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20 Electrochemistry Solutions to Exercises 2Fe(s) + O₂(g) + (aq) + E° = 1.23 V - (-0.440 V) = 1.67 V 2Cu(s) + O₂ + (aq) E° = 1.23 V- (0.337 V) = 0.893 V Note, however, that Fe has a more negative than Cu so when the two are in contact Fe acts as the sacrificial anode, and corrosion (of Fe) occurs preferentially. This is verified by the larger E° value for the corrosion of Fe, 1.67 V, relative to the corrosion of Cu, 0.893 V. When the three metals Zn, Fe, and Cu are in contact, oxidation of Zn will happen first, followed by oxidation of Fe, and finally Cu. 20.88 The principal metallic component of steel is Fe. for Fe, -0.763 V, is more negative than that of Cu, 0.337 V. When the two are in contact, Fe acts as the sacrificial anode and corrodes (oxidizes) preferentially in the presence of O₂(g). 2Fe(s) + O₂ + E° = 1.23 V - (-0.440 = 1.67 V 2Cu(s) + O₂(g) + E° = 1.23 V - (0.337 V) = 0.893 V Both reactions are spontaneous, but the corrosion of Fe has the larger E° value and happens preferentially. Electrolysis; Electrical Work (section 20.9) 20.89 (a) Electrolysis is an electrochemical process driven by an outside energy source. (b) Electrolysis reactions are, by definition, nonspontaneous. (c) (g) + (d) When an aqueous solution of NaCl undergoes electrolysis, is preferentially reduced to form H₂(g). 20.90 (a) An electrolytic cell is the vessel in which electrolysis occurs. It consists of a power source and two electrodes in a molten salt or aqueous solution. (b) It is the cathode. In an electrolysis cell, as in a voltaic cell, electrons are consumed (via reduction) at the cathode. Electrons flow from the negative terminal of the voltage source and then to the cathode. (c) A small amount of (aq) present during the electrolysis of water acts as a change carrier, or supporting electrolyte. This facilitates transfer of electrons through the solution and at the electrodes, speeding up the reaction. (Considering (aq) as the substance reduced at the cathode changes the details of the half-reactions, but not the overall E° for the electrolysis. cannot be oxidized.) (d) If the active metal salt is present as an aqueous solution during electrolysis, water is reduced [to (g)] rather than the metal ion being reduced to the metal. This is true for any active metal with an value more negative than -0.83 V. 636