1. Oxidation-Reduction Reactions
Section 4.4
Oxidation-Reduction Reactions

2. 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

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

4. Oxidation-Reduction Reactions
One cannot occur without the other.

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

7. 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

8. 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)

9. 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

10. Oxidation Numbers

11. 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?

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

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

14. 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)

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

16. 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

17. Activity Series

18. Homework
and 4.58