MR. SURRETTE
VAN NUYS HIGH SCHOOL
CHAPTER 18: OXIDATION AND REDUCTION
CLASS NOTES
REDOX REACTIONS
One type of chemical reaction involves the transfer of electrons from one species (species means atoms
or groups of atoms) to another. These reactions are called oxidation reduction reactions.
REDOX REACTIONS
The species that loses electrons is oxidized and the species gaining electrons is reduced. Oxidation
reduction reactions are also known as redox reactions (red = reduction, ox = oxidation).
OXIDATION AND REDUCTION
Oxidation and reduction reactions take place at the same time. This is because electrons from one
species need to find another species to accept them.
OXIDATION NUMBERS
Oxidation numbers are assigned to each element in a redox reaction. Oxidation numbers help
determine which element is oxidized and which is being reduced.
OXIDATION NUMBERS
If the oxidation number of an element increases (becomes more positive) the element is oxidized. If the
oxidation number of an element decreases, the element is reduced.
ANIONS AND CATIONS
Ions are species that have net electric charges. Ions are negative if they have extra electrons and
positive if they have lost electrons. Negative ions are called anions and positive ions are called cations.
OXIDATION NUMBERS AND LDS DIAGRAMS
Sometimes it is useful to assign oxidation numbers to elements found in polar covalent species. By
creating Lewis Dot Structures (LDS) diagrams for each element, it is possible to determine their
oxidation numbers.
OXIDATION NUMBERS AND LDS DIAGRAMS
Next, combine the LDS diagrams for the elements and determine which electrons are shared.
OXIDATION NUMBERS AND LDS DIAGRAMS
Once the number and types of shared electrons has been determined, assign each shared electron to the
more electronegative element.
ELECTRONEGATIVITY
An element’s ability to attract electrons is its electronegativity. In general, the halogens and group 16
atoms have the highest electronegativity. The metals on the left side of the periodic table tend to donate
electrons to the high electronegative elements.
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CHEMISTRY
MR. SURRETTE
VAN NUYS HIGH SCHOOL
LDS DIAGRAM EXAMPLE
Example 1. Draw a Lewis Dot Structure (LDS) diagram for water.
1A.
Hydrogen has one valence electron and oxygen has six:
Because the molecular formula for water is H2O, combine two hydrogen LDS diagrams atoms with a
single oxygen LDS diagram. This creates the following LDS diagram:
Example 2. Assign oxidation numbers to hydrogen and oxygen based on the LDS diagram for water.
2A.
Oxygen is the more electronegative element (it is in group 16). It shares a pair of electrons with both
hydrogen atoms. This means oxygen has 2 “extra” electrons that give it a – 2 oxidation number.
Each hydrogen in the water molecule has a single electron pulled away from it. Therefore, both
hydrogen atoms have a +1 oxidation number.
PURE ELEMENTS
All pure elements, for example H2 and Cl2, are assigned a + 0 oxidation number.
HYDROGEN
When combined with elements of higher electronegative values, non-elemental hydrogen is assigned a
+ 1 oxidation number.
GROUP 1: ALKALI METALS
Non-elemental alkali metals are assigned a + 1 oxidation number.
GROUP 2: ALKALINE METALS
Non-elemental alkaline metals are assigned a + 2 oxidation number.
OXYGEN
Non-elemental oxygen is assigned a - 2 oxidation number.
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CHEMISTRY
MR. SURRETTE
VAN NUYS HIGH SCHOOL
GROUP 17: HALOGENS
Non-elemental halogens are assigned a - 1 oxidation number.
OXIDATION NUMBER RULES
For more complicated atoms, apply the following rules.
OXIDATION NUMBER RULES
Rule 1. As shown earlier, the oxidation number of atoms in a pure element is defined as zero:
C(0) Fe(0) H2(0)
Rule 2. A single atom is assigned an oxidation number equal to its electrical charge. For metals,
electrical charges are assigned to the metal’s number of valence electrons. Examples are Na+ Mg+2
and Fe+3.
Rule 3. An ionic species must have oxidation numbers that sum to the electrical charge of the species.
Example 3. Assign oxidation numbers to sulfur and oxidation in the sulfite ion SO3-2.
3A.
(1) Non-elemental oxygen is assigned a -2 oxidation number. Since there are three oxygen atoms in
the sulfite ion, all three add up to – 6 oxidation number.
(2) In order for the entire sulfite ion to have a -2 charge, the single sulfur atom must be assigned a + 4
oxidation number.
(3) Double-check charges: - 6 + 4 = - 2 (SO3-2).
Rule 4. A neutral molecule has oxidation numbers adding to zero.
In Example 2, both hydrogen atoms added a + 2 oxidation number to the water molecule. The single
oxygen atom added a - 2 oxidation number. Double check charges: + 2 – 2 = 0 (neutral water
molecule).
OXIDATION NUMBERS
LDS diagrams are not always needed to predict oxidation numbers. As shown above, the four rules of
oxidation can also be used to predict oxidation numbers.
Example 4. Use the rules of oxidation to predict the oxidation numbers for carbon and oxygen within
the carbon dioxide molecule.
4A.
The molecular formula for carbon dioxide is CO2. Non-elemental oxygen is assigned - 2 charge. Since
there are two oxygen atoms, both combine to a - 4 oxidation number.
Since CO2 is a neutral molecule, the charge on the carbon atom must balance out the oxidation numbers
found on both oxygen atoms. Therefore, the carbon atom has a + 4 oxidation number.
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CHEMISTRY
MR. SURRETTE
VAN NUYS HIGH SCHOOL
Example 5. Predict the oxidation numbers for nitrogen and hydrogen within the ammonia molecule.
5A.
The molecular formula for ammonia is NH3. Even though the rules of oxidation do not tell us the
oxidation number for nitrogen, we know the oxidation number for non-elemental hydrogen is + 1.
Since there are three hydrogen atoms in NH3, the hydrogen atoms must have a combined oxidation
number of + 3.
The nitrogen atom must have a - 3 charge to balance out the 3 hydrogen atoms.
Double check charges: + 3 - 3 = 0 (neutral ammonia molecule).
REDOX CHEMICAL EQUATIONS
Sometimes we are given balanced chemical equations. In these cases, we need to determine the
oxidation numbers for both reactants and products.
REDOX CHEMICAL EQUATIONS
One very important note… In redox reactions, the oxidation numbers within the reactants are different
from the oxidation numbers within the products.
Examples 6 - 8. Aluminum and oxygen react to form aluminum oxide:
4 Al(s) + 3 O2(g)  2 Al2O3(s)
6. Predict the before and after oxidation numbers for aluminum and oxygen.
6A.
Reactants:
For Al: Aluminum is a pure element so it has a + 0 oxidation number.
For O2: Oxygen is a pure element so it has a + 0 oxidation number.
Product:
For Al2O3: Oxygen is assigned a – 2 oxidation number. Since there are three oxygen atoms in Al2O3,
the oxygen atoms have a combined – 6 oxidation number. The remaining two aluminum atoms must
have + 3 oxidation numbers to balance out the oxygen oxidation numbers.
Final oxidation numbers
4 Al(0) + 3 O2(0)  2 Al2(+3)O3(-2)
7. Identify the element that is reduced.
7A.
Because oxygen gains electrons, it is reduced.
8. Identify the element that is oxidized.
8A.
Because aluminum loses electrons, it is oxidized.
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CHEMISTRY