1. Aim: How to write half reactions

2. Half Reaction
Half reaction shows the exchange of electrons in a redox reaction
One half reaction shows oxidation; the other reduction

3. Example of a Reduction Half Reaction
Fe3+ + 3e-  Fe0 * Electrons on the left side, gained in the reaction

4. Example of an Oxidation Half Reaction
Fe0  Fe e-
*Electrons are the right hand side, loss of electrons in the reaction
** Always add electrons to the side of the reaction that has a higher total charge

5. Following the law of conservation
Half reactions follow:
Law of conservation of mass; same # of atoms on both sides of the reaction
Conservation of charge; net charge must be the same on both sides of the equation (# of electrons lost = # of electrons gained)

6. Rules for setting up half reactions
Assign oxidation numbers
Mg + ZnCl2  MgCl2 + Zn +2 -1 +2 -1

7. Rules for setting up half reactions
2. Identify oxidation and reduction
Mg + ZnCl2  MgCl2 + Zn +2 -1 +2 -1

8. Rules for setting up half reactions
3. Begin to set up half reaction. Use element symbol and assigned charge. *only for diatomic molecules you use the subscripts Mg + ZnCl2  MgCl2 + Zn +2 -1 +2 -1

9. Rules for setting up half reactions
4. Balance masses (change coefficients) and balance charge Mg + ZnCl2  MgCl2 + Zn Oxidation: Mg0  Mg2+ + 2e- Reduction: Zn2+ + 2e-  Zn +2 -1 +2 -1

10. Example 2 (balance masses) Hg + I2  HgI Example 3 (balance charges) Cu + AgNO3  Cu(NO3)2 + Ag