HW Chp20 - 20.1 In a Brnsted-Lowry acid-base reaction H is...

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20.1 In a Brønsted-Lowry acid-base reaction, H + is transferred from the acid to the base. In a redox reaction, the substance being oxidized (the reductant) loses electrons and the substance being reduced (the oxidant) gains electrons. Furthermore, the number of electrons gained and lost must be equal. The concept of electron transfer from reductant to oxidant is clearly applicable to redox reactions. (The path of the transfer may or may not be direct, but ultimately electrons are transferred during redox reactions.) 20.2 (a) If a Zn(s) strip was placed in a CdSO 4 (aq) solution, Cd(s) would form on the strip. Although º red E for Zn 2 + (aq), –0.763 V, and Cd 2 + (aq), –0.403 V, are both negative, the value for Cd 2 + is larger (less negative), so it will be the reduced species in the redox reaction. (b) If a Cu(s) strip was placed in a AgNO 3 (aq) solution, Ag(s) would form on the strip. Although º red E for Cu 2 + (aq), 0.337 V, and Ag + (aq), 0.799 V, are both positive, the species with the larger º red E value, Ag + , will be reduced in the reaction. 20.3 Ni 2 + (aq) + 2e Ni(s), V, 28 . 0 E º red = cathode Fe 2 + (aq) + 2e Fe(s), V, 44 . 0 E º red = anode Ni 2 + (aq) has the larger , E º red so it will be reduced in the redox reaction. Reduction occurs at the cathode, so Ni 2 + (aq) and Ni(s) will be in the cathode compartment and Fe 2 + (aq) and Fe(s) will be in the anode compartment. The voltmeter will read: V 16 . 0 V) 044 ( V 28 . 0 anode) ( E cathode) ( E E º red º red º cell = = = 20.4 The species with the largest º red E is easiest to reduce, while the species with the smallest, most negative º red E is easiest to oxidize. (a) The species easiest to oxidize is at the bottom of Figure 20.14. (b) The species easiest to reduce is at the top of Figure 20.14. 20.5 A(aq) + B(aq) A (aq) + B + (aq) (a) A gains electrons; it is being reduced. B loses electrons; it is being oxidized. (b) Reduction occurs at the cathode; oxidation occurs at the anode. A(aq) + 1e A (aq) occurs at the cathode. B(aq) B + (aq) + 1e occurs at the anode. (c) In a voltaic cell, the anode is at higher potential energy than the cathode. The anode reaction, B(aq) B + (aq) + 1e , is higher in potential energy.
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20.6 The main difference between a fuel cell and a battery is that a fuel cell is not self-contained. That is, there is a continuous supply of fuel (reductant) and oxidant to the cell, and continuous exhaust of products. The fuel cell produces electrical current as long as reactants are supplied. It never goes ”dead.” 20.7 Zinc, V, 763 . 0 E º red = is more easily oxidized than iron, V. 440 . 0 E º red = If conditions are favorable for oxidation, zinc will be preferentially oxidized, preventing iron from corroding. The protection lasts until all the Zn coating has reacted. 20.8 Unintended oxidation reactions in the body lead to unwanted health effects, just as unwanted oxidation of metals leads to corrosion. Antioxidants probably have modes of action similar to anti-corrosion agents. They can preferentially react with oxidizing agents (cathodic protection), create conditions that are unfavorable to the
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