Beacon Lesson Plan Library

The Great Gas Race

Coleen Pemberton

Description

Students improve their understanding of Graham's Law by using properties of gases to evaluate the rate of effusion of two compounds as they vaporize.

Objectives

The student knows that connections (bonds) form between substances when outer-shell electrons are either transferred or shared between their atoms, changing the properties of substances.

The student knows the difference between an element, a molecule, and a compound.

Materials

Sets of the following for each lab team:
-Glass tubing 40 - 60 cm in length and 10 - 15 mm in diameter
-Small cotton balls or Q-tips
-Glass marking pen
-Chemical forceps
-2 watch glasses
-Rubber stoppers to fit tubing ends
-Concentrated NH3 in dropper bottle
-Concentrated HCl in dropper bottle
-Metric ruler

Preparations

1. Use a funnel to transfer concentrated HCl and NH3 into enough sets of dropper bottles for each team of two students.
2. Label the dropper bottles with Con. HCl or Conc. NH3, the date of preparation, and your initials.
3. Cut sections of glass tubing as close to 50 cm in length as possible in the appropriate quantity for your students. Be sure to flame the ends so no students get cut.
4. Arrange to have cotton balls or Q-tips in enough quantity that the procedure can be run a second time if you wish students to take averages.
5. Perform the lab before your students do to be sure the formation of ammonium chloride is visible through the selected tubing and to have an idea of the distances and ratios they should record and calculate.
NOTE: If time or supplies are limited a large scale demonstration can also be used.

Procedures

SAFETY ALERT: Before beginning this lab be sure you are wearing goggles and working under a fume hood. Concentrated ammonia and hydrochloric acid can cause painful burns if they come in contact with your skin.

Introduction: Effusion of a gas is a process that occurs when a gas escapes through a small hole or pathway in its container. According to Graham's Law of Effusion the rate of effusion of a gas is inversely proportional to the square root of its formula mass. This same relationship is also true of diffusion.

Student Instructions:
1. Obtain a piece of glass tubing from your teacher and make sure it is completely dry.

2. Separate the cotton ball into two pieces or break your Q-tip in half.

3. Place your cotton to be used into either end of the tubing and make a mark with your glass marker to indicate the starting line for the great gas race.

4. Take the cotton back out and place one piece on each of the two watch glasses.

5. You will need to coordinate the next step with your partner. One partner places 5 drops of NH3 on one cotton ball while the other partner does the same procedure with the HCl.

6. Using chemical forceps quickly place the cotton balls back onto their starting line at the same time and stopper the ends.

7. Hypothesize which gas you think will move faster while you watch the tube for any sign of change.


8. When a white ring appears mark the point with your glass marker. The new compound formed at the point where the two gases meet is called ammonium chloride.

9. Remove the stoppers and cotton balls and place the cotton balls in the waste container designated by your teacher.

10. Measure the distance traveled by each gas in cm.

11. Calculate the ratio between the two gases.

12. Use Graham's Law to determine which gas actually moved faster. NOTE: Graham's law of effusion states that the rate of effusion of a gas is inversely proportional to the square root its molecular mass.

13. Clean your tubing with a buret brush, pipecleaner, or acetone (NOTE: acetone is very flammable). Return all materials to the appropriate location and leave your lab station clean and neat.

Teacher Instructions:
14. Debrief by discussing the activity as a class prior to having students complete a lab report (see Assessment).
Make sure that the discussion includes the difference between an element, a molecule, and a compound, and reviews how bonds are formed.

Assessments

Students create and complete a laboratory report with a Title, Purpose, Hypothesis, Experimental procedure summary, Data, Data Analysis, and Conclusion. The data and analysis if correctly done should reflect confirmation of Graham's Law. A valid, justifiable conclusion will reflect that ammonia should effuse about 1.5 times faster than gaseous HCl.

Through the lab report, the teacher should find evidence that students know the difference between an element, a molecule, and a compound, and that students know that connections (bonds) form between substances when outer-shell electrons are either transferred or shared between their atoms, changing the properties of substances.

Students who do not demonstrate this will need additional feedback and more instruction.

Web Links

Web supplement for The Great Gas Race
Think Quest

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