Chapter 17, Section 1, pages 602-611
Oxidation, Reduction and Electrochemistry
Objectives
To identify atoms that are oxidized or reduced through electron transfer
To assign oxidation numbers to atoms in compounds and ions
To identify redox reactions
To balance equations for oxidation-reduction reactions
Input
Demo-battery and light circuit, page 603
Electron Transfer and Chemical Reactions
2Na + Cl2 -----------> 2NaCl
New compound that is formed is made up of ions, Na+ and Cl-. This is due to the
electron transfer from Na to Cl. Even in polar covalent compounds there is electron
transfer as in CO
Oxidation Involves the Loss of an Electron
Oxidation- a reaction in which one or more electrons are removed from a
substance such that the valence or oxidation state increases.
Na -----------> Na+ (Na is oxidized)
Reduction is a reaction in which one or more electrons are gained wither by
the removal of oxygen, the addition of hydrogen or the addition of electrons.
Cl -------------> Cl- (Cl is reduced)
Oxidation and Reduction Occur Together
Normally oxidation, the loss of an electron, happens at the same time as reduction,
the gain of an electron. This is a oxidation –reduction reaction, or redox reaction.
2HgO ------> 2Hg + O2
Hg +2 O-2, Hg is + because it has lost an electron to O---oxidation
O is – because it has gained an electron from Hg--reduction
Hg is reduced (gains an electron) while O is oxidized (loses an electron)
Oxidation Numbers
The number of electrons that must be added to or removed from an atom in a
combined state to convert the atom into an elemental form.
Rules for Assigning Oxidation Numbers
1. Write the formula
2. Assign know oxidation numbers (use periodic table)
a. Any free atom has an oxidation of zero
b. Ions have an oxidation number equal to the charge on the ion
3. Sum of all oxidation numbers must be zero
Sample Problem A, page 607
Assign oxidation numbers to the sulfur and oxygen atoms in the pyrosulfate ion,
S2O7 -2
Oxygen is always -2
O7, -2 x 7 = 14
Overall charge on the ion is -2
So, 2 S + -14 =-2
Then, S = +6
Practice Problems, page 607, #1
Identify Redox Reactions
In an oxidation reaction oxidation number increases
In a reduction reaction oxidation numbers decreases.
2HCl + Zn ---------> Zn +2 Cl -1 Cl -1 + H2
Half-Reactions
Zn goes from 0 to +2---it is oxidized, oxidation number increases, + 2eH goes from +1 to 0---it is reduced, oxidation number decreases, - 2eCl no change, does no take part in the reaction
Sample Problem B
8H3O+ + MnO4- + Fe +2 ------> Mn +2 + Fe +3 + 12H2O
Ox. Numbers +1-2
+7 -2
+2
+2
+3
0
Half Reactions + 5 e- + 8H3O+ + MnO4- + Fe +2 ---> Mn +2 + Fe +3 + 12H2O +e-5
+8
-1
+2
+2
+3
0
-1
Must balance charges on each side of the reaction equation.
Identifying Agents in Redox Reactions
The substance that gains electrons in a redox reaction is known as an oxidizing
agent.
The substance the losses electrons in a redox reaction is known as a reducing agent.
Conclusion
Review key concepts
Complete handout given out in class
Homework
Complete handout