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Unit Four: Chemical Reactions and Stoichiometry

Lesson 1: Introduction to chemical reaction

Competencies

Material

Acetic acid, sucrose (table sugar), ammonium chloride, NaHCO3 (baking soda), H2SO4, barium chloride BaCl2, Ba(OH)2, test tubes, beakers, spatula, stirrer, test tube holder, goggle

Note for the teacher

The challenge in teaching chemical reaction is to enable students understand the interaction between substances at a particle level. Even if we assist them conduct experiments on chemical reactions, the phenomena could not be clear for the students. They fail to understand that chemical reaction is the rearrangement of atoms, which involves bond breaking and bond formation. They think that in chemical reactions like combustion, there is destruction of matter.

So, we need to use different atomic and molecular models to describe interactions between particles.

This lesson is designed to make students attentive and arouse their curiosity through the whole unit. It enables them to understand the difference between physical and chemical processes clearly.

Starter activity (6minutes)

You may start the lesson by asking the students to list some physical and chemical changes. Let them try to identify the difference between the two changes. From unit two in grade 7 chemistry, they may remember a number of changes like changes of state, melting, boiling, condensing, freezing of water and changes like breaking of clay, tearing of paper, cutting of wood etc as physical changes.

On the other hand, they might mention burning of paper, rusting of iron, souring of milk, fermentation of injera paste etc as chemical changes. You should appreciate any change they mentioned and list them on the chalk board.

Let the students discuss the basic differences and characteristics of each type of changes across their table with friends and forward their idea to the class.
Make a table of the following kind so that students use it to compare and contrast characteristics of the two changes.


Physical change

Chemical change

 

 

Some students may not be able to understand some of the properties of physical and chemical changes. In this case you may help them by explaining points like reversibility, changes in composition, formation of new substances and changes of energy.

You may inform the students that chemical changes involve change in composition and changes in composition can be described by using symbols and formulas.
Example: the rusting of iron,

Iron changes to rust, meaning Fe changes to Fe2O3.H2O by taking oxygen and moisture from the air. In souring of tella, the alcohol (C2H5OH) undergoes oxidation to an acid known as acetic acid (CH3COOH)

Evaluation

What are the characteristics of a chemical change? How do you describe the chemical changes in substances?

Main activity (28 minutes)

The following activities can serve as starter activities for the students to explore chemical reactions. If you have materials and chemicals available for groups of students it will be preferable to make students conduct the activities by themselves. Otherwise, it is possible to demonstrate the reactions so that the students follow up all the steps and record their observations.

Procedure

  1. prepare a worksheet in advance ( if you are unable to  provide a print-out worksheet, you can write the format on the blackboard)
    1. What are the starting materials in the chemical change?
    2. Write down what you expect when the starting materials are subjected to change.
    3. Write down the appearance of the starting materials before the change.
    4.  Write down your observations during the changes. After each reaction, identify any indicators of a chemical reaction— change of color, change of energy, change of odor and evolution of gas.
    5. What are the final products of the changes?
    6. Try to describe the changes using words, symbols and formulas.
    7.  Give reasons for the changes you observed.
    8. Write down your conclusion.
  2. Activities
    1. Baking soda and vinegar (NaHCO3 and CH3COOH)
      1. Set out two beakers.
      2. Pour about two table spoons of vinegar into the first beaker.
      3. Now, take about two spatula of baking soda into the second beaker.
      4. Pour the vinegar from the first beaker into the second beaker.
      5. Watch as the two chemicals react.
      6. Clean up any spills.
    2. Sucrose (table sugar) and sulphuric acid ( C12H22O11 and H2SO4)
      1. Take a tea spoon of table sugar on a watch glass.
      2. Use a dropper to add some concentrated sulphuric acid to the sugar.
      3. Observe and register your observation in your worksheet.

      Caution concentrated sulphuric acid is highly corrosive. Use gloves when you are working with it

    3. Ammonium chloride and barium hydroxide (calcium hydroxide) (NH4Cl and BaCl2 or CaCl2)
      1. Prepare solutions of ammonium chloride (a white paste material in a dry cell battery) and barium hydroxide (calcium hydroxide can be used alternatively) in separate beakers.
      2. Hold a test tube with a test tube holder and add about 15ml of ammonium chloride.
      3. Then slowly add barium hydroxide and observe what happens.
    4. Barium chloride and sulphuric acid (BaCl2 and H2SO4)
      1. Dissolve some barium chloride in a beaker.
      2. Add about 20ml of the solution to a test tube
      3. Add 20ml of diluted sulphuric acid to the barium chloride solution in the test tube.
      4. Now observe what happens.

Encourage the students to complete their worksheet and appreciate the different characteristics of chemical reactions. Let them discuss on the changes they observed.

Evaluation

Concluding activity (6minutes)

What are the characteristics of a chemical reaction? How do we represent a chemical reaction? Define a chemical reaction using your own words.