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Unit Four: Chemical Reactions and Stoichiometry

Lesson 5: Oxidation reduction reaction

Competencies

Materials: rusted iron materials, salt solutions of less active metals preferably copper sulphate or silver nitrate and metal strips like iron, zinc and Al.

Starter Activity (6 minutes)

Let the students’ brain storm about rusting of iron. Ask them why does iron rust.

After discussing round their table let them share their ideas to the class.

You may write down students’ ideas on the chalk board. Challenge the students with the points they forwarded by asking them to reason out their views. You may remind the students how metallic elements react during the formation of ionic compounds.

Let them explain what exactly happens to the metal atoms during the reaction. This may lead them to understand the process of electron losing and gaining.

Evaluation

Is rusting of iron physical or chemical change?

Does iron lose or gain electrons during rusting?

What will happen to the charge of an iron atom?

Feedback

Students know from their lower grades that rusting is a chemical change. They may also know that rusting is the reaction of iron with oxygen. As this reaction is an example of a reaction between a metal and a non-metal, students may tell that iron loses electrons in the process and develops a positive charge.

Main Activity (28 minutes)

Demonstration of oxidation reduction reactions

Procedure

  1. Fill a test tube with a solution of copper sulphate or silver nitrate to a one third.
  2. Insert one third of an iron strip/nail into the solution.
  3. Wait for about three minutes and observe what happens.
  1. What change is observed on the part of the iron nail that is immersed in the solution?
  2. Why do you think this change happens?
  3. Is the change chemical or physical?
  4. Describe the change using a chemical equation.
  5. Does this reaction resemble the displacement reaction you studied in types of reactions? Explain.

After students discussed and responded to the questions you can use the opportunity to explain the electron transfer process. Definitions can be given for oxidation, reduction, oxidizing agent and reducing agents in terms of copper ion and iron atom.

You may start your explanation with the observation. When Zn or Fe is introduced in the solution of CuSO4, the observations are:

CuSO4 (aq)+ Zn(s) → ZnSO4(aq) + Cu (s)

To show the change in the charge or oxidation number of the species, you can write ionic equation for the reaction.

Cu+2SO4-2 (aq) + Zn0(s) → Zn+2SO4-2(aq) + Cu 0(s)

Now identify the actual changes that occur in the reaction.

  1. Cu+2   → Cu0
  2. Zn →  Zn+2

Let the students try to explain the meanings of these changes. After they forwarded their opinion, explain that a decrease in charge/oxidation number of the ions means an acceptance of electrons and an increase in the oxidation number is a loss of electrons. The electrons lost by one of the species of a chemical reaction are accepted by the other species.

Therefore, the species which gain electron or whose oxidation number decreased are said to be reduced. In our particular example Cu+2 is reduced because its oxidation number is reduced from +2 to 0. This happens because copper ion gained two electrons. The process is called reduction.

On the other hand, the species which loses electrons or whose oxidation number increased is said to be oxidized and the process is known as oxidation. Zn is changed to Zn+2 by losing two electrons or its oxidation number is changed from 0 to +2.

The students should then understand lose of electrons is oxidation and gaining of electrons is reduction.

Let again students discuss the other aspect of the oxidation and reduction.   Ask them what caused copper ion to be reduced and what caused zinc ion to be oxidized.

What caused Cu+2 to be reduced? Or what donated electrons to Cu+2 to make Cu0? Help the students to point out that it is Zn atom which gave two electrons to Cu+2 to make Cu0. A species which causes reduction of the other species by giving electrons is called a reducing agent. Therefore, Zn is a reducing agent in the above example.

What caused Zn to be oxidized? Or what takes its two electrons so that it becomes Zn+2?  by the same token students can mention it is Cu+2 which accepted the two electrons. Therefore, a species which accepts electron of the other species to make it oxidized is called oxidizing agent. As the result Cu+2 in the above example is an oxidizing agent.

Students should also be aware that whenever there is a donor of an electron, there is always an acceptor of an electron. In the other words, oxidation can never happen in the absence of reduction. Oxidation and reduction occur simultaneously in the same reaction and the reaction is called oxidation reduction reaction or in short redox reaction. You can note the reaction equation for the above activity.

CuSO4 (aq)+ Zn(s) → ZnSO4(aq) + Cu (s)

To avoid misunderstanding, assist students to bear in mind that there are two reactants that play opposing roles in the redox reaction. One is the reduced substance which causes oxidation of the other and called oxidizing agent. The second is the oxidized substance which causes the other substance to be reduced and called reducing agent.

Evaluation

Let the students define the following in terms of electron transfer.

Concluding activity

Let the students review their lesson and classify the following terms into the two categories, oxidation and reduction.