Oxidation-Reduction Reaction Review

Oxidation-Reduction (“Redox”) Reactions

• Most common reaction • Process often written as two “half-reactions”—separating the

oxidation and reduction

• Deals with electron transfer between ionic compounds

• One compound wants to GIVE electrons, other compound wants to TAKE electrons.

• Identified by a change in oxidation number

2 chemical processes happening at the same time ! 1) Reduction—

• GAIN of electrons in element• Compound becomes more NEGATIVE• DECREASE in oxidation number

2) Oxidation• LOSS of electrons • Compound becomes more POSITIVE • INCREASE in oxidation number

•Oxidation and Reduction MUST happen together—can’t have one without the other.

•electrons gained = electrons lost

Example 1:

•2Na + Cl2 2Na+ Cl-

Practice• Which equations are reduction half-reactions and which

are oxidation half-reactions.

1)Zn+2 + 2e- Zn

2)Ag Ag+ + e-

3)Cu+2 + 2e- Cu

4)Al Al+3 + 3e-

Oxidation Numbers/State

• Allows us to identify redox reactions

• A change in these numbers------ REDOX REACTION ! ! !

• Acts like all chemical compounds are ionic----gives atom a charge it would have IF it was ionic

Rule # 1 Examples

• Br2

• MgCl2• C6H12O6

• Cu• Ag• Fe

Oxidation Number Rules

2) Sum of oxidation numbers for an ion = ion’s charge

Examples:

•SO4-2

•NH4+

•Al ion •Cl ion

Oxidation Number Rules

3) Group 1A metals = +1

Group 2A metals = +2

Examples: MgSO4

K3PO4

Oxidation Number Rules

4) For the majority of chemical compounds—• F = -1• H = +1 (sometimes will have -1) • O = -2

Examples: NH3

H2O

CO

HF

KF

Oxidation Number Rules

5) Group 7A Elements = -1

Group 6A Elements = -2

Group 5A Elements = -3

**When combined with a metal in a binary compound !

Rule #5 Examples

• CaF2

• Li2S

• Mg3N

• K3P

Let’s Practice !

• Calculate the oxidation number of chromium in each

1)CrO4-2

2)Cr2O7-2

3)CrO2

More Practice! • State which reactions are oxidation-reduction reactions.

State which compound is being oxidized and which compound is being reduced.

1)Cu + 2Ag+ Cu+2 + 2Ag

2)CH3CO2H + OH- CH3CO2- + H2O

3)SF4 + F2 SF6

4)CuSO4 + BaCl2 BaSO4 + CuCl2

Homework

Read pp. 755-758

Oxidation-Reduction Review Sheet