (II) sulfate solution. Mg (s) + Cu. SO 4 (aq) Mg. SO 4 (aq) + Cu (s)

Making a Galvanic Cell 0 Balance the reaction using the half-reaction method +2+6 -2 0 Mg (s) + Cu. SO 4 (aq) Mg. SO 4 (aq) + Cu (s)

Making a Galvanic Cell 0 Balance the reaction using the half-reaction method +2+6 -2 0 Mg (s) + Cu. SO 4 (aq) Mg. SO 4 (aq) + Cu (s) Mg (s) Mg 2+ (aq) + 2 e. Cu 2+ (aq) + 2 e- Cu (s)

Making a Galvanic Cell 0 Balance the reaction using the half-reaction method +2+6 -2 0 Mg (s) + Cu. SO 4 (aq) Mg. SO 4 (aq) + Cu (s) 1(Mg (s) Mg 2+ (aq) + 2 e-) 1(Cu 2+ (aq) + 2 e- Cu (s)) Mg (s)+Cu. SO 4 (aq)+2 e- Mg. SO 4 (aq)+Cu (s)+2 e-

Making a Galvanic Cell

Making a Galvanic Cell Anode Cathode Salt Bridge Flow of electrons

Measuring the Output of a Galvanic Cell 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

Measuring the Output of a Galvanic Cell 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

Measuring the Output of a Galvanic Cell 1(Mg (s) Mg 2+ (aq) + 2 e-) E˚ox= +2. 37 V 1(Cu 2+ (aq) + 2 e- Cu (s)) E˚red= +0. 342 V Mg (s)+Cu. SO 4 (aq)+2 e- Mg. SO 4 (aq)+Cu (s)+2 e. E˚cell = +2. 71 V

Writing a Line Notation Oxidation||Reduction X(s)|X+(aq)||Y+(aq)|Y(s) Mg(s)|Mg 2+(aq)||Cu 2+(aq)|Cu(s) 1(Mg (s) Mg 2+ (aq) + 2 e-) 1(Cu 2+ (aq) + 2 e- Cu (s)) Mg (s)+Cu. SO 4 (aq)+2 e- Mg. SO 4 (aq)+Cu (s)+2 e-

Writing a Line Notation Cu(s)|Cu 2+(aq)||Ag 1+(aq)|Ag(s) 1(Cu (s) Cu 2+ (aq) + 2 e-) E˚ox= -0. 342 V 2(Ag 1+ (aq) + 1 e- Ag (s)) E˚red= +0. 800 V Cu(s)+2 Ag 1+(aq)+2 e- Cu 2+(aq)+2 Ag (s)+2 e. E˚cell= +0. 458 V

Batteries Series of electrochemical cells connected to each other Completes the circuit Dry cell ◦ Flashlight battery ◦ Watch battery Wet Cell ◦ Car battery

Batteries Carbon-Zinc Battery ◦ ◦ Zinc casing…anode Carbon rod…cathode Mn. O 2 is actually reduced Alkaline battery…has KOH rather than NH 4 Cl

Batteries Carbon-Zinc Battery ◦ Zn(s) Zn 2+(aq) + 2 e- ◦ 2 NH 41+(aq) + 2 Mn. O 2(s) + 2 e- Mn 2 O 3(s) + 2 NH 3(g) + H 2 O(l)

Batteries Lead-Acid Storage Battery Pb(s) + Pb. O 2(s) + H 2 SO 4(aq) Pb. SO 4(s) + H 2 O(l) Spontaneous & nonspontaneous

Concentration Cells Not 1 M Require additional calculations Can manipulate potential to a particular V

Concentration Cells Nernst Equation ◦ Ecell = E˚cell – {(0. 0592/n)(log. Q)} n Q

Concentration Cells A 0. 500 M solution of copper (II) sulfate is reacted with magnesium metal. A 0. 750 M solution of magnesium sulfate is one of the products. What is the cell potential? ◦ ◦ ◦ Write two half reactions Write balanced equation Determine E˚cell Use Nernst to solve for Ecell Mg(s)|Mg 2+(aq)||Cu 2+(aq)|Cu(s)

Concentration Cells 1(Mg (s) Mg 2+ (aq) + 2 e-) E˚ox= +2. 37 V 1(Cu 2+ (aq) + 2 e- Cu (s)) E˚red= +0. 342 V Mg (s)+Cu. SO 4 (aq)+2 e- Mg. SO 4 (aq)+Cu (s)+2 e. E˚cell = +2. 71 V

Concentration Cells 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

Concentration Cells Cu(s)|Cu 2+(0. 0100 M)||Ag 1+(0. 0250 M)|Ag(s)