Objectives• Assign oxidation numbers to reactant and

product species.• Define oxidation and reduction.• Explain what an oxidation-reduction reaction

(redox reaction) is.

19.1 Oxidation & Reduction

• Also called REDOX reactions• Always involve a TRANSFER of

ELECTRONS

Redox Reactions:• 2 half-reactions occur at the same time

2Na(s) + Cl2(g) → 2NaCl(s)

LEO – loss of electrons is oxidationAlkali metals: Na → Na+ + 1e-

• electron(s) are product• Sodium is oxidized

GER – gain of electrons is reductionHalogens: Cl + 1e- → Cl-

• electron(s) are reactant• Chlorine is reduced

Remember: “LEO the Lion says GER!”

A

B

A

B

Oxidation: A loses 2 electrons

Reduction: B gains 2 electrons

Reducing agent

Oxidizing agent

A is oxidizedB is reduced

Reducing & Oxidizing Agents

Reducing agent – causes another to be reduced by donating electrons

• is oxidized (loses electrons)

Oxidizing agent – causes another to be oxidized by accepting electrons

• is reduced (gains electrons)

Oxidation of Zinc• Zn loses electrons.

Is zinc oxidized or reduced? Is it the oxididizing or reducing agent?

• Zinc is oxidized. It is a reducing agentHalf-reaction: Zn → Zn2+ + 2e-

• Copper gains electrons.Is copper oxidized or reduced? Is it the oxididizing or reducing agent?

• Copper is reduced. It is an oxidizing agent. Half-reaction: Cu2+ + 2e- → Cu

Oxidation States• Redox reactions can be identified and understood by assigning

oxidation numbers (aka oxidation states) to reactants and products

Oxidation numbers/states = • artificial “bookkeeping” devices used to keep track of overall

electron distribution• not physical characteristics of atoms• hypothetical charge that an atom would have if all bonds to atoms

of different elements were 100% ionic.

• Transition metals often have more than one oxidation stateEx. Oxidation States of Mn

• In Potassium Permanganate, Mn is in oxidation state +7. This gives a purple salt.

• In alkaline conditions, and in the presence of a sugar, Mn is slowly reduced to its +6 and +4 oxidation state. (+6 being green, and +4 being yellow)

Rules for Assigning Oxidation Numbers

1. Uncombined elements = 02. Monoatomic ions = charge3. More electronegative element in binary

compound is assigned number equal to its charge if it were an ion

4. Fluorine in compound is always -15. Oxygen is -2 unless combined with F when its

+1 or +2, or in a peroxide when its -1

6. Hydrogen is usually +1 unless combined with a metal when it is -1

7. Group 1 & 2 and Aluminum have oxidation numbers of +1, +2, and +3 respectively

8. The sum of the oxidation numbers of all atoms in a neutral compound is zero.

9. The sum of the oxidation numbers of all atoms in a polyatomic ion is equal to the charge on the ion.

Sample Problem E p. 233

Assign oxidation numbers to each atom in the following compounds:UF6

Start with known: F is always -1Compounds must add up to zeroU is +6

Sample Problem E p. 233H2SO4

Oxygen & sulfur more electronegativeHydrogen assigned +1.

Oxygen not combined with fluorine or in peroxide.Oxygen assigned -2.

Sum must be zero.2(+1) + 4(-2) + 1(x) = 0

x = +6 Sulfur atom assigned +6

ClO-3

Oxygen is more electronegative and is assigned -2

Sum must be equal to charge of polyatomic ion = -1

3(-2) + 1(x) = -1x = +5Chlorine is assigned an oxidation number of +5

Homework: “Assigning Oxidation Numbers” worksheet