Chapter10 - Solutions for Chapter 10 End-of-Chapter...

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April 2005 ACS Chemistry Chapter 10 suggested solutions 1 Solutions for Chapter 10 End-of-Chapter Problems Problem 10.1. (a) We have seen (Investigate This 10.2) that electrolysis of a dilute aqueous solution of an ionic compound (magnesium sulfate) produces a gas at both electrodes and a basic solution at the cathode and acidic solution at the anode, just as the problem statement says is observed here for a dilute aqueous NaCl solution. Thus, we might assume that the electrolysis half reactions are the same in this case as in the investigation: cathode (reduction): 4H 2 O (l) + 4 e (from electrode) 2H 2 (g) + 4OH (aq) anode (oxidation): 2H 2 O (l) O 2 (g) + 4H + (aq) + 4 e (to electrode) The gas produced at the anode is oxygen, which does not burn, so this is also consistent with the observations reported in the problem statement. (b) In the concentrated aqueous NaCl solution, the cathode reaction produces a basic solution, so the half reaction is still probably the same as in the electrolysis of the dilute solution. At the anode, however, the gas produced causes a choking sensation when inhaled. The only elements present in the solution are H, O, Na, and Cl. The anode reaction is an oxidation and the most likely species to be oxidized (aside from water) is the chloride ion, Cl (aq) , in the solution. The product of the oxidation is chlorine gas, Cl 2 (g) , which produces a choking sensation when inhaled. Solutions of chlorine gas also act as bleach, so the bleaching of the indicator dye color at the anode (reported in the problem statement) is consistent with this identification of the product. The reactions are: cathode (reduction): 2H 2 O (l) + 2 e (from electrode) H 2 (g) + 2OH (aq) anode (oxidation): 2Cl (aq) Cl 2 (g) + 2 e (to electrode) The ease of oxidation of water and chloride ion are similar with chloride being a bit easier to oxidize. When there is only a small amount of chloride in the solution, the large number of water molecules relative to chloride ions overwhelms the chloride and water is oxidized. When the concentration of chloride ion is large, it can compete successfully with water and is oxidized. At both concentrations, the gas at the anode is probably a mixture that contains both oxygen and chlorine, but in quite different proportions in the two cases. Problem 10.2. (a) When an aqueous solution of KI is electrolyzed, bubbles of gas are formed at the cathode and the solution around the anode becomes orange. The orange color at the anode appears to be I 3 (aq) (which the problem statement tells us is formed in a solution containing iodine, I 2 (aq) , and iodide, I (aq) ), so iodine, I 2 (aq) , must have been produced at this electrode and reacted with I (aq) . Thus, the anodic reaction is probably the oxidation of I (aq) to give I 2 (aq) : 2I (aq) I 2 (aq) + 2 e The cathodic reaction must be the reduction of water to yield hydrogen gas and hydroxide in solution: 2H 2 O (l) + 2 e H 2 (g) + 2OH (aq) This is the reaction that occurs at the cathode in aqueous solution whenever there is no species
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