Download elements of chemistry unit

MR. SURRETTE
VAN NUYS HIGH SCHOOL
CHAPTER 19: 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. 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. 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. Next, combine the LDS diagrams for the elements and determine which electrons
are shared. Once the number and types of shared electrons has been determined, assign each shared
electron to the more electronegative element.
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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.
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.
PURE ELEMENTS
All pure elements, for example H2 and Cl2, are assigned a 0 oxidation number.
HYDROGEN
Hydrogen is assigned a + 1 oxidation number.
GROUP 1: ALKALI METALS
Alkali metals are assigned a + 1 oxidation number.
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VAN NUYS HIGH SCHOOL
GROUP 2: ALKALINE METALS
Alkaline metals are assigned a + 2 oxidation number.
GROUP 6 ELEMENTS
Elements in group 6 are assigned a - 2 oxidation number.
GROUP 7: HALOGENS
The halogens are assigned a - 1 charge.
OXIDATION NUMBER RULES
For more complicated atoms, apply the following rules.
OXIDATION NUMBER RULES
1. As shown earlier, the oxidation number of atoms in an element is defined as zero:
C(0) Fe(0) H2(0)
OXIDATION NUMBER RULES
2. A single atom is assigned an oxidation number equal to its electrical charge. Examples are Na+ Mg+2
and Fe+3.
OXIDATION NUMBER RULES
3. An ionic species must have oxidation numbers that sum to the electrical charge of the species.
OXIDATION NUMBER RULES
4. A neutral molecule has oxidation numbers adding to zero.
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.
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VAN NUYS HIGH SCHOOL
Example 3. Use the rules of oxidation to predict the oxidation numbers for carbon and oxygen within
the carbon dioxide molecule.
3A.
The molecular formula for carbon dioxide is CO2. Since oxygen is a group 16 element, both oxygen
atoms are assigned - 2 charges for a total oxidation number of – 4. Since CO2 is a neutral species, the
charge on the carbon atom must balance out the charges found on the oxygen atoms. Therefore, the
carbon atom has a + 4 oxidation number.
Example 4. Predict the oxidation numbers for nitrogen and hydrogen within the ammonia molecule.
4A.
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 hydrogen is + 1. Since there are 3
hydrogen atoms in NH3, they must have a combined oxidation number of + 3. Therefore, the nitrogen
atom must have a - 3 charge to balance out the 3 hydrogen atoms.
REDOX CHEMICAL EQUATIONS
Sometimes we are given balanced chemical equations and we need to determine the oxidation numbers
for both the reactants and the products. In redox reactions, the oxidation numbers within the reactants
are different from the oxidation numbers within the products.
Example 5. Aluminum and oxygen react to form aluminum oxide:
4 Al(s) + 3 O2(g)  2 Al2O3(s)
5a. Predict the before and after oxidation numbers for aluminum and oxygen.
A.
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.
For Al2O3: Oxygen is a group 16 element, so each oxygen atom has a – 2 oxidation number. Since
there are 3 oxygen atoms in Al2O3, the O3 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(-2)  2 Al2(+3)O3(-2)
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5b. Identify the element that is reduced.
A. Oxygen
5c. Identify the element that is oxidized.
A. Aluminum
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