Learning Objectives

Oxidation-Reduction Reactions

An oxidation-reduction reaction is a reaction where electrons are transferred between two reactants. Whenever a substance is oxidized, another substance must be reduced. The reverse is also true. When a substance is reduced, another substance must be oxidized. The substance that is oxidized is called the reducing agent. The substance that is reduced is called the oxidizing agent. Electrons are transferred from the reducing agent to the oxidizing agent in these reactions.

Oxidation-reduction reactions include some of the most important chemical changes.

Definitions of Oxidation-Reduction

Historically, oxidations were first defined as the combination of oxygen with some other element or compound and reductions were defined as the loss of oxygen from a compound. This simple definition worked fine with reactions such as the oxidation of lithium and the burning of sulfur and hydrocarbons.

4 Li   +   O₂   ®   2 Li₂O

S   +   O₂   ®   SO₂

CH₄   +   2 O₂   ®   CO₂   +   2 H₂O

The definition also works for the loss of oxygen from a metal oxide in the smelting process.

2 Fe₂O₃   +   3 C   ®   4 Fe   +   3 CO₂

If sodium metal is reacted with chlorine, sodium chloride will be formed as the product. This reaction is an example of an oxidation-reduction reaction that is not covered by the previous definition. The sodium in this reaction is oxidized and the chlorine is reduced but oxygen is not involved at all in the process.

2 Na   +   Cl₂   ®   2 NaCl

The product is the ionic compound sodium chloride. The sodium atom is a cation and the chloride atom is an anion. The sodium metal has given up an electron to the chlorine and this is the modern definition of oxidation.

Oxidation is the loss of an electron or electrons. Reduction is the gain of an electron or electrons.

A simple memory device for these definitions is "LEO (the lion) says GER". Loss of Electrons is Oxidation and Gain of Electrons is Reduction

How can you recognize whether an oxidation-reduction reaction occurred? Simply note whether the atoms involved in the reaction changed their oxidation state or charge. The sodium atom went from a neutral atom to a cation and the chlorine atom went from a neutral atom to an anion.

Oxidation States

The oxidation state or number is a measure of the electric charge that an atom or group possesses. There are rules that can help in determining the oxidation number of an atom or group.

Example Reactions

Let's look at the examples we used for combination reactions.

4 Na   +   O₂   ®   2 Na₂O

In this reaction, the sodium metal went from an oxidation number of zero to an oxidation number of +1. The oxygen atoms both went from an oxidation number of zero to an oxidation number of -2. Therefore the metal lost an electron to the oxygen atom and was oxidized. The oxygen atoms gained electrons from the metal and were reduced.

C   +   O₂   ®   CO₂

In this reaction, the carbon atom went from an oxidation number of zero to an oxidation number of +4. The oxygen atoms both went from an oxidation number of zero to an oxidation number of -2. Therefore the carbon atom lost 4 electrons to the oxygen atoms and was oxidized. The oxygen atoms gained electrons from the carbon atom and were reduced.

Now let's look at the single replacement reaction.

Remember that this is a reaction where one metal reacts with a compound and replaces a different metal in that compound. This behavior is predicted by the activity series.

Some examples of this type of reaction include the following. Notice the relative positions of each metal in the activity series.

Fe   +   CuSO₄   ®    FeSO₄   +   Cu

Sn   +   2 HCl   ®    SnCl₂   +   H₂

Zn   +   2 AgNO₃   ®   Zn(NO₃)₂   +   2 Ag

Electrochemical Cells

Batteries produce electrical energy from oxidation-reduction reactions. A battery is more properly called an electrochemical cell. A collection of electrochemical cells wired in series is properly called a battery. A flashlight battery is really a single electrochemical cell. A car battery is really a battery since it is three electrochemical cells in series. The cathode of an electrochemical cell is the electrode where reduction occurs. The anode is the electrode where oxidation occurs. Electrons flow in a closed circuit from the anode to the cathode. There are many kinds of electrochemical cells such as; zinc-carbon cells, alkaline cells, mercury cells, lead-acid batteries and nickel-cadmium cells.

Completing and Balancing Oxidation-Reduction Reactions

If the reaction is a combination reaction, the question really is just a nomenclature question...what is the formula of the compound comprised of the two elements that are the reactants.

If the reaction is a single replacement reaction...just use the activity series and remember the charge on the metal ion that is in the ionic compound and the ion that will form from the metal.

Problems

1. Identify the oxidizing agent and the reducing agent in the following reaction. Is the iron oxidized or reduced? Is the bromine oxidized or reduced?

2 Fe + 3 Br₂ ® 2 FeBr₃

2. Identify the oxidizing agent and the reducing agent in the following reaction. Is the manganese oxidized or reduced? Is the oxygen oxidized or reduced?

Mn + O₂ ® MnO₂

3. Identify the oxidizing agent and the reducing agent in the following reaction. Is the aluminum oxidized or reduced? Is the chromium in the chromium oxide oxidized or reduced?

Cr₂O₃ + 2 Al ® 2 Cr + Al₂O₃

4. Complete and balance the following chemical reactions.

Na   +   O₂   ® answer

Mg   +   O₂   ® answer

Al   +   O₂   ® answer

Fe   +   O₂   ® _{high valence state of iron} answer

Al   +   Cl₂   ® answer

Fe   +   Br₂   ® _{high valence state of iron} answer

Mg   +   S₂   ® answer

Cu   +   F₂   ® _{high valence state of copper} answer

Ca   +   I₂   ® answer

5. Complete and balance the following chemical reactions using the activity series.

Na   +   HCl   ®   answer

Zn   +   CuSO₄   ®   answer

Mg   +   AlCl₃   ®   answer

Cu   +   MgCl₂   ®   answer

Pb   +   KBr   ®   answer

Mg   +   SnCl₄   ®   answer

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