Oxidation-Reduction Reactions Section 4.4 Oxidation-Reduction Reactions

Oxidation-reduction or REDOX reactions Review of Reactions Precipitation: cations and anions form insoluble ionic compound Neutralization: H+ and OH- ions form H2O A third kind of reaction involves electron transfers between reactants Oxidation-reduction or REDOX reactions

Oxidation-Reduction Reactions An oxidation occurs when an atom or ion loses electrons. A reduction occurs when an atom or ion gains electrons.

Oxidation-Reduction Reactions One cannot occur without the other.

Oxidation Numbers Oxidation number: assigned to each element in a neutral compound or charged entity Oxidation # = oxidation Oxidation # = reduction

4 Rules for Oxidation Numbers 1. Elements in their elemental form have an oxidation number of 0. -Ex: H in H2 2. The oxidation number of a monatomic ion is the same as its charge. -Ex: K+ = +1; S2- = -2

3. Nonmetals tend to have negative oxidation numbers, except: Oxygen is −2, except in the peroxide ion (O2 2-) in which it is −1. Hydrogen is −1 when bonded to a metal, +1 when bonded to a nonmetal. Fluorine always is −1. The other halogens are −1, BUT positive when combined with oxygen (oxyanions)

4. The sum of the oxidation numbers in a neutral compound is 0 4. The sum of the oxidation numbers in a neutral compound is 0. The sum of the oxidation numbers in a polyatomic ion is the charge on the ion. -Ex: H3O+ = 3(+1) + (-2) = +1

Oxidation Numbers

Oxidation of Metals by Acids/Salts Many kinds of redox reactions Focus: redox reaction of metals with acid or salt A + BX  AX + B Ex: Zn(s) + 2HBr(aq)  ZnBr2 (aq)+ H2 (g) What type of reaction does this look like?

Displacement Reactions The ion in a solution is displaced through oxidation of an element.

Displacement with Metals and Acids Metal + acid  salt + H2 Mg(s) + 2HCl(aq)  MgCl2(aq) + H2(g) 0------------------------+2 +1------------------------------0

Displacement with Metals and Salts Metals can be oxidized by aqueous solutions of salts Molecular Equation Fe(s) + Ni(NO3)2(aq)  Fe(NO3)2 (aq) + Ni(s) Net Ionic Equation Fe(s) + Ni 2+ (aq)  Fe 2+ (aq) + Ni(s)

Remember… Whenever one substance is oxidized (loses electrons/becomes more positive), some other substance must be reduced (gains electrons/becomes less positive).

Activities Series List of metals arranged in order of decreasing ease of oxidation Alkali and alkaline earth metals: Active metals (most easily oxidized ) Transition metals at the bottom of list: Noble metals (low reactivity) Any metal on the list can be oxidized by the ions of elements below it

Activity Series

Homework 4.49-4.52 and 4.58