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Oxidation Numbers Oxidation state: the net charge that an atom would have if the electrons in a covalent bond belonged entirely to the more electronegative atom Oxidation number: + or − value (integer or fraction) representing an atom’s oxidation state Common Oxidation Numbers: a) Any element = 0 b) H (in compounds) = +1 c) O (in compounds) = −2 d) Any monoatomic ion = its charge Assign oxidation numbers to each atom/ion shown below HCl(aq) + NaOH(aq) → H2O(l) + NaCl(aq) Zn(s) + CuCl2 (aq) → Cu(s) + ZnCl2 (aq) Two Exceptions to the Common Oxidation Numbers: In metal hydrides (ex. LiH), H = −1 In peroxides (ex. H2O2), O = −1 Rules for Assigning Other Oxidation Numbers: All of the oxidation numbers in a molecule/ion must add up to equal the charge on the molecule/ion In compounds with two unknown oxidation numbers, the more electronegative atom is given an oxidation number equal to the negative charge it usually has as a simple anion Assign an oxidation number to each atom/ion: (a) HClO4 (b) Cr2O72− (c) SiBr4 Identifying Redox Reactions With Oxidation Numbers A chemical’s oxidation number changes when it gains or loses electrons oxidation: an increase in the oxidation number Fe(s) → Fe2+(aq) + 2 e 2Cl(aq) → Cl2(g) + 2 e (oxidation # becoming more positive or less negative) reduction: a decrease in the oxidation number Cl2(g) + 2e− → 2Cl−(aq) Sn4+(aq) + 2e− → Sn2+(aq) (oxidation # becoming less positive or more negative) Change in the oxidation number oxidation -3 -2 -1 0 +1 reduction +2 +3 In a redox reaction, the OA is always reduced. the OA is the reactant whose oxidation number has decreased In a redox reaction, the RA is always oxidized. the RA is the reactant whose oxidation number has increased A chemical reaction is not a redox reaction when there is no change in any of the oxidation numbers Is the combustion of methane a redox reaction? If so, identify the OA and RA. Assign oxidation numbers and identify the OA and RA. Cr2O72−(aq) + H+ (aq) + C2H5OH(aq) → Cr3+(aq) + H2O(l) + CH3COOH(aq) In disproportionation reactions, only one type of atom is changing its oxidation number The same reactant acts as both the OA and RA HS2O3−(aq) + H+(aq) → S8(s) + H2O(l) + HSO4−(aq)