A New Unit
Oxidation &
Reduction
(Redox)
Do you remember the five
kinds of reactions?
A + B → AB
AB → A + B
A + O2 → AO
AB + C → AC + B
AB + CD → AD + CB
Synthesis
Decomposition
Combustion
Single replacement
Double replacement
Do you remember the five
kinds of reactions?
Most of these reactions involve the
transfer of electrons from one atom
to another.
Do you remember what
makes a “happy atom”?
•Plus and minus charges balance
•Electron shells are exactly full.
Do you remember what
makes a “happy atom”?
Sodium is unhappy
It has one more electron than will
fit in its second shell
Chlorine is unhappy
It is missing one electron from its
third shell.
Do you remember what
makes a “happy atom”?
Sodium gives its extra electron to
chlorine
They are still unhappy. Why?
Sodium has a positive charge
Chloride has a negative charge.
Do you remember what
makes a “happy atom”?
But if they stay attached to one
another, they can make each other
happy.
In an ionic bond, the total charges
balance out.
Do you remember what
makes a “happy atom”?
Na
Cl
Do you remember what
makes a “happy atom”?
Na
Cl
Do you remember what
makes a “happy atom”?
Na
Cl
Do you remember what
makes a “happy atom”?
Na
Cl
Do you remember what
makes a “happy atom”?
NaCl
Do you remember what
makes a “happy atom”?
NaCl
Do you remember what
makes a “happy atom”?
2Na + Cl2 → 2NaCl
Redox
A reaction in which electrons are
transferred from one atom to another
is called an oxidation-reduction
reaction, or “redox” for short.
Redox
Originally, the word “oxidation”
referred to reactions with oxygen.
In combustion reactions, oxygen
steals electrons from other atoms.
Redox
Today, “oxidation” refers to any kind
of atom stealing electrons from
another.
Redox
Redox
Ger…
Redox
Ger…
Leo says “Ger”
Redox
Ger…
L
E
O
G
E
R
Redox
Ger…
Loss of
Electrons is
Oxidation
G
E
R
Redox
Ger…
Loss of
Electrons is
Oxidation
Gain of
Electrons is
Reduction
Oxidation Number
The oxidation number of an
atom in an ionic compound is
the number of electrons lost
or gained when it forms an
ion.
Oxidation Number
Ca + Br2 → CaBr2
Oxidation Number
Ca + Br2 → CaBr2
Ca + Br2 →
2+
Ca
+
2Br
Oxidation Number
2+
Ca + Br2 → Ca + 2Br
Oxidation Number
2+
Ca + Br2 → Ca + 2Br
2+
Ca charge =
2+
Br
1-
charge =
Oxidation Number
2+
Ca + Br2 → Ca + 2Br
2+
Ca charge =
oxidation number =
Br
charge =
oxidation number =
+2
+2
-1
-1
Oxidation Number
2+
Ca + Br2 → Ca + 2Br
Calcium is oxidized by bromine.
Oxidation Number
2+
Ca + Br2 → Ca + 2Br
Calcium is oxidized by bromine.
Bromine is reduced by calcium.
Oxidation Number
2+
Ca + Br2 → Ca + 2Br
Calcium is oxidized by bromine.
Bromine is reduced by calcium.
An oxidizing agent takes electrons.
Oxidation Number
2+
Ca + Br2 → Ca + 2Br
Calcium is oxidized by bromine.
Bromine is reduced by calcium.
An oxidizing agent takes electrons.
A reducing agent gives electrons.
Oxidation Number
2+
Ca + Br2 → Ca + 2Br
Calcium is oxidized by bromine.
Bromine is reduced by calcium.
An oxidizing agent takes electrons.
A reducing agent gives electrons.
(LEO says GER)
Oxidation Number
Covalent bonds can also
involve redox reactions if
the bonds are polar.
Homework
Read pages 634-640
Answer questions 1-3 on page 640