Chapter 17 Worksheet #2 Name ______
Electrochemical Cells - Redox reactions can be used in a controlled manner to make a battery. A galvanic cell (voltaic cell or battery) converts the chemical energy of the reactants into electrical energy.
BATTERY:
Anode -
AN OX, RED CAT Cathode -
Salt Bridge - A tube containing a salt (such as KCl or NaNO3) solution that is used to connect two half-cells in an electrochemical cell; allows the passage of ions (maintains charge neutrality), but prevents the mixing of half-cell electrolytes.
Shorthand notation for a galvanic cell:
Zn(s)│Zn2+(aq)║Cu2+(aq)│Cu(s) where the anode is on the left, the cathode on the right, │ indicates the interface between the metal and solution, and ║ indicates the salt bridge.
In many cells, the electrode itself does not react but serves only as a channel to direct electrons to or from the solution where a reaction involving other species takes place. The electrode itself is unaffected. Platinum and graphite are inert in most (but not all) electrochemical reactions.
The Cu electrode could be replaced by a platinum or graphite electrode in the Zn/Cu battery:
Zn(s)│Zn2+(aq)║Cu2+(aq)│Pt(s)
Construct a battery from the reaction: Cr(s) + Pb2+(aq) Cr3+(aq) + Pb(s)
Construct a galvanic cell using platinum electrodes and the reaction:
- - + 2+ 10 Br (aq) + 2 MnO4 (aq) + 16 H (aq) 5 Br2(ℓ) + 2 Mn (aq) + 8 H2O(ℓ)
A salt bridge is not required in a battery in which the reactants are physically separated from each other. e.g.: Pb-storage battery
Pb(s) + PbO2(s) + 2 H2SO4(aq) 2 PbSO4(s) + 2 H2O(ℓ)
The lead-acid storage battery is conveniently recharged by applying a voltage in the reverse direction greater than the cell potential.
2 PbSO4(s) + 2 H2O(ℓ) Pb(s) + PbO2(s) + 2 H2SO4(aq)
Different species have varying degrees of ability to be oxidized or reduced. For example, sodium is very easily oxidized while gold is oxidized with great difficulty. A tabulation of reduction half-reaction potentials is listed + o in your text on pages APP-28 to APP-30. These are measured with respect to the H /H2 electrode ( E = 0 ).
When two half-cells are combined to make a galvanic cell, reduction will occur in the half-cell with the greater reduction potential (making it the cathode). Oxidation will occur in the other half-cell (making it the anode).
The standard state cell potential (voltage or emf) is given by:
E = E(reduction) + E(oxidation) OR E = Ered(cathode) - Ered(anode)
- - + 2+ Determine E for: 10 Br (aq) + 2 MnO4 (aq) + 16 H (aq) 5 Br2(ℓ) + 2 Mn (aq) + 8 H2O(ℓ)
Construct a galvanic cell for the reaction: 2 Al(s) + 3 Fe2+(aq) 2 Al3+(aq) + 3 Fe(s)
(a) Determine the cell potential under standard state conditions. (b) Label the anode and the cathode. (c) Show the electron flow and the flow of ions from salt bridge. (d) Which electrode could be replaced by an inert electrode without affecting the battery's operation? (e) Write the shorthand notation for the cell.
Determine the standard state potentials for the following reactions:
(a) Cu(s) + 2 Ag+(aq) Cu2+(aq) + 2 Ag(s)
2- + 3+ 3+ (b) Cr2O7 (aq) + 14 H (aq) + 2 Al(s) 2 Cr (aq) + 2 Al (aq) + 7 H2O(ℓ)
(c) 3 Mg2+(aq) + 2 Au(s) 3 Mg(s) + 2 Au3+(aq)
Positive emf's (voltages) indicate a spontaneous process. A negative voltage indicates the reaction is spontaneous in the reverse direction.
Name a species which will oxidize Al to Al3+ but not Pb to Pb2+. ______
- - Name a species which will reduce Cl2 to Cl but not Br2 to Br . ______
Name a species which is a better oxidizing agent than Cu2+. ______
Name a species which is a better reducing agent than Zn. ______
Since spontaneity is related to ΔGo, it follows that Eo is related to ΔGo:
o o ΔG = -n E
The charge on one electron is 1.602x10-19 coulombs. The charge on one mole of electrons is 96,485 coulombs. This quantity is called a Faraday ().
coulombs joule = 96,485 = 96,485 mole volt mole
A volt is a unit of electrical potential. An electrical potential of one volt indicates one coulomb of charge has an increase in energy of 1 joule.
Determine ΔG (in kJ) for the reaction at 25oC:
2 Ag+(aq) + Fe(s) 2 Ag(s) + Fe2+(aq)
Determine the equilibrium constant for the reaction at 25oC:
Ca2+(aq) + 2 Na(s) Ca(s) + 2 Na+(aq)
Nernst Equation: 푅푇 0.0257 퐸 = 퐸° − ln(푄) = 퐸° − ln(푄) 푛퐹 푛 Determine the cell potential for the following battery:
Cr(s)│Cr3+(0.050 M)║Cu2+(1.50 M)│Cu(s)