Oxidation-Reduction (Redox) Reactions

Oxidation-Reduction ReactionsElectron transfer between ionic compounds, change in oxidation

numbers

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

Reduction— GAIN of electrons in element/compound Compound becomes more NEGATIVE DECREASE in oxidation number

Oxidation LOSS of electrons in element/compound Compound becomes more POSITIVE INCREASE in oxidation number

Oxidation CANNOT happen without Reduction.

Reduction CANNOT happen without Oxidation

Both have to happen in a redox reaction ! ! !

Example 1:Mg (s) + Cu+2 (aq) Mg+2

(aq) + Cu (s)

Ex. 2: 2Mg + O2 2MgOWhere does reduction happen?Which does oxidation happen?

Example 3:

2 Na + Cl2 2NaCl

Assign oxidation numbersIdentify where oxidation occurs, where reduction occurs.

Determine whether each of these reactions is a oxidation-reduction reaction. If so, identify what is oxidized and what is reduced.

1) Mg + 2HCl MgCl2 + H2

2) I2 + 3Cl2 2 ICl3

3) NaOH + HCl NaCl + H2O

4) 2Na + 2H2O 2NaOH + H2

Half-Reaction Describes the oxidation or reduction process going on in a

redox reaction

2 half-reactions make ONE redox reaction

Contains “half” of a redox reaction

Half-Reactions

A method of separating the OXIDATION part of a chemical reaction from the REDUCTION part.

Oxidation process---one equationReduction process---one equation

Then we put it all together.

Half-Reaction Example Cu+2

(aq) + 2e-1 Cu (s) Reduction

Mg (s) Mg+2 (aq) + 2e-1 Oxidation

Total: Mg(s) + Cu+2(aq) Mg+2

(aq) + Cu(s)

Example 4: Oxidation: Cu (s) Cu+2

(aq) + 2e -1 Reduction: Ag+1

(aq) + 1e -1 Ag (s)

*When electrons do not balance, multiply one or both half-reactions when needed to balance the amount of electrons gained and lost in the reaction. **

How do we write half-reactions? Original Equation: 2 Na + Cl2 2NaCl

Oxidation Half-Reaction: 2Na 2Na+ + 2e-

Show electrons “lost” through process, 2 because you have 2 Na

Reduction Half-Reaction:Cl2 + 2e- 2Cl- Show electrons “gained” through process, 2 because you

have 2 Cl**Electrons MUST equal in number for BOTH half-reactions!!!

Putting Redox Equation back together……Original Equation:

2 Na + Cl2 2NaClAssign Oxidation Numbers Identify where oxidation/reduction happening

Half-Reactions:Oxidation: 2Na 2Na+ + 2e-

Reduction: Cl2 + 2e- 2Cl- Cancel elements and electrons existing on both sides, Add half-reactions together.

RESULT ! !2 Na + Cl2 2NaCl

Example 5: 2Mg + O2 2MgO

Example 6: Cu(s) + NO3 Cu+2 + NO

Example 7: Zn + 2HCl ZnCl2 + H2

Practice!

Write half-reactions and balance if the reaction is redox !

1) Mg + 2HCl MgCl2 + H2

2) I2 + 3Cl2 2 ICl33) NaOH + HCl NaCl + H2O4) 2Na + 2H2O 2NaOH + H2