Oxidation-Reduction (Redox) Reactions

Oxidation-Reduction Reactions Electron 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  2MgO Where does reduction happen? Which does oxidation happen? Oxidation between Magnesium Reduction between oxygen Assign Oxidation Numbers

Example 3: 2 Na + Cl2  2NaCl Assign oxidation numbers Identify 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. Mg + 2HCl  MgCl2 + H2 I2 + 3Cl2  2 ICl3 NaOH + HCl  NaCl + H2O 2Na + 2H2O  2NaOH + H2 Magnesium oxidized, hydrogen reduced Iodine oxidized, chlorine reduced Not a redox reaction Sodium oxidized, hydrogen reduced

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 equation Reduction 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  2NaCl Assign 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 ! Mg + 2HCl  MgCl2 + H2 I2 + 3Cl2  2 ICl3 NaOH + HCl  NaCl + H2O 2Na + 2H2O  2NaOH + H2