Oxidation-Reduction
An oxidation-reduction (redox)
reaction involves the transfer
of electrons (e - ).
Sodium transfers its electrons to chlorine
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Chemists need a way to keep
track of what happens in a
redox reaction - ie who gains
electrons and who loses
electrons
They use a book-keeping system
called oxidation numbers
Oxidation numbers look
like the charge on the ion
but are written with the - or
+ in front of the number!
Oxidation numbers are
sometimes the ionic
charge on an element but
must be determined from
a set of rules.
Find the oxidation numbers of
each element in a reaction and
see which ones have changed.
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Rules for assigning oxidation number
– Any uncombined element is 0.
– A monatomic ion equals the charge of the ion
– Fluorine is -1
– Oxygen is -2, unless it is combined with F,
when it is +2, or it is in a peroxide such as H2O2
when it its -1
– Hydrogen is usually +1, unless combined with
a metal, then it is -1
– Group 1 is +1, Group 2 is +2, Aluminum is +3
– The sum of the oxidation numbers of all atoms
in a neutral compound is 0.
– The sum of the oxidation number of all atoms
in a polyatomic ion equals the charge of the ion.
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The sum of the oxidation numbers
of all the atoms in a compound is
zero.
CuO
Oxygen is -2
The oxidation number of
copper must be
calculated
X + -2 = 0
X = +2
Na2SO4
– Na is +1 because it is a
group 1 metal
– O is -2
– The oxidation number
of Sulfur must be
calculated
2(+1) + X + 4(-2) = 0
(2 ) + X + (-8) =0
X = +6
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The sum of the oxidation numbers
in an ion is equal to the charge of
the ion.
Monatomic
Cl-1
Fe3+
+3
Polyatomic
NO3– O is -2
– The oxidation number
of nitrogen must be
calculated
X + (3)(-2) = -1
X + (-6) = -1
X = +5
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The same element
can have a different
oxidation number in
different substances
Practice Question
What is the oxidation number of potassium in
K2Cr2O7 ?
(a) +1
(b) +2
(c) -2
(c) -7
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Practice Question
What is the oxidation number of oxygen in
K2Cr2O7 ?
(a) +1
(b) +2
(c) -2
(d) -7
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Practice Question
What is the oxidation number of chromium in
K2Cr2O7 ?
(1) +12
(2) +2
(3) +3
(4) +6
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Practice Question
In which substance does chlorine have an
oxidation number of +1?
(1) Cl2
(2) HCl
(3) HClO
(4) HClO2
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Cl2 = 0
Rule #1
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HCL
H = +1 Rule #6
+1 + X = 0 Rule #8
Cl = -1
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HClO
H = +1 Rule #6
O = -2 Rule #5
+1 + X + -2 = 0 Rule #8
X: Cl = +1
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HClO2
H = +1 Rule #6
O = -2 Rule #5
+1 + X + (2x-2) = 0 Rule #8
X: Cl = +3 Rule #8
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Continue with where we left off yesterday
(page 81 or 78 or beyond)
Identifying Redox Reactions
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During redox, one atom is
oxidized while another atom is
reduced. Reduction and
oxidation happen together.
The oxidation numbers of the atoms will change….
one goes up (0 to +1) and one goes down (0 to -2)
Oxidation
0
0
2H2 + O2
+
2H2O
2-
Reduction
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Oxidation numbers (states) can
be assigned to atoms and ions.
Changes in oxidation numbers
indicate that oxidation and
reduction have occurred.
0
+2
+5 -2
+2 +5 -2
0
Zn + Cu(NO3)2 Zn(NO3)2 + Cu
Find the oxidation numbers and see which ones change.
Nitrate NO3 is -1
Zn0
Zn2+ + 2e- Oxidation
Cu2+ + 2eCu0 Reduction
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Oxidation is the loss of
electrons.
Metal atoms lose electrons to become + ions
Oxidation Number INCREASES
Cr2+
Cr4+ + 2e-
2N3-
N20 + 6e22
A half-reaction can be written
to represent oxidation.
Zn0
Zn2+ + 2eIn oxidation half reactions,
electrons are written on the right
because electrons are lost
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Reduction is the gain of
electrons.
Metals gain electrons to form new ions
or elements
Oxidation Number DECREASES
Fe3+ + 3e-
→ Fe0
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A half-reaction can be written
to represent reduction.
Cu2+ + 2e-
Cu0
In reduction half reactions,
electrons are written on the left
because electrons are gained
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Practice Question
Given the equation:
2 Al + 3 Cu2+
2 Al3+ + 3Cu
The reduction half-reaction is
(1) Al
Al 3+ + 3e –
(2) Cu 2+ + 2e –
Cu
(3) Al + 3e –
Al 3+
(4) Cu 2+
Cu + 2e –
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LEO (the lion) says GER
Loss of
Electrons
Oxidation
Gain of
Electrons
Reduction
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Practice Question
Given the reaction:
Mg(s) + 2H+(aq) + 2Cl–(aq)
H2(g)
Mg2+(aq) + 2Cl–(aq) +
Which species undergoes oxidation?
(1) Mg(s)
(2) H+(aq)
LEO says GER
(3) Cl– (aq)
(4) H2 (g)
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Practice Question
Given the equation:
C(s) + H2O(g)
CO(g) + H2 (g)
Which species undergoes reduction?
(1) C(s)
(2) H +
LEO says GER
(3) C2+
(4) H2 (g)
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Agents
Reducing Agent: element or a
compound in a redox reaction that
reduces another species.
It becomes oxidized, is the electron
donor in the redox.
A reducing agent is oxidized.
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• Oxidizing Agent: element or a
compound in a redox reaction that
oxidizes another species.
It becomes reduced, is the electron
acceptor in the redox.
An oxidizing agent is reduced.
Practice Question
In any redox reaction, the substance that undergoes
reduction will
(1) lose electrons and have a decrease in oxidation
number
(2) lose electrons and have an increase in oxidation
number
(3) gain electrons and have a decrease in oxidation
number
(4) gain electrons and have an increase in oxidation
number
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An electrochemical cell can be
either voltaic or electrolytic. In
an electrochemical cell,
oxidation occurs at the anode
and reduction at the cathode.
Red Cat – An Ox
Reduction at the
Cathode
Oxidation at the
Anode
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