Soutions_Chapter11 - CHAPTER 11 ELECTROCHEMISTRY Galvanic...

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440 CHAPTER 11 ELECTROCHEMISTRY Galvanic Cells, Cell Potentials, and Standard Reduction Potentials 15. Electrochemistry is the study of the interchange of chemical and electrical energy. A redox (oxidation-reduction) reaction is a reaction in which one or more electrons are transferred. In a galvanic cell, a spontaneous redox reaction occurs that produces an electric current. In an electrolytic cell, electricity is used to force a nonspontaneous redox reaction to occur. 16. Magnesium is an alkaline earth metal; Mg will oxidize to Mg 2+ . The oxidation state of hydrogen in HCl is +1. To be reduced, the oxidation state of H must decrease. The obvious choice for the hydrogen product is H 2 (g), where hydrogen has a zero oxidation state. The balanced reaction is Mg(s) + 2 HCl(aq) MgCl 2 (aq) + H 2 (g). Mg goes from the 0 to the +2 oxidation state by losing two electrons. Each H atom goes from the +1 to the 0 oxidation state by gaining one electron. Since there are two H atoms in the balanced equation, a total of two electrons are gained by the H atoms. Hence two electrons are transferred in the balanced reaction. When the electrons are transferred directly from Mg to H + , no work is obtained. In order to harness this reaction to do useful work, we must control the flow of electrons through a wire. This is accomplished by making a galvanic cell that separates the reduction reaction from the oxidation reaction in order to control the flow of electrons through a wire to produce a voltage. 17. A galvanic cell at standard conditions must have a positive overall standard cell potential ). 0 E ( o cell The only combination of the half-reactions that gives a positive cell potential is: Cu 2+ + 2e Cu E°(cathode) = 0.34 V Zn Zn 2+ + 2e E°(anode) = 0.76 V Cu 2+ (aq) + Zn(s) Cu(s) + Zn 2+ (aq) o cell E = 1.10 V a. The reducing agent causes reduction to occur since it always contains the species which is oxidized. Zn is oxidized in the galvanic cell, so Zn is the reducing agent. The oxidizing agent causes oxidation to occur since it always contains the species which is reduced. Cu 2+ is reduced in the galvanic cell, so Cu 2+ is the oxidizing agent. Electrons will flow from the zinc compartment (the anode) to the copper compartment (the cathode). b. From the work above, o cell E = 1.10 V. c. The pure metal that is a product in the spontaneous reaction is copper. So the copper electrode will increase in mass as Cu 2+ (aq) is reduced to Cu(s). The zinc electrode will decrease in mass for this galvanic cell as Zn(s) is oxidized to Zn 2+ (aq).