Chapter 21


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Unformatted text preview: 21-1CHAPTER 21ELECTROCHEMISTRY: CHEMICAL CHANGE AND ELECTRICAL WORK 21.1 Oxidation is the loss of electrons (resulting in a higher oxidation number), while reduction is the gain of electrons (resulting in a lower oxidation number). In an oxidation-reduction reaction, electrons transfer from the oxidized substance to the reduced substance. The oxidation number of the reactant being oxidized increases while the oxidation number of the reactant being reduced decreases. 21.2 An electrochemical process involves electron flow. At least one substance must lose electron(s) and one substance must gain electron(s) to produce the flow. This electron transfer is a redox process. 21.3 No, one half-reaction cannot take place independent of the other because there is always a transfer of electrons from one substance to another. If one substance loses electrons (oxidation half-reaction), another substance must gain those electrons (reduction half-reaction). 21.4 O2-is too strong a base to exist in H2O. The reaction O2-+ H2O →2OH-occurs. Only species actually existing in solution can be present when balancing an equation. 21.5 Multiply each half-reaction by the appropriate integer to make e-lost equal to e-gained. 21.6 To remove protons from an equation, add an equal number of hydroxide ions to both sides to neutralize the H+and produce water: H+(aq) + OH-(aq) →H2O(l). 21.7 No, add spectator ions to the balanced ionic equation to obtain the balanced molecular equation. 21.8 Spontaneous reactions, ∆Gsys< 0, take place in voltaic cells, which are also called galvanic cells. Nonspontaneous reactions take place in electrolytic cells and result in an increase in the free energy of the cell (∆Gsys> 0). 21.9 a) Trueb) Truec) Trued) False, in a voltaic cell, the system does work on the surroundings. e) Truef) False, the electrolyte in a cell provides a solution of mobile ions to maintain charge neutrality. 21.10 a) To decide which reactant is oxidized, look at oxidation numbers. Cl-is oxidized because its oxidation number increases from -1 to 0. b) MnO4-is reduced because the oxidation number of Mn decreases from +7 to +2. c) The oxidizing agent is the substance that causes the oxidation by accepting electrons. The oxidizing agent is the substance reduced in the reaction, so MnO4-is the oxidizing agent. d) Cl-is the reducing agent because it loses the electrons that are gained in the reduction. e) From Cl-, which is losing electrons, to MnO4-, which is gaining electrons. f) 8 H2SO4(aq) + 2 KMnO4(aq) + 10 KCl(aq) →2 MnSO4(aq) + 5 Cl2(g) + 8 H2O(l) + 6 K2SO4(aq) 21-221.11 2 CrO2-(aq) + 2 H2O(l) + 6 ClO-(aq) →2 CrO42-(aq) + 3 Cl2(g) + 4 OH-(aq) a) The CrO2-is the oxidized species because Cr increases in oxidation state from +3 to +6....
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