Redox Reactions: Oxidation and
Reduction
I. ELECTRON TRANSFER AND REDOX
REACTIONS
• A. Reactions have the ability to gain or lose
electrons when reacting with other elements
• B. Transfer of electrons from one atom to
another
– 1. 2Na + Cl2  2Na+ + 2Cl– 2. electron is transferred from the sodium atoms
to the Cl2 molecule
• C. Magnesium Oxide
1. 2Mg + O2  2MgO
2. Each magnesium transfers 2 eTo each oxygen atom
• D. Oxidation – Reduction reaction- a reaction
in which electrons are transferred from one
atom to another aka REDOX REACTIONS
Consider this: Single-Replacement
• 2KBr + Cl2  2KCl + Br2
– Re-write as net ionic equation
– 2Br- + Cl2  Br2 + 2Cl• Chlorine gets electrons from bromine
• When bromine loses electrons, the two form a covalent
bond to make Br2
• Formation of covalent bond by sharing electrons is also
a redox reaction
How do oxidation and reduction
differ?
• Oxidation- loss of electrons from atoms of a
substance
– Na  Na+ + e– Sodium is oxidized because it loses an electron
– For oxidation to take place the electrons lost must be
accepted by another substance
• Reduction- the gain of electrons by atoms of a
substance
– Cl2 + 2e-  2Cl– Chlorine is reduced because it gains electrons
• LEO the lion says GER
– LEO GER
– Loss of Electrons is Oxidation, Gain of Electrons is
Reduction
Changes in Oxidation Number
• Oxidation number- the number of electrons lost
or gained by an atom when it forms ions
– Complete: 2K + Br2  2KBr
– Net: 2K + Br2  2K+ + 2Br– Each Potassium loses 1 electron so its oxidation
number is +1. It changes from 0 to +1.
– Each Bromine gains 1 electron so its oxidation number
is -1. It changes from 0 to -1.
• When an atom or ion is reduced its numerical
value or oxidation is reduced, which is why its
called reduction.
II. OXIDIZING AND REDUCING
AGENTS
• Oxidizing agents- the substance that oxidizes
another substance by accepting its electrons
• Reducing agent- the substance that reduces
another substance by losing its electrons
oxidized
• 2K + Br2  2KBr
reduced
• Bromine is the oxidizing agent, potassium is
the reducing agent
III. REDOX AND
ELECTRONEGATIVITY
• Some redox reactions involve changes in
molecular substances or polyatomic ions, in
which atoms are covalently bonded to other
atoms
– Ammonia N2 + 3H2  2NH3
– Does not evolve ions or obvious electron transfers
– Nitrogen is the oxidizing agent and hydrogen is the
reducing agent
– The more electronegative atom is reduced by
gaining electrons
IV. DETERMINING OXIDATION
NUMBERS
Rules for determining oxidation
numbers
• 1. the oxidation number of an uncombined atom
is 0.
– O2, Cl2, etc
• 2. the oxidation number of a monoatomic ion is
equal to the charge on the ion
– Ca2+ is +2.
• 3. the oxidation number of the more
electronegative atom in a molecule or a complex
ion is the same as the charge it would have if it
were an ion.
– NH3 – N is more electronegative and has a charge of -3
• 4. the most electronegative element, fluorine,
always has an oxidation number of -1 when it
is bonded to another element
• 5. The oxidation number of oxygen in a
compound is always -2, except in peroxides,
such as hydrogen peroxide (H2O2) where it is 1. When it is bonded to fluorine, the only
element more electronegative than oxygen,
the oxidation number is +2.
• 6. The oxidation number of hydrogen in most
of its compounds is +1.
– Exceptions: LiH, NaH, CaH and AlH3, it is -1
• 7. The metals of groups 1A and 2A and
aluminum in group 3A form compounds in
which the metal atom always has a positive
oxidation number equal to the number of its
valence electrons (+1, +2, +3 respectively)
• 8. The sum of oxidation number in a neutral
compound is 0.
• 9. The sum of oxidation number of the atoms
in polyatomic ions is equal to the charge on
the ion.
– NH4+ , N -3 H +1(4) = +1
Use the rules to determine the
oxidation numbers of each element
• KClO3
• SO32-
V. OXIDATION NUMBER IN REDOX
REACTIONS
• 2KBr + Cl2  2KCl + Br2
– Assign numbers to all elements in the balanced
equation
– Determine the change in numbers
– Determine either oxidation, reduction, or none