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76 Chapter 4 Chemical Quantities and Aqueous Reactions (c) Skeletal reaction: HBr(aq) + Na₂S(aq) NaBr(aq) + gas Balanced reaction: 2 HBr(aq) + Na₂S(aq) + H₂S(g) (d) Skeletal reaction: + Li₂CO₃(aq) H₂CO₃(aq) + + + decomposes gas Balanced reaction: 2 + Li₂CO₃(aq) + + Oxidation-Reduction and Combustion 4.93 (a) Ag. The oxidation state of Ag = 0. The oxidation state of an atom in a free element is 0. (b) The oxidation state of Ag+ = +1. The oxidation state of a monatomic ion is equal to its charge. (c) CaF₂. The oxidation state of Ca = +2, and the oxidation state of F = -1. The oxidation state of a group 2A metal always has an oxidation state of +2, and the oxidation of F is 1 because the sum of the oxidation states in a neutral formula unit = 0. (d) H₂S. The oxidation state of H = +1, and the oxidation state of = The oxidation state of H when listed first is +1, and the oxidation state of because S is in group 6A and the sum of the oxidation states in a neutral molecular unit = 0. (e) The oxidation state of C = +4, and the oxidation state of = -2. The oxidation state of 0 is normally -2, and the oxidation state of C is deduced from the formula because the sum of the oxidation states must equal the charge on the ion. (C OX state) OX state) = -2; (C state) + 3(-2) = -2, so C state = +4. (f) The oxidation state of Cr = +6, and the oxidation state of The oxidation state of is normally -2, and the oxidation state of Cr is deduced from the formula because the sum of the oxidation states must equal the charge on the ion. (Cr OX state) + state) = -2; (Cr state) + 4(-2) = -2, so Cr OX state = +6. 4.95 (a) CrO. The oxidation state of Cr = +2, and the oxidation state of = The oxidation state of O is nor- mally -2, and the oxidation state of Cr is deduced from the formula because the sum of the oxidation states must = 0. (Cr OX state) + state) = 0; (Cr OX state) + (-2) = 0, so Cr = +2. (b) The oxidation state of Cr = +6, and the oxidation state of = -2. The oxidation state of is normally -2, and the oxidation state of Cr is deduced from the formula because the sum of the oxidation states must = 0. (Cr OX state) + state) = 0; (Cr OX state) + (-2) = 0, so Cr = +6. (c) Cr₂O₃. The oxidation state of Cr = +3, and the oxidation state of = The oxidation state of 0 is nor- mally 2, and the oxidation state of Cr is deduced from the formula because the sum of the oxidation states must = 0. (Cr OX state) + state) = 0; 2(Cr OX state) + 3(-2) = 0, so Cr = +3. 4.97 (a) 4Li(s) + 2Li₂O(s) Oxidation states; 0 0 This is a redox reaction because Li increases in oxidation number (oxidation) and decreases in number (reduction). O₂ is the oxidizing agent, and Li is the reducing agent. (b) Mg(s) + Mg²⁺ (aq) + Fe(s) Oxidation states; 0 +2 +2 0 This is a redox reaction because Mg increases in oxidation number (oxidation) and Fe decreases in number (reduction). Fe²⁺ is the oxidizing agent, and Mg is the reducing agent. (c) Pb(NO₃)₂(aq) + PbSO₄(s) + Oxidation states; +2+5-2 +1+6-2 +2+6-2 +1+5-2 This is a not a redox reaction because none of the atoms undergoes a change in oxidation number. (d) HBr(aq) + KOH(aq) + KBr(aq) Oxidation states; +1-1 +1-2+1 +1-2 +1-1 This is a not a redox reaction because none of the atoms undergoes a change in oxidation number. 4.99 (a) Skeletal reaction: S(s) + SO₂(g) Balanced reaction: S(s) + Copyright © 2017 Pearson Education, Inc.