1
Chemistry
Redox Reactions
Redox Reactions: are oxidation-reduction reactions (similar to acid base reactions).
Oxidation-Reduction Reaction: is a reaction that involves the transfer of electrons (e-).
Oxidation: Loss of electrons
Reduction: Gain of electrons
*Note: Each reaction by itself is considered a half reaction because 2 half reactions are
needed to form a whole reaction (redox reaction).
Redox Reaction
Example:
Half reaction (1)
Cu(s) → Cu+2(aq) + 2e-
The half reaction here tells you that solid copper (Cu(s)) is being oxidized, losing an e-, to
form the copper ion with a plus two charge (Cu+2).
*Consider that:
(a) The reaction is balanced on both sides
(b) We have copper on both sides
(c) The charges are balanced
The e- means that there is a free electron with a negative charge which can reduce some
other species in the next half reaction.
Half Reaction (2)
2Ag+ (aq) + 2e- → 2Ag(s)
This second half reaction tells you that two silver ions (2Ag+) are being reduced through
the summation of two electrons which will form solid silver (2Ag(s))
Both of the half reactions (1&2) can then be combined to form a redox equation.
Cu(s) → Cu+2(aq) + 2e+ 2Ag+ (aq) + 2e- → 2Ag(s)
Cu(s) + 2Ag+ (aq) + 2e- → Cu+2(aq) + 2Ag(s) + 2e-
*Note: when a chemical oxidizes another substance it is considered to be the oxidizing
agent, and oxidants get reduced by a reducing agent.
Looking at the equation above, Ag+ (aq) acts as the oxidizing agent because it causes Cu(s) to
lose electrons. Therefore, Cu(s) is the reducing agent and causes Ag+ (aq) to gain electrons.
Hence, in a redox reaction you have the oxidizing agent and the reducing agent in the same
side of the equation.
2
Chemistry
Basic Steps:
1. Separate the equation into a half oxidation and a half reduction reaction.
2. Balance:
(a) The elements other than H and O
(b) The O by adding H2O
(c) The charge by adding e3. Multiply each half reaction by an integer in a way that the number of e- lost in one
half equation equals the number gained in the other half equation.
4. Combine the reactions and cancel
5. Add OH- to each side until all H+ are gone and then cancel the equation again.
How oxidation numbers of a compound work
Oxidation Number: A number assigned to each atom to help keep track of the electrons
during a redox reaction.
*An increase in the oxidation number corresponds to oxidation; a decrease in oxidation
corresponds to reduction.
In general, the oxidation number gives an idea of the strength or tendency of the compound
to be oxidized or reduced. Given this information, we can decide which best serves as the
oxidizing agent or the reducing agent.
Rules
1.
2.
3.
4.
5.
Atoms in their elemental form, the oxidation number is 0
For ions, the oxidation is equal to their charge
For single Hydrogen, the number is +1 but can be -1in some cases
For oxygen, it is usually -2
The sum of oxidation number of all the atoms in the molecule, or ion, is equal to its
total charge