Transcript Document

Reduction Oxidation
Reduction Oxidation reactions are often
referred to as REDOX reactions.
Definition
 A redox reaction occurs when 1 or 2 elements
change their oxidation number during a
chemical reaction.
 The next two frames contain 4 common
examples fo redox reactions. Which ones are
you familiar with?
Examples
 Metals rust when
exposed to water or
oxygen.
 Batteries produce
electricity to run a
flashlight or to start a
car.
More examples
 An apple turns brown
after a bite is taken.
 A piece of jewelry is
plated with a thin layer
of gold or silver.
How about in Chemistry class?
 One of the most interesting aspects of redox
chemistry is the different colors.
 Many transition metals lose different numbers
of electrons. Each different oxidation number
usually has a different color.
 Click to the next frame to see some examples
Colors of Transition Metals
Assigning Oxidation Numbers
 The oxidation number of a free element is 0.
 The charge of an ion is it’s oxidation number.
 The sum of the oxidation numbers in a
compound is equal to 0.
 H in a compound has an oxidation number of
+1, except in metallic hydrides.
 Oxygen in a compound has an oxidation
number of -2, except in peroxides.
 The sum of the oxidation numbers in a
polyatomic ion is equal to it’s charge.
The oxidation number of a free
element is 0.
 Free elements are elements that are not
bonded to another element.
 Any element has the same number of (+)
protons and (-) electrons, so they do not have
a charge.
 Diatomic elements are also considered free
elements. We all love ClIF H. BrON
The charge of an ion is it’s oxidation
number.
 The elements in column I like to lose 1
electron.

They have an oxidation number of +1
 The elements in column II like to lose 2
electrons.

They have an oxidation number of +2
 The elements in column III like to lose 3
electrons.

They have an oxidation number of +3
The charge of an ion is it’s oxidation
number.
 The elements in column VII like to gain 1
electron.

They have an oxidation number of -1
 The elements in column VI like to gain 2
electrons.

They have an oxidation number of -2
 The elements in column V like to gain 3
electrons.

They have an oxidation number of -3
Column IV
 The elements in column IV often share
electrons.
 Sometimes they lose 4 electrons.
 Sometimes they gain 4 electrons.
 Therefore, their oxidation number can be
either +4 or -4.
The sum of the oxidation numbers in a
compound is equal to 0.
 Transition Metals can lose different numbers
of electrons. Therefore, they have different
oxidation numbers. To determine their
oxidation number, add up the oxidation
numbers in the compound and set the sum
equal to zero.
 For instance, what is the oxidation number of
the elements in Nickel(II) chloride?
Nickel(II) chloride is NiCl2
 To start this problem, we know that chlorine is
in column VII, therefore it has an oxidation
number of -1.
 Since there are two chlorine atoms, that is a
total charge of -2.
 For the sum of the oxidation numbers to
equal 0, what is the ox # of the nickel?
The ox # of Ni is +2
 +2 from the nickel and 2 chlorines, each -1 adds up
to a total of zero. In other words, the number of
electrons lost by the nickel is equal to the number of
electrons gained by the chlorines.
What about Rust?
 Rust has the formula Fe2O3. What is the
oxidation number of each element in rust?
 First, we know that oxygen has an oxidation
number of -2 in a compound. There are 3
oxygen atoms for a total charge of -6.
 The oxidation numbers of a compound must
add up to zero. So what is iron’s ox #?
Iron has an ox # of +3
 Each iron atom loses three electrons for an
oxidation number of +3.
2(+3) + 3(-2) =0
Fe2O3
Practice
 What is the oxidation of each transition metal
in the following compounds?

FeCl3

MnO

Cu2O

CoBr2
Answers
 Fe in FeCl3 is +3
 Mn in MnO is +2
 Cu in Cu2O is +1
 Co in CoBr2 is +2
The sum of the oxidation numbers in a
polyatomic ion is equal to it’s charge.
To help understand this rule, let’s look at the
polyatomic ion sulfate, SO4-2.
Oxygen atoms have an oxidation number of -2
? + 4(-2) = -2
SO4
It must add up to -2
6 + 4(-2) = -2
SO4
 Therefore the oxidation number of the sulfur
in sulfate is +6
What about CrSO4?
 Since this is a compound, the sum must be 0
+2
+6 4(-2) = 0
CrSO4
The Cr is +2, S is +6 and the O is -2
One last problem…….
 What is the oxidation number of each
element in Cu(NO3)2?
 NO3-1 has an oxidation number of -1 and
there are two of them.
 The compound must add up to zero.
What is copper’s oxidation number?
+2
2(-1)
=0
Cu(NO3)2
Each nitrate is -1, so the copper must be +2
So what is the nitrogen?
+2
??
6(-2)
=0
Cu(NO3)2
There are six oxygen atoms, each one with an
oxidation number of -2.
The compound must add up to zero.
+2
2(+5) 6(-2)
=0
Cu(NO3)2
There are two nitrogen atoms and the must
have a total charge of +10.
Therefore each nitrogen has an oxidation
number of +5
Final answer………
Copper has an oxidation number of +2
Nitrogen has an oxidation number of +5
Oxygen has an oxidation number of -2