Transcript oxidation number

Redox Reactions
Redox Reactions
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Redox is the abbreviated way to write reductionoxidation reaction.
Redox reactions are a very broad type of reaction
and many of the types we have already discussed
can also be classified as a redox reaction.
In a redox reaction, reduction and oxidation must
occur together.
But what is reduction and oxidation?
Reduction – chemical change in which electrons are
gained.
Oxidation – a chemical change in electrons are
removed.
So, we need to Review Oxidation
Numbers
What is the oxidation number?
 Book definition – the number of electrons that
must be added to or removed from an atom
in a combined state to convert the atom into
the elemental form.
 In other words, the number of electrons
gained or lost when a chemical bond is made.
 Where can we find possible oxidation
numbers? The Periodic Table
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Assigning Oxidation Numbers – The
steps from page 606
1.
2.
Identify the chemical formula
If no formula is provided, write the formula
of the formula or ion from the name.
Assign known oxidation numbers
- The oxidation number of an atom of any free
element in atomic or molecular form is zero.
Examples: Cu, H2, Ag, Na, S8
- The oxidation number of monatomic ion is
equal to the charge of the ion
Assigning Oxidation Numbers –
Continued
- Fluorine always has an oxidation number of
-1 since it is the most electronegative
element.
- In a compound:
Group 1 atoms = +1
Group 2 atoms = +2
Aluminum = +3
- Hydrogen = +1, unless it is with a metal,
then it is -1
Assigning Oxidation Numbers –
Continued
- Oxygen is usually -2, exceptions:
- If combined with Fluorine, Oxygen’s
oxidation number becomes +2
- In peroxides, H2O2, Oxygen’s oxidation
number becomes -1
- When comparing nonmetals, the more
electronegative atom (page 194) gets the
negative charge based on the column it is
located in on the periodic table.
- Group 17 = -1 , Group 16 = -2 , Group 15 = -3
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Assigning Oxidation Numbers –
Continued
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Calculate any remaining numbers, and verify
results.
– Use the total oxidation number of each
element’s atoms = oxidation number times
subscript
– The sum of the oxidation numbers for all the
atoms in a molecule is zero.
– The sum of the oxidation numbers for all the
atoms in a polyatomic ion is equal to the
charge on that ion.
Examples
-3 +4 = +1
+2 +6 -8= 0
+2 -2 = 0
-3 +1
+2 +6 -2
+1 -2
NH4+
PbSO4
0
0
Al
H2
Ca(OH)2
+6 +12 -18 = 0
+3 +4 -2
Fe2(CO3)3
+2 -2 = 0
+1 +5 -6 = 0
+2 +5 -8 = -1
+1 -2
+1 +5 -2
+1 +5 -2
H2O
+2
Pb+2
KClO3
H2PO4-1
+3 -3 = 0
+12 -14 = -2
+3 -1
+6 -2
BF3
S2O7-2
Now that we reviewed oxidation
numbers, what do we do with them?
Remember, in a redox reaction, something
has to be oxidized (loses electrons,
oxidation number increases) and
something else has to be reduced (gains
electrons, oxidation number decreases).
 If we assign oxidation numbers to all
atoms in a chemical equation, we can
determine whether it is a redox reaction.
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Are the following redox reactions?
2 H3O+ + Zn  H2 + 2H2O + Zn+2
C3H8 + 5O2  3CO2 + 4H2O
2KI + Pb(NO3)2  PbI2 + 2KNO3
2K + Cl2  2KCl
Some other terms
Oxidizing agent – the substance that gains
electrons in an redox reaction and is
reduced.
 Reducing agent - the substance that loses
electrons in an redox reaction and is
oxidized.
 Notice that the oxidation agent oxidizes
another substance and becomes reduces in
the process and visa versa
 Also note that these refer to the whole
compound or ion.
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Let’s look at the following redox
reaction
First, write the oxidation number for each atom:
8 H3O+ + MnO4- + 5Fe+2  Mn+2 + 5Fe+3 + 12 H2O
What
What
What
What
was reduced? Mn
was oxidized? Fe
is the oxidizing agent? MnO4is the reducing agent? Fe+2
Homework
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Page 611: 6, 7, 8, 9