1. Aim: How to write half reactions
Do Now: 1. What is the oxidation number of carbon in NaHCO3? a. +6 b. +2 c. -4 d. +4
2. Given the reaction: 2Li(s) + Cl2(g) -> 2LiCl(s) As the reaction takes place, the Cl2(g) will a. gain electrons b. lose electrons c. gain protons d. lose protons.

2. Identifying if a reaction is a redox reaction
Are electrons transferred? To answer this question you'll have to write down the chemical equation for the process and assign oxidation numbers to the atoms in all of the reactants and products. If the oxidation numbers for any of the atoms in the reactants are different from the oxidation number for the same atoms on the product side, electron transfer must have occurred, and you have a redox reaction

3. HCl(aq) + NaOH(aq)  NaCl(aq) + H2O(l)
Redox
Not Redox
HCl(aq) + NaOH(aq)  NaCl(aq) + H2O(l)

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

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

6. 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**

7. 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)

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

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

10. 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

11. 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

12. Practice
Which half-reaction correctly represents reduction? a. Cr3+ + 3e- -> Cr(s)
b. Cr3++ -> Cr(s) + 3e-
c. Cr(s) -> Cr3+ + 3e-
d. Cr(s) + 3e- -> Cr3+

13. Given the balanced equation: 2Al(s) + 6H+(aq) -> 2Al3+(aq) + 3H2 When 2 moles of Al(s) completely reacts, what is the total number of moles of electrons transferred from Al(s) to H+(aq)? a. 5 b. 6 c. 3 d. 4

14. Given the reaction:     __Mg + __Cr3+ -> __Mg2+ + __Cr When the equation is correctly balanced using smallest whole numbers, the sum of the coefficients will be a. 10
b. 7
c. 5
d. 4

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