Exp. 32 Galvanic Cells, the Nernst Equation p. 357

1. Exp. 32 � Galvanic Cells, the Nernst Equation p. 357 To measure the relative reduction potentials for a number of redox couples To develop an understanding of the movement of electrons, anions, and cations in a galvanic cell. To study factors affecting cell potentials. To estimate the concentration of ions in solution using the Nernst equation.

2. Exp. 32 � Introduction When iron corrodes, a change in the oxidation number of the iron atoms occurs. When gasoline burns, a change in the oxidation number of the carbon atoms occurs. A change in the oxidation number of an atom in a chemical reaction is the result of an exchange of electrons between the reactants.

3. Exp. 32 � Introduction A chemical reaction that involves the transfer of electrons from one substance to another is an oxidation-reduction (redox) reaction. If you place copper wire into a silver ion solution, copper atoms spontaneously loose electrons (are oxidized) to the silver ions which are reduced.

4. Exp. 32 � Introduction Silver ions migrate to the copper atoms to pick up electrons and form silver atoms at the copper metal/solution interface (picture, p. 357). The reaction that occurs at the interface is: Cu(s) + 2Ag+(aq) ? 2Ag(s) + Cu2+(aq)

5. Exp. 32 � Introduction Redox reactions can be divided into oxidation and reduction half-reactions. Each half-reaction, called a redox couple, consists of the reduced state and the oxidized state of the substance: Cu(s) + 2Ag+(aq) ? 2Ag(s) + Cu2+(aq)= Cu(s) ? Cu2+(aq) + 2e- oxidation � rxn 2 Ag+(aq) + 2e- ? 2 Ag(s) reduction � rxn

6. Exp. 32 � Introduction A galvanic cell uses this spontaneous transfer of electrons. Instead of electrons being transferred at the interface between copper metal and silver ions, a galvanic cell separates the copper metal from the silver ions, forcing the electrons to pass externally through a wire.

7. Exp. 32 � Introduction The two redox couples are placed in separate compartments called half-cells (fig. 32.1). Each half-cell consists of an electrode, usu. the metal of the redox couple and a solution containing the corresponding cation of the redox couple. The electrodes are connected by a wire and the solutions are connected by a salt bridge.

8. Exp. 32 � Introduction The electrode at which reduction occurs is called the cathode (+). The electrode at which oxidation occurs is called the anode (-) Electrons flow from the anode to the cathode.

9. Exp. 32 � Introduction Different metals have different tendencies to oxidize; similarly, their ions have different tendencies to undergo reduction. Cell potential (Ecell) � the difference in tendencies of the two metals to oxidize (lose electrons) or of their ions to reduce (gain electrons).

10. Exp. 32 � Introduction Reduction potential � a common measurement of the tendency for a substance to gain electrons or, the value used to identify the relative ease of reduction for a half-reaction. Potentiometer � a gauge that measures the Ecell

11. Exp. 32 � Introduction Ecell = EAg+,Ag � ECu2+,Cu The higher the reduction potential (more positive), the greater the tendency to undergo reduction. In our current example, this is Ag/Ag+. Therefore, since Ecell is positive, EAg+,Ag is placed before ECu2+,Cu.

12. Exp. 32 � Introduction Standard Cell Potential (Eocell) � is measured when Tsoln = 25oC and [ions] = 1 mol/L. The Nernst equation is applied to redox systems that are not at standard conditions: Ecell = Eocell � (0.0592/n)logQ Where Q = [Cu2+]/[Ag+]2 and n = # e- Cu(s) + 2Ag+(aq) ? 2Ag(s) + Cu2+(aq)

13. Exp. 32 � Procedural Notes Part A.1.� Set up as in Fig. 32.3. Use 50 mL beakers (obtain from stockroom). I have the potentiometers. Please try to keep the alligator clips out of the solutions. Part C. � Omit

14. Exp. 32 � Report Sheet Questions 1, 2, 4