Unit Four: Chemical Reactions and Stoichiometry

Lesson 6: Rate of Chemical Reactions

• Comptencies
• Starter Activities
• Main Activites

Competencies

• Define a reaction rate
• Measure the rate of a chemical reaction
• Describe reaction rates using graphs

Materials

Gas syringe, conical flask, rubber tube, rubber stopper with one hole, magnesium ribbon, diluted hydrochloric acid graph paper

Starter activity ( 7 minutes)

Let students discuss the following issues in groups:

1. Do all chemical reactions have the same speed or different speed? Explain your view point by giving examples
2. Arrange the following processes according to the speed of reaction: rusting of iron, burning of splints of wood, burning of log of wood, fermentation of teff pate, burning of benzene.
3. Do you think that the speed of a chemical reaction has effect on the industrial production of goods? How?

Allow the group representatives to report their findings to the class. Encourage them to discuss and ask each other question. Students can successfully mention that chemical reactions vary in speeds, but may unable to give examples. The second discussion point can provide them a hint to explain how different process occurs at different speeds. Help students to come to the consensus that burning of benzene, burning of splint, burning of log, fermentation (which usually take about three days) and rusting of iron are the correct order of the process in decreasing speed. They should know that the speeds of chemical reactions differ in such a way that some take place immediately as the reactants come in contact, some have moderate speed and others take days and weeks and even more.

Students should also able to explain that the speeds at which chemical reactions take place influence production of industrial products. The more is the time taken during the reaction, the higher is the cost of the product.

Evaluation

How do the speeds of chemical reactions vary?

Main activity (28 minutes)

Measuring speed of chemical reaction

Procedure

1. Provide a conical flask, gas syringe, rubber topper, delivery tube, connector, magnesium ribbon and diluted hydrochloric acid to each group
2. Help them to assemble the apparatus as in the figure.
3. Let them add about 50ml of diluted hydrochloric acid to the conical flask
4. Then drop 5cm magnesium ribbon in the acid and stopper with the rubber stopper attached to the syringe.

   

5.  Record volume of the gas at 20 seconds time interval as follows.

   Time (s)	20	40	60	80	100	120	140	160	180	200	220
   Volume of gas(cm3)
   V/t

6. Draw a graph of time  the (x-axis) against volume of the gas evolved(y-axis)

Analysis

Let the students discuss and respond to the following questions.

1. How the volume does vary in all the 20s intervals? (Constant, increase or decrease)?
2. What does the ratio of the change in volume to change in time represents?
3. How do you compare the speed of the reaction in the first 20s and in the last 20s?
4. Write down a chemical equation for the reaction.
5. What happens to the amount of the reactants as time passes?
6. How about the amount of the product?
7. What do you call the speed with which the amounts of the reactants and products change?

Based on their observations students should give the following responses.

• The volume at different intervals continuously decreases
• The ratio the change in volume to change in time gives the rate of a chemical reaction
• The speed continuously decreases
• Mg + 2HCl → MgCl2 + H2
• The amount of the reactants decreases and the amount of the products increases as time elapsed.
• The speed of change of amount of the reactants and the products are the measures of the rate of chemical reactions.

Conclusion

Encourage students to write down conclusions

1. How do you measure the speed of reaction of magnesium and hydrochloric acid?
2. Does the rate of the reaction of magnesium with hydrochloric acid increase or decrease as time elapses?
3. What is the ratio of change in volume of hydrogen to change in time during the reaction of magnesium with hydrochloric acid?

Generalization

Let them write how the rate of a chemical reaction can be determined and what happens to the reactants and products during the reaction. Let them also interpret the graph.

Evaluation

Students may be asked the following questions:

What is the rate of a chemical reaction? How is the rate of a chemical reaction measured?

Concluding activity (5 minutes)

Let the students write down mathematical expression for the rate of a chemical reaction in terms of change in concentration of the reactants or the products.