1. Chemical Reactions SCH3u/4C Ms. Richardson
Electrochemistry
Chemical Reactions
SCH3u/4C
Ms. Richardson

2. Reduction-Oxidation Reactions
Reduction and oxidation reactions (redox reactions) involve the transfer of electrons.
This either is caused by, or produces an electric current.

3. Reduction Reactions
According to modern theory, a reduction reaction involves a gain of electrons.
Ag+(aq) + e-  Ag(s)
We demonstrate this gain of electrons by placing the electrons on the reactant side of the equation

4. Oxidation Reactions
According to modern theory, an oxidation reaction involves a loss of electrons.
Ag(s)  Ag(aq) + e-
We demonstrate this loss of electrons by placing the electrons on the product side of the equation

5. Redox Reactions
Oxidation and reduction reactions are separate processes that occur simultaneously.
The total number of electrons gained must be equal to the total number of electrons lost.
The overall equation includes both processes and must be balanced!

7. Assigning Oxidation Numbers
Free elements: each atom has an oxidation # of zero
Monatomic ions: oxidation # is equal to the ion’s charge
Oxygen: oxidation # is -2, unless in peroxides it is -1
Hydrogen: when bonded to non-metal is +1, and when bonded to a metal is -1
Fluorine: always oxidation # of -1
In neutral compounds: Sum of all oxidation #’s is zero
Oxidation States
An oxidation number (or state) indicates how many electrons may be lost by an atom in a compound.
We can follow this set of rules to predict oxidation numbers:

8. 0 = oxidation of H + oxidation of S
Examples:
What is the oxidation number of sodium in sodium chloride?
NaCl
Na = +1
Cl = -1
What is the oxidation number of sulfur in H2S?
H = +1
S = -2
Make sure its balanced!
0 = 2(+1) + (-2)
0 = oxidation of H + oxidation of S

9. Oxidation of Ca + Oxidation of S + Oxidation of O = 0
Example
What is the oxidation number for each element in CaSO4
Ca = +2
S = -2
O = -2
Make sure its balanced!
Oxidation of Ca + Oxidation of S + Oxidation of O = 0
+2 + (?) + 4(-2) = 0
? = 6

10. Predicting Reduction and Oxidation
In redox reactions, oxidation numbers for particular elements change from reactants to products.
When oxidation #’s increase, oxidation has occurred.
When oxidation #’s decrease, reduction has occurred

11. Examples Which of the following are Redox reactions? 2K + Cl2  2KCl
N2 + 3H2  2NH3
S + 3Cl2  SCl6
Al + Cu2+ Cu + Al3+
Al(OH)3 + 3NaCl  3NaOH + AlCl3

12. Half Reactions
Half reactions are used to show which components of the overall reaction are being oxidized and which are being reduced in separate, smaller equations.
Example: FeO + Al  Al2O3 + Fe
Oxidation half reaction: Fe  Fe2+ + 2e-
Reduction half reaction: Al3+ + 3e-  Al