Redox Reaction Redox Reaction • one or more elements change oxidation number • • • • all single displacement, and combustion, some synthesis and decomposition always have both oxidation and reduction split reaction into oxidation half-reaction and a reduction half-reaction aka electron transfer reactions half-reactions include electrons oxidizing agent is reactant molecule that causes oxidation contains element reduced reducing agent is reactant molecule that causes reduction contains the element oxidized Oxidation & Reduction • oxidation is the process that occurs when oxidation number of an element increases element loses electrons compound adds oxygen compound loses hydrogen half-reaction has electrons as products • reduction is the process that occurs when oxidation number of an element decreases element gains electrons compound loses oxygen compound gains hydrogen half-reactions have electrons as reactants Rules for Assigning Oxidation States • rules are in order of priority free elements have an oxidation state =  Na = and Cl2 = in Na(s) + Cl2(g) monatomic ions have an oxidation state equal to their charge  Na = +1 and Cl = -1 in NaCl (a) the sum of the oxidation states of all the atoms in a compound is  Na = +1 and Cl = -1 in NaCl, (+1) + (-1) = Rules for Assigning Oxidation States (b) the sum of the oxidation states of all the atoms in a polyatomic ion equals the charge on the ion  N = +5 and O = -2 in NO3–, (+5) + 3(-2) = -1 (a) Group I metals have an oxidation state of +1 in all their compounds  Na = +1 in NaCl (b) Group II metals have an oxidation state of +2 in all their compounds  Mg = +2 in MgCl2 Rules for Assigning Oxidation States in their compounds, nonmetals have oxidation states according to the table below  nonmetals higher on the table take priority Nonmetal Oxidation State Example F -1 CF4 H +1 CH4 O -2 CO2 Group 7A -1 CCl4 Group 6A -2 CS2 Group 5A -3 NH3 Oxidation and Reduction • oxidation occurs when an atom’s oxidation state increases during a reaction • reduction occurs when an atom’s oxidation state decreases during a reaction CH4 + O2 → CO2 + H2O -4 +1 +4 –2 +1 -2 oxidation reduction Oxidation–Reduction • oxidation and reduction must occur simultaneously  if an atom loses electrons another atom must take them • the reactant that reduces an element in another reactant is called the reducing agent  the reducing agent contains the element that is oxidized • the reactant that oxidizes an element in another reactant is called the oxidizing agent  the oxidizing agent contains the element that is reduced Na(s) + Cl2(g) → Na+Cl–(s) Na is oxidized, Cl is reduced Na is the reducing agent, Cl2 is the oxidizing agent Tro, Chemistry: A Molecular Approach Identify the Oxidizing and Reducing Agents in Each of the Following H2S + NO3– + H+ 3S + NO + H2O MnO2 + HBr MnBr2 + Br2 + H2O Identify the Oxidizing and Reducing Agents in Each of the Following red ag ox ag +1 -2 +5 -2 H2S + NO3– + H+ 3S + NO + H2O +1 +2 -2 +1 -2 oxidation reduction ox ag red ag +4 -2 +1 -1 MnO2 + HBr MnBr2 + Br2 + H2O +2 -1 +1 -2 oxidation reduction 10