CHEMISTRY 4
CHAPTER 19
OXIDATION-REDUCTION (REDOX) REACTIONS
1 ELECTRON-TRANSFER REACTIONS
Let us consider a system which consists of two beakers.
One of the beakers contains a solution of copper sulfate and a copper rod
The other beaker contains a zinc sulfate solution and a zinc rod.
A connection is made between the two solutions by means of a salt bridge, a strip of
paper containing a solution of potassium sulfate.
When the two metallic rods are connected, zinc rod starts to dissolve
Zn (s)  Zn2+ (aq) + 2e( at the zinc rod)
At the copper rod, copper (II) ions leave the solution and metallic copper is deposited.
Cu2+ (aq) + 2e-  Cu (s)
( at the copper rod)
The overall oxidation-reduction reaction can be separated into two half reactions.
Zn (s)  Zn2+ (aq) + 2e( at the zinc rod)
Cu2+ (aq) + 2e-  Cu (s)
( at the copper rod)
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Zn (s)
+
Cu2+ (aq)
 Zn2+ (aq) + Cu (s)
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The process of loosing electrons is called oxidation. The chemical change that occurs
at the zinc electrode is oxidation, zinc is oxidized to Zn2+.
The reduction is a gain of electrons. Copper (II) cations are reduced to Cu at the
copper rod.
2 ELECTROLYTIC AND VOLTAIC CELLS
A voltaic cell (galvanic cell ) is a device in which chemical energy is changed to
electrical energy. The reaction in the voltaic cell occurs spontaneously.
The electrode where oxidation occurs is called anode. In the above example the
chemical change that occurs at the zinc electrode is oxidation, so zinc rod is the anode of
the electrochemical cell. When zinc ions leave the electrode, the electrons stay in the
metal, so zinc electrode is negative
The electrode where the reduction occurs is called cathode. Copper (II) cations are
reduced at the copper rod, so copper electrode is the cathode. The copper electrode gives
electrons to copper cations, so the copper electrode is the positive electrode.
Electrons flow through the external circuit from negative electrode (anode) to positive
electrode (cathode).
An electrolytic cell is a device where an external electric source is used to carry out a
chemical change.
Example: Electrolysis of liquid sodium chloride. Two electrodes are connected to an
external electric source and dipped in liquid sodium chloride.
Sodium ions Na+ are attracted to the negative electrode, accept electrons from the
electrode and are reduced to sodium. The reaction is reduction so the electrode is called
cathode.
Na+ + e-  Na
Chloride ions Cl- are attracted to the positive electrode, where they are oxidized to
chlorine gas
2 Cl-
+ 2 e- 
Cl2
The reaction at the positive electrode is oxidation, so the positive electrode is an anode.
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3 OXIDATION NUMBERS AND REDOX REACTIONS
Electron-transfer reaction is called reduction-oxidation or redox reaction.
The reactant that gains electrons is called the oxidizing agent (oxidizer).
The reactant that loses electrons is called the reducing agent (reducer ).
In the reaction, the oxidizing agent is reduced, and the reducing agent oxidized.
To determine which species has gained and which has lost electrons, it is useful to
determine the oxidation states, or oxidation numbers of reactants and products.
The rules for determination of oxidation numbers:
The oxidation number of any elemental substance is 0
The oxidation number of a monatomic ion is the same as the charge on the ion
The oxidation number of combined oxygen is -2, except in peroxide(-1), superoxide(1/2), and OF2 (+2)
The oxidation number of combined hydrogen is +1(except hydride ion , H- ).
In any molecular or ionic species the sum of the oxidation numbers of all atoms in a
molecule or polyatomic ion is equal to the charge on the species.
Oxidation is an increase in oxidation number.
Reduction is a reduction in oxidation number.
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4 STRENGHTS OF OXIDIZING AGENTS AND REDUCING AGENTS
The strength of an oxidizing agent is measured by its ability to accept electrons. Strong
oxidizing agent has a strong attraction for electrons. The strength of a reducing agent is
measured by its ability to to give up electrons. A strong reducing agent releases electrons
readily.
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5 PREDICTING REDOX REACTIONS
A strong oxidizing agent has a strong attraction for electrons. In an redox reaction, the
strong oxidizing agent is reduced and became a weak reducing agent.
A strong reducing agent releases electrons readily. In a redox reaction the strong
reducing agent is oxidized and became a weak oxidizing agent.
The reaction between a strong oxidizing agent and a strong reducing agent to produce a
weak oxidizing agent and a weak reducing agent is spontaneous.
(strong oxidizing agent)1
+
(strong reducing agent)2 --->
(weak oxidizing agent)2 + (Weak reducing agent)1
The strength of a oxidizing agent can be measured by the value of its standard reduction
potential.
The standard potential of the reduction of hydrogen cation to hydrogen gas is assigned a
value zero to the scale.
The more positive the value of the standard reduction potential the stronger the oxidizing
agent.
The more negative the standard reduction potential, the stronger the reducing agent.
6 WRITING REDOX EQUATIONS
1 Identify the element oxidized and write the oxidation half reaction.
Balance the atoms by adding water molecules and hydrogen ion when necessary.
Balance the charge by adding electrons to the more positive side.
Recheck the equation to be sure that it is balanced in both atoms and charge.
2 Identify the element reduced and write the reduction half reaction.
Balance the atoms by adding water molecules and hydrogen ion when necessary.
Balance the charge by adding electrons to the more positive side.
3 Multiply both half reactions by the necessary coefficients so each half reaction transfers
the same number of electrons. Add the two half reactions and simplify both sides.
4 Check that the atoms and charges are balanced.
Example: Reaction in acidic solution between Fe2+ and NO3Fe2+ + NO3-  Fe3+ + NO
Example: Reaction in acidic solution between Fe2+ and Cr2O72Fe2+ + Cr2O72-  Fe3+ + Cr3+
Example: Reaction in acidic solution between Cl- and MnO4Cl- + MnO4-  Mn2+ + Cl2
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