Oxidation numbers answers 1. Determine the oxidation number of the specified element in each of the following. a. N in NF3 NF3 does not contain H or O, ox. # of F (highest EN) = -1 Because NF3 is a compound, ox.# = 0. Let ox.# of N be x: x + 3(-1) 0 x - 3 0, x 3 The ox.# of nitrogen is +3. b. S in S8 Because S8 is the formula of a pure element, the ox.# of S in S8 is 0. c. Cr in CrO42O has its usual ox.# of -2. ox.# = -2. Let ox.# of Cr be x. x + 4(-2) -2 x - 8 2, x 6 d. The ox.# of chromium is +6. P in P2O5 O has its usual ox.# of -2. ox.# = 0. Let ox.# of P be x. 2x + 5(-2) 0 2x - 10 0, 2x 10, x 5 e. The ox.# of phosphorus is +5. C in C12H22O11 ox.# of H is +1. ox.# of O is -2. ox.# = 0. Let ox.# of C be x. 12x + 22(+1) + 11(-2) 0 12x + 22 - 22 0, 12x 0, f. x0 The ox.# of carbon is 0. C in CHCl3 ox.# of H is +1. ox. # of Cl (highest EN) = -1 x. ox.# = 0 Let ox.# of C be x + 1(+1) + 3(-1) 0 x + 1 - 3 0, x - 2 0, x 2 2. The ox.# of carbon is +2. Determine the oxidation number of each element in each of the following. a. H2SO3 ox.# of H is +1. ox.# of O is -2. ox.# = 0. Let ox.# of S be x. 2(+1) + x + 3(-2) 0 2 + x - 6 0, x - 4 0, x 4 b. The ox.# of sulfur is +4 OHox.# of H is +1. ox.# of O is -2. ox.# = -1. To check if our assignment is right: 1(-2) + 1(+1) -1, c. -2 + 1 -1 Yeah! HPO42ox.# of H is +1. ox.# of O is -2. ox.# = -2. Let ox.# of P be x. 1(+1) + x + 4(-2) -2 1 + x - 8 -2, x - 7 -2, x5 The ox.# of phosphorus is +5 3. As stated in rule 4, oxygen does not always have its usual oxidation number of -2. Determine the oxidation number of oxygen in each of the following. a. the compound oxygen difluoride, OF2 Oxygen has an electronegativity of 3.44. Fluorine has an electronegativity of 3.98. Therefore, ox.# of F is -1. ox.# = 0. Let ox.# of O be x. x + 2(-1) 0, x - 2 0, x 2 b. The ox.# of oxygen is +2. the peroxide ion, O22ox.# = -2. Let ox.# of O be x. 2x -2, x -1, 4. The ox.# of oxygen is -1. Determine the oxidation number of each element in each of the following ionic compounds by considering the ions separately. a. Al(HCO3)3 The compound Al(HCO3)3 contains Al3+ ions and HCO3- ions. ox.# of Al is +3, ox.# of H is +1, ox.# of O is -2. ox.# of HCO3- = -1. Let ox.# of C be x. 1(+1) + x + 3(-2) -1 1 + x - 6 -1, x - 5 -1, x 4. b. The ox.# of carbon is +4 (NH4)3PO4 The compound(NH4)3PO4 contains NH4+ ions and PO43- ions. ox.# of H is +1, ox.# of NH4+ = +1. Let ox.# of N be x. x + 4(+1) +1 x + 4 +1, x +1 - 4, x -3 ox.# of O is -2, ox.# of The ox.# of nitrogen is -3. PO43- = -3. Let ox.# of P be y. y + 4(-2) -3 y - 8 -3, y -3 + 8, y 5 c. The ox.# of phosphorus is +5 K2H3IO6 The compound K2H3IO6 contains K+ ions and H3IO62- ions. ox.# of K is +1. H3IO62- : ox.# of H is +1, O is -2. Let ox.# of I be x. ox.# of H3IO62- = -2 3(+1) + x + 6(-2) -2 3 + x - 12 -2, x - 9 -2, x 7 The ox.# of iodine is +7. 5.Use oxidation numbers to determine whether each reaction is a redox reaction. a. H2O2 + 2Fe(OH)2 2Fe(OH)3 +1 -1 +2 -2 +1 +3 -2 +1 The ox.# of O that originates in Fe(OH)2 is -2 on both sides. The ox.# of O that originates in H2O2 from -1 to -2. The ox.# of Fe from +2 to +3 the reaction is a redox reaction. b. PCl3 + 3 H2O H3PO3 + 3 HCl +3-1 +1 -2 +1+3-2 +1-1 No elements undergo changes in ox.#, so the reaction is not a redox reaction. 6. Identify the oxidizing agent and the reducing agent for the redox reaction(s) in the previous question. a. 7. The ox.# of O in H2O2 from -1 to -2, H2O2 is reduced; ox. agent. The ox.# of Fe in Fe(OH)2. from +2 to +3 Fe(OH)2 is oxidized; red. agent. For the following balanced net ionic equation, use oxidation numbers to identify the reactant that undergoes oxidation and the reactant that undergoes reduction. Br2 + 2 ClO2- 2 Br- + 2 ClO2 0 +3 -2 -1 +4 -2 The ox.# of O is the same on both sides. The ox.# of Cl in ClO2- from +3 to +4 ClO2- is oxidized. The ox.# of Br in Br2 from 0 to -1. Br2 is reduced. 8. Nickel and copper are two metals that have played a role in the economy of Newfoundland and Labrador. Nickel and copper ores usually contain the metals as sulfides, such as NiS and Cu2S. Do the extractions of these pure elemental metals from their ores involve redox reactions? Explain your reasoning. In NiS, Ni has an ox.# of +2. Metallic nickel, Ni, is an element with an ox.# of 0. In Cu2S, Cu has an ox.# of +1. Metallic copper, Cu, is an element with an ox.# of 0.Because each of these metals undergoes a in its ox.# during extraction, the extraction process must involve reduction. Oxidation and reduction always occur together, so the extraction processes must be redox reactions. We do not need to know the other reactants to know that this conclusion is true.

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