Oxidation & Reduction
Reactions in which some elements
change their oxidation number.
Oxidation Numbers
• An oxidation number is the charge an atom
would have if electrons in its bonds belonged
completely to the more electronegative atom.
ClO2
Cl can have several oxidation numbers:
-1,+1,+3,+5,+7
Redox Reaction
• A chemical reaction in species change their
oxidation number:
Na + Cl2 => NaCl
What’s the point ?
REDOX reactions are important in …
• Electrical production (batteries, fuel cells)
Rules
Determine the oxidation number of each atom in
each of the following formulas:
(a) O2
(b) CH4
(c) NaCl
(d) SO4-2
(e) KMnO4
Practice Assigning
Oxidation Numbers
NO2
N2O5
HClO3
HNO3
Ca(NO3)2
KMnO4
Practice Assigning
Oxidation Numbers
Fe(OH)3
K2Cr2O7
CO32CNK3Fe(CN)6
Practice Assigning
Oxidation Numbers
NO2
N= +4, O = -2
N2O5
N = +5, O = -2
HClO3
H=+1, Cl=+5, O = -2
HNO3
H=+1, N = +5, O = -2
Ca(NO3)2
Ca=+2, N =+5, O= -2
KMnO4
K=+1, Mn=+7, O= -2
Practice Assigning
Oxidation Numbers
Fe(OH)3
Fe =+3, O=-2, H=+1
K2Cr2O7
K=+1, Cr=+6, O=-2
CO32-
C=+4, O =-2
CN-
C=+4, N=-5
K3Fe(CN)6
K=+1, Fe=+3, C=+4, N=-5
Lab: Corrosion of Iron
• An important aspect of the use of some metals,
particularly of iron, is the possibility of corrosion.
• Rust is apparently a hydrated form of iron(III)oxide
Fe2+(aq) + O2(g) + H2O(l) → Fe2O3•H2O(s)
rust
Galvanic Corrosion
• Galvanic corrosion is an electrochemical
process in which one metal corrodes
preferentially to another when both metals are
in electrical contact.
Zinc
What do we do with
oxidation numbers?
•
The oxidation numbers of atoms in an
equation allow us to determine which
element is oxidized and which is reduced
in a redox reaction.
6Cd + 2 FeBr3 => 3 Cd2Br2 + 2 Fe
LEO
LEO says
GER!
LEO
says
GER!
Loss of Electrons = Oxidation
Gain of Electrons = Reduction
Oxidation and Reduction
• Apply oxidation numbers to all particles and
identify the oxidized and the reduced elements.
Mg
F2
+
HCl =>
+
AlBr3
MgCl2 +
=>
H2
AlF3 + Br2
• An increase in the oxidation number indicates
that an atom has lost electrons and therefore
oxidized.
• A decrease in the oxidation number indicates
that an atom has gained electrons and therefore
reduced
Zn
+
CuSO4  ZnSO4 + Cu
Using Oxidation Numbers
• An increase in the oxidation number indicates
that an atom has lost electrons and therefore
oxidized.
• A decrease in the oxidation number indicates
that an atom has gained electrons and therefore
reduced
Zn
+
CuSO4  ZnSO4 + Cu
Exercise
For each of the following reactions find
the element oxidized and the element
reduced
Cl2
Cu
+
KBr

KCl +
Br2
+ HNO3  Cu(NO3)2 + NO2 + H2O
HNO3 +
I2
 HIO3 +
NO2
Exercise
For each of the following reactions find
the element oxidized and the element
reduced
Cl2
+
KBr

KCl +
Br2
Exercise
For each of the following reactions find
the element oxidized and the element
reduced
Cu
+ HNO3  Cu(NO3)2 + NO2 + H2O
Oxidation-Reduction
Reactions (Redox)
• All oxidation reduction reactions
have element reduced and one one
element oxidized.
Why is the Statue of Liberty
green?
Replica – Ellis Island
• Acid Rain causes the copper to turn into copper oxide, copper
sulfate, copper hydroxide or copper chloride because of the
oxidation-reduction reaction and this is basically copper salts
or in other words, tarnish.
Things like statues, pennies,
or pipes made of Copper turn green
over a period of time.
This happens due to oxidation.
Corrosion of steel, whether in atmospheric,
soil, water, or chemical exposure, annually
costs the economy about 3.1% of the Gross
Domestic Product (GDP). Estimates show
25-30% of corrosion could be eliminated if
proper corrosion protection methods, such
as: galvanizing, painting, stainless or
weathering steel were employed.
Steel
• Steel is an alloy made by combining iron and
another element, usually carbon. When carbon is
used, its content in the steel is between 0.2% and
2.1% by weight, depending on the grade. Other
alloying elements sometimes used are manganese,
chromium, vanadium and tungsten.
Corrosion of Iron
• When iron rusts a spontaneous redox
reaction occurs, between the oxygen and
iron. If water is added the rusting occurs
more rapidly.
Oxidation :
Fe(s) → Fe2+(aq) + 2e-
Equation 1
Reduction :
O2(g) + 2H2O(l) + 4e- → 4OH-(aq)
Equation 2
4Fe(OH)2(s) + O2(g) + xH2O(I)→ 2Fe2O3•(x)H2O(s)
RUST
Equation 5
3Fe2+(aq) + 2Fe(CN)6 3-(aq) → Fe3[Fe(CN)6]2 (s) Equation 6
Yellow
Prussian blue
Corrosion of Iron
• Corrosion of iron is called rusting. Rust is
Fe2O3x.H2O.
Many metals like iron, steel, polymers,
ceramic etc are damaged due to some
natural process.
Corrosion is one of them.
http://tdwhs.nwasco.k12.or.us/staff/bfroemming/CorrosionIron.html