Transcript Ch. 5 Oxidation and Reduction

Oxidation and Reduction
Overview
• Oxidation and reduction reactions always
occur together (redox reactions)
• You can’t have one without the other
• Includes: extracting metal from an ore
• Salt on roads
– Salt accelerates the reaction between oxygen
and metal
– Makes electron transfer easier
Oxygen
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Most abundant element on Earth
21% of our atmosphere (pure)
89% of Earth’s water (compound)
45.5% of Earth’s crust (compound)
2/3rd’s of the human body (compound)
Foods (compound)
Rust
Chemical Properties of Oxygen
• Combines with many elements and
compounds
• Rust (reacts with metals)
• Other oxides (reacts with nonmetals)
• Methane (reacts with other compounds)
– Bunsen burner
Hydrogen
• In the universe, 95% of all atoms are
hydrogen
• Seldom found alone
• Hydrocarbons
• Hindenburg…
3 ways of looking at oxidation and
reduction
• 1. oxidation is a gain of oxygen atoms,
reduction is a loss of oxygen atoms
• 2. oxidation is a loss of hydrogen atoms,
reduction is a gain of hydrogen atoms
• 3. oxidation is a loss of electrons,
reduction is a gain of electrons
– Most fundamental explanation, what we will
be dealing with the most
Oxidation
Reduction
Gains oxygen
Lose Oxygen
Oxygen
Lose Hydrogen
Gains hydrogen
Hydrogen
Lose Electrons
Gains electrons
e-
In relation to our lab…
• Remember that reduction is a loss of
oxygen from a compound
• We converted iron ore to metallic iron
– We removed oxygen from the iron(III) oxide
– 2Fe2O3 + 3C  4Fe + 3CO2
– Carbon was oxidized because it gained an
oxygen
Leo the Lion!
• LEO the lion says GER
– Loss of electrons is oxidation, gain of
electrons is reduction
Examples
• Is the reactant oxidized or reduced?
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Pb  PbO3
SnO2  SnO
KClO3 KCl
C2H6O  C2H4O
C2H2  C2H6
Pertaining to LEO…
• Mg + S  MgS
• Mg + S  Mg2+ + S2• Magnesium is oxidized
– Said to be the reducing agent
– Substance in the reaction that loses electrons
• Sulfide sulfur atom is reduced
– Said to be the oxidizing agent
– Substance in the reaction that gains electrons
Oxidation Numbers
• A count of the electrons transferred or
shared in the formation or breaking of
chemical bonds
• You must assign each element in the
reaction an oxidation number
• Follow a set of rules…
Oxidation Number Rules
1. The total of the oxidation numbers of all
the atoms in a neutral molecule, an
isolated atom, or a formula unit is 0
– Examples
2. In their compounds, the Group 1A metals
all have an oxidation number of +1, and
the Group 2A metals have an oxidation
number of 2+.
– Examples
Rules Con’t
3. In its compounds, hydrogen has an oxidation number of
+1 (except in metal hydrides such as NaH, where it is
-1)
– Examples
4. In its compounds, oxygen has an oxidation number of -2
(except in peroxides such as H2O2, where it is -1)
– Examples
5. In their binary compounds with metals, Group7A
elements have an oxidation number of -1. Group 6A
elements have an oxidation number of -2, and Groups
5A elements have an oxidation number of -3.
– Examples
Problems
• What is the oxidation number of each
element?
• I2
• Cr2O3
• AlCl3
• Na2SO4
• CaH2
Identifying Redox Reactions
0 +3 -2
0 +3 -2
• 2 Al + Fe2O3  2 Fe + Al2O3
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Al increases from 0 to +3, it is ______
Oxidized!
Fe decreases from +3 to 0, it is _______
Reduced!
Problems
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Zn  Zn2+
Fe3+  Fe2+
CaCO3  CaO + CO2
AgNO3  Ag
• Do Practice Exercises
Oxidizing and Reducing Agents
• Now the confusing part…
• CuO + H2  Cu + H2O
• Cu goes from +2 to 0
– Cu is reduced, therefore it is called an oxidizing
agent because it causes some other substance to be
oxidized
• H goes from 0 to +1
– H is oxidized, therefore it is called a reducing agent
because it causes some other substance to be
reduced.
Identifying Agents in an Equation
Reduction: CuO is
the oxidizing agent
• CuO + H2  Cu + H2O
Oxidation: H2 is
the reducing
agent
Some common oxidizing agents
• Oxygen!
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Oxidized coal in electric power
Gas in automobiles
Wood in campfires
Food we eat
• Antiseptics
– Hydrogen Peroxide
– Benzoyl peroxide
• Disinfectants
– Chlorine
Some common reducing agents
• Metals
• Antioxidants
– Ascorbic acid is used to prevent the browning
of fruits by inhibiting air oxidation
– Many antioxidants are believed to retard
various oxidation reactions that are potentially
damaging to vital components of living cells