Expt 028 -- Gas Diffusion: Graham's Law

Description

Hydrogen chloride and ammonia gas are mixed in a closed container. By measuring the distance between the points of origin of the two gases and the place where the formation of solid ammonium chloride is first noticed, a quantitative relationship between average speed and molar mass is confirmed.

Background

Safety

Concentrated hydrochloric acid and concentrated ammonia are toxic and corrosive; their fumes are toxic and noxious. Avoid inhaling the fumes. Work in a well ventilated room. Wear goggles and apron. Wash spills immediately. Wash hands after experiment.

Procedure

  1. Place a 12-cm clear plastic ruler on a dark background. Obtain pipets of concentrated HCl and NH3, a marking pen, and a plastic cover from a 96-well plate. Use only one drop of each reagent.
  2. You must work quickly once you begin. Both solutions must be added at the same time. Read all of the instructions before you begin.
  3. Avoid parallax errors by viewing from directly above while marking the lines. Viewing from the side will introduce errors.
  4. Using a small plastic transfer pipet, simultaneously place 1 drop of concentrated HCl on the 1.00 cm line, and 1 drop of concentrated ammonia on the 11.00 cm line. Quickly cover the ruler with the plastic cover from a 96-well plate.
  5. After a relatively short time, solid white NH4Cl forms inside the plastic cover. In fact, the solid is a remarkably sharp line of demarcation. Mark that point when the solid FIRST appears. Watch carefully to note this first point. Align the center of this line with the white front. Thermal currents will cause the line to swirl and mix with time. The line will disperse, broaden, and drift with time. Only the first point is significant.
  6. Use a marking pen to mark the exact location of the drops under the plastic cover. The lines from markers have considerable width. Align the edge, which is closest to the center, with the drop to minimize linewidth problems.
    !!!Click here to See Movie.
  7. Wet a paper towel. Remove the cover and quickly dab each spot of concentrated reagent with the wet towel. Wipe up the chemicals. Rinse the towel at the sink. Discard the wet towel with ordinary solid trash.
  8. Measure the distance between each starting point and the spot where solid first forms.
    !!!Click here to See Picture.
  9. Determine the ratio of these distances. Compare this ratio to the inverse square root of the molar masses of HCl and NH3.

Questions

  1. Is the solid that is formed physically closer to the HCl or the NH3 droplet? Explain.
  2. Suppose HCl is replaced with HBr, but all other things are equal. Predict the ratio of distances expected. Indicate whether the solid formed will be closer to or further from the center than in the HCl/NH3 cases.
  3. Suppose small drops of bromthymol blue, a pH indicator with a color change near pH 7, were placed along the ruler. Predict the outcome of repeating this experiment.

Handout

Name ___________________________ Class ________

Teacher__________________________

SmallScale 028 Gas Diffusion: Graham's Law

Makeup students should read the distances from the picture. Be sure to measure to the center edge of the lines marking the drops.

NH3 distance ___________
HCl distance ___________
Ratio of the distances ___________
Ratio of inverse square root of molar masses ______
% difference between the ratios above ____________

Curriculum-

This experiment fits well when gases are discussed. The activity can be performed as a demonstration on an overhead projector when clear, colorless rules are use. The activity also can be used when acids and bases are discussed. Sometimes students notice a white solid on the outside of hydrochloric acid bottles, and the experiment can be performed after a student makes this "discovery."

Safety-

Time-

Teacher Preparation: 5 minutes

Class Time: 15 minutes

Materials-

Disposal-

Because the amounts are so small, and the gathering procedure mixes acid with base, disposal at the sink with very large amounts of water is appropriate. Discard the wet paper towels with ordinary solid trash.

Data Analysis-

The experimental result is remarkably close to that predicted from the square root of the ratio of the molar masses.

Answers-

Q1. Is the solid that is formed physically closer to the HCl or the NH3 droplet? Explain.
A1. The spot is closer to the HCl. This molecule is more massive than is NH3, so it moves more slowly.
Q2. Suppose HCl is replaced with HBr, but all other things are equal. Predict the ratio of distances expected. Indicate whether the solid formed will be closer to or further from the center than in the HCl/NH3 cases.
A2. The ratio is (80.9/17)0.5 = 2.2. This is a larger number than for HCl (36.5/17)0.5 = 1.47. The spot is further from the center than the HCl/NH3 cases. HBr does not travel as far in the same amount of time.
Q3. Suppose small drops of bromthymol blue, a pH indicator with a color change near pH 7, were placed along the ruler. Predict the outcome of repeating this experiment.
A3. The acid color should appear near the HCl. The base color should appear near the NH3. The intermediate color should appear at roughly the predicted ratio (1.5) -- 40% of the distance between the spots but closer to the HCl side.

CoopLearn-

Average the experimental ratios obtained from all of the groups in the class.

Literature Data-

Graham's law is complicated by the presence of air molecules and the concentration of the concentrated reagents. For a discussion of the issues, see Shakhashiri, Chemical Demonstrations, University of Wisconsin Press, Madison, WI, 1985, volume 2, section 5.15, pp. 59-62.

Key Words 1-

gas, diffusion, Graham's law, relative speed

Elements-

H Cl N