Chapter 20
Oxidation-Reduction Reactions
Anything in black letters = write it in
your notes (‘knowts’)
20.1 – The Meaning of Oxidation and Reduction
Original definitions, still useful…
Oxidation
Combining with oxygen
Reduction
Loss of oxygen
Oxidation cannot occur without reduction.
Circle the substance that is being oxidized
CH4(g) +
4Fe(s)
2O2(g) →
+
3O2(g)
CO2(g) + 2H2O(g)
→
2Fe2O3(s)
2Fe2O3(s) + 3C(s) → 4Fe(s) + 3CO2(g)
Oxidation
Reduction
Combining with oxygen
Loss of oxygen
Modern definitions, more useful…
Oxidation
Loss of electrons
Reduction
Gain of electrons
Helpful acronym…
‘LEO the lion says GER’
ger
Loss of Electrons is Oxidation
Gain of Electrons is Reduction
A substance can be oxidized without reacting with oxygen.
Mg(s)
+
S(s)
heat
MgS(s)
Mg is oxidized because it lost 2 e-
Oxidation
Reduction
Loss of electrons
Gain of electrons
Oxidizing Agent – causes another substance
to be oxidized.
Reducing Agent – causes another substance
to be reduced.
Oxidizing Agent
2C + O2  2CO
Reducing Agent
Carbon is oxidized, oxygen is reduced.
The oxidizing agent will always be reduced
Circle the oxidizing agent
2AgNO3(aq) + Cu(s) →
Cu(NO3)2(aq) + 2Ag(s)
20.2 – Oxidation Numbers
This section is really a review…
Oxidation Number – charge on an ion.
Elements & compounds have an oxidation # of zero.
0
0
+2 -1
Mg + Cl2  MgCl2
Mg0  Mg+2 + 2e- Loss of e- = oxidation
Cl20 + 2e-  2Cl-1
Gain of e- = reduction
These should make sense…don’t write them all down!
Rules for Assigning Oxidation Numbers
1.
The oxidation number of a monatomic ion is equal in
magnitude and sign to its ionic charge. For example, the
oxidation number of the bromide ion (Br1–) is –1; that of
the Fe3+ ion is +3.
2.
The oxidation number of hydrogen in a compound is +1,
except in metal hydrides, such as NaH, where it is –1.
3.
The oxidation number of oxygen in a compound is –2,
except in peroxides, such as H2O2, where it is –1, and in
compounds with the more electronegative fluorine,
where it is positive.
4.
The oxidation number of an atom in uncombined
(elemental) form is 0. For example, the oxidation number
of the potassium atoms in potassium metal (K) or of the
nitrogen atoms in nitrogen gas (N2) is 0.
5.
For any neutral compound, the sum of the oxidation
numbers of the atoms in the compound must equal 0.
6.
For a polyatomic ion, the sum of the oxidation numbers
must equal the ionic charge of the ion.
What is the oxidation number of each kind of
atom in the following ions and compounds?
a. SO2
b. Na2SO4
c. CO32–
d. Cu2S
ASSIGNMENT:
Chapter 20 Worksheet #1
20.3 – Describing Redox Rxns
Not covered…