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Chapter 19 Electrochemistry 423 Additional electrolytes promote more corrosion. The presence of an electrolyte (such as sodium chloride) on the surface of iron promotes rusting because it enhances current flow. This is why cars rust so quickly in cold climates where roads are salted and in areas directly adjacent to beaches where salt water mist is present. The presence of acids promotes corrosion. Because H⁺ ions are involved as a reactant in the reduction of oxygen, lower pH enhances the cathodic reaction and leads to faster corrosion. Problems by Topic Balancing Redox Reactions 19.37 Conceptual Plan: Separate the overall reaction into two half-reactions: one for oxidation and one for reduction. Balance each half-reaction with respect to mass in the following order: (1) Balance all elements other than H and (2) balance 0 by adding and (3) balance H by adding Balance each half-reaction with respect to charge by adding electrons. (The sum of the charges on both sides of the equation should be made equal by adding electrons as necessary.) Make the number of electrons in both half-reactions equal by multiplying one or both half-reactions by a small whole number. Add the two half-reactions, canceling electrons and other species as necessary. Verify that the reaction is balanced with respect to both mass and charge. Solution: (a) Separate: K(s) K⁺(aq) and Cr³⁺(aq) Cr(s) Balance elements: K(s) and Cr³⁺(aq) Cr(s) Add electrons: K(s) + e⁻ and + Cr(s) Equalize electrons: 3K(s) + and + Cr(s) Add half-reactions: 3K(s) + (aq) + + + Cr(s) Cancel electrons: 3K(s) + Cr³⁺ 3 + Cr(s) Check: Reactants Products 3 K atoms 3 K atoms 1 Cr atom 1 Cr atom +3 charge +3 charge (b) Separate: Al(s) and Fe(s) Balance elements: Al(s) and Fe(s) Add electrons: Al(s) + 3e⁻ and + Fe(s) Equalize electrons: 2 + and (aq) + Add half-reactions: 2 Al(s) + 3 Fe²⁺ (aq) + 2 (aq) + + Cancel electrons: 2 Al(s) + 3 Fe²⁺ (aq) 2 + Check: Reactants Products 2 Al atoms 2 Al atoms 3 Fe atoms 3 Fe atoms +6 charge +6 charge (c) Separate: Br⁻(aq) and N₂H₄(₈) Balance non H & O elements: Br⁻(aq) and N₂(g) Balance 0 with H₂O: Br⁻(aq) + and Balance H with Br⁻(aq) and N₂H₄(g) Add electrons: Br⁻(aq) + and Equalize electrons: 2 + 12 + 2 + and Add half-reactions: + + + + 6 H₂O + Cancel electrons & others: (aq) + + + Copyright © 2017 Pearson Education, Inc.