1. Chemistry 141 Friday, October 6, 2017 Lecture 14
Solution Chemistry, Part 4:
Redox Reactions

2. Objectives for today
Begin to gain chemical intuition about reactions in solution
Assign oxidation states to elements in compounds
Identify the elements that are oxidized and reduced in redox reactions
Write half reactions and balance redox reactions
Perform stoichiometric calculations with solutions

3. Rules for determining oxidation states
The oxidation state of an atom in its elemental form is 0.
The oxidation state of a monatomic (free) ion is equal to its charge.
The sum of the oxidation numbers of the atoms in any uncharged compound is 0. The sum of the oxidation numbers of the atoms in a charged species (such as a polyatomic ion) is equal to the charge of the species.
Within compounds, the following rules apply in order :
Alkali metals have oxidation number +1 (e.g., NaCl).
Alkaline earth metals have oxidation number +2 (e.g., BaCl2).
Hydrogen (H) has oxidation number +1, except in compounds with alkali metals or alkaline earth metals.
Fluorine (F) has oxidation number –1.
Oxygen (O) has oxidation number –2, except in compounds with fluorine.
The other halogens have oxidation number –1, except in compounds with fluorine or oxygen.

4. Oxidation – Reduction (Redox) Reactions
Oxidation – Reduction reactions involve a change in oxidation states
Two elements must change oxidation states
electrons are transferred
total number of electrons must be conserved
If one element is oxidized, another must be reduced
Oxidation – loses electron(s), oxidation state increases
Reduction – gains electron(s), oxidation state decreases
The oxidizing agent is reduced, and reducing agent is oxidized in the reaction

5. Displacement Reactions
2 AgNO3 (aq) + Cu (s)  Cu(NO3)2 (aq) + 2 Ag (s)

6. Activity Series
Elements higher on the activity series are more reactive.
They are more likely to exist as ions.

7. Balancing Redox Equations
Make two half-reactions (oxidation and reduction).
Balance atoms other than O and H. Then, balance O and H using H2O/H+.
Add electrons to balance charges.
Multiply by common factor to make electrons in half-reactions equal.
Add the half-reactions.
Simplify by dividing by common factor or converting H+ to OH– if basic.
Double-check atoms and charges balance!

8. Stoichiometry with solutions
How many mL of M HCl are needed to completely neutralize 50.0 mL of M Ba(OH)2 solution? Ba(OH)2 + 2 HCl  BaCl2 + 2 H2O