Grahams gas law


Block 8
April 20th, 2005
To observe how masses of atoms and molecules affect their rate of diffusion, and explain our observations in terms of mass, velocity, kinetic energy and temperature.

Warm-up Question:
1.What is the definition of temperature?
Temperature is a measure of the average Kinetic Energy of the particles composing a material.

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2.What is the definition of diffusion?
Diffusion is a spontaneous spreading of particles until they are uniformly distributed in their container.

3.If we increase the temperature of sample of gas, then what about the gas molecules will have to change?
The gas molecules would begin to increase in speed since the temperature would increase.

4.The element iodine (I2) diffuses more slowly than ethanol (CH3CH2CH2OH). Why is this?
Iodine is heavier in atomic mass then ethanol, slowing down its process to diffuse. Iodine weighs 253.6u while ethanol weighs 60.03u, giving ethanol a faster diffusions rate.

Procedure A:
1.Obtain a glass tube and wash it with water. Dry the outside of the tube with a paper towel. Next, squirt some acetone into the tube. A small amount is all that is needed. Then dry the inside of the tube by passing air through the tube (use a fan, the window, blow through it, etc.)
2.Use chalk to label one end of your lab station, “HC1,” and the other end, “NH3.”
3.Lay the glass tube on the table and place the thermometer on the table next to the tube.

5.Obtain 1 small bottle of HCI and 1 small bottle of NH3. Place each bottle in their respective ends of the lab table. Obtain forceps, label them, “HC1” or “NH3″with tape, and place them at their respective end of the lab table.

6.Record the temperature of the air around the tube in the data table.

7.Separate the cotton ball into pieces small enough to stuff into the ends of the glass tube. Do not put them into the tube yet.
8.While holding the cotton ball with the forceps, place 4 drops of HCl or NH3 (whichever corresponds to the label on the forceps).

9.Repeat step 8 with the other chemical. You should now have two small cotton balls, one with 7 drops of HCI, one with 7 drops of NH3. Keep them at their respective ends of the lab station.

10.Record the time in minutes and seconds.

11.Using the forceps, insert each ball into its respective end of the tube. Do not shove them all the way in – leave a bit hanging out so that you can remove it from the tube when necessary.

12.Observe the tube for a smoke ring to appear. When it does, record the following:
a.The time taken for the smoke ring to appear.

b.The distance (in cm) that the ring was from each cotton ball.

13.Remove the cotton balls from the tube, and dispose of each in the appropriate container.

14.Clean out the tube as in step 1.

Procedure B:
1.This set up is the same as in procedure A but a heat lamp is used to warm the tube. Set the lamp up so that it is shining over the entire tube and thermometer.

2.Record the temperature.

3.Repeat steps 1 -13 from procedure A.

Procedure C:
1.This set up is the same as in A and B, except ice .is used to cool the tube. Obtain a tray with ice in it, cover the ice with a layer of paper towel, place the tube and the thermometer on the ice, and after it has had time to cool, record the temperature of the tube and its surroundings.

2.Record the temperature.

3.Repeat steps 1 – 13 from procedure A.

Procedure AProcedure BProcedure C
Temperature25 C57 C0 C
Time taken for ring to form70 seconds65 seconds120 seconds
Distance from NH312 cm13.5 cm13 cm
Distance from HCl14 cm13 cm12.6 cm
Follow-up Questions:
1.Rank the three procedures in terms of time from fastest to slowest to form the ring.

Fastest – B
Middle – A
Slowest – C
2.In procedure B, the temperature of the gases should have been higher that in procedures A and C. Refer to warm-up question #1 and rank the three procedures in terms of kinetic energy from highest to lowest.

Highest – B
Middle – A
Lowest – C
3.What two properties of the particles of a gas are responsible for its kinetic energy?
The two properties of the particles of a gas that are responsible for its kinetic energy are mass and velocity.

4.Which of the two properties you listed in #3 must have been changed in these three procedures? Explain your answer.

Out of the two properties the only one that could change was velocity. Mass cannot be changed because it already has a defined weight. When velocity changes that means there was an increase or decrease in temperature. Mass has not changed though but it does play a major role in how fast the gas diffuses.
5.Did the ring generally form closer to the NH3 or the HCl? Explain why this is so.

The ring generally formed closer to the HCl because of its mass. HCl is heavier in mass than NH3 which causes the HCl to diffuse slower in the tube. The ring formed where the two gases met and we can see that the NH3 diffused or traveled faster in the tube because it was farther away from its start position.

6.Which gas diffuses more quickly? Why?
As I stated in question # 5 the NH3 diffused more quickly and was farther from the start position because the NH3 was lighter then the HCl and the rate of its diffusion is related to the mass. Lower Mass = Higher Diffusion or Higher Mass = Lower Diffusion.

7.If we were to replace the NH3 with NaOH, would there be any difference in the position of and time to from the ring?
NH3 weighs 17.01 u while NaOH weighs 39.99 u and to repeat the experiment with HCl which weighs 36.45 u, there would be a definite difference because ring would be practically be in the center between the two. Since the two gases weigh almost the same they will diffuse at similar rates. The rate would be very slow for both to diffuse and cause the ring not to appear for a long period of time.

8.If you wanted to make the ring appear exactly in the middle of the tube between the two cotton balls, what could you do? You can change any part of the lab procedure or materials for your answer.

In order to accomplish this goal the gases would have to be exactly equal in mass. If this is accomplished the two gases will diffuse at the same rate causing the gases to travel in the tube at the same speed and make the ring appear in the center of the tube.
9.When wearing perfume, women usually place it on their pulse points, where the blood is a little closer to the skin surface than most body parts. Why do they place it there?
The reason to place perfume on pulse points is because we are warm blooded and when perfume is placed on a pulse point the heat from the pulse and blood will cause a higher kinetic energy. When the temperature and kinetic energy is raised the scent will spread faster. We can compare this to our experiment under the heated lamp. The higher temperature excited the gas particles and allowed for a higher kinetic energy to speed up the process of diffusion for both gases.

10.You may have noticed that in the winter, there aren’t many smells outside, but in the summer, you can smell thousands of different things in one day. Why is that?
In the winter the temperature is much lower than in the summer. Because of the lower temperature the particles from all the smell around us slow down because the particles do not have as much kinetic energy and are not excited as much to spread everywhere. Now in the summer, when the heat really comes, we can smell hundreds of different things because the particles are excited and their velocity has increased due to the temperature increase so that they can diffuse further, faster.
This lab was simple to understand. We had to find out how the mass and temperature of the atoms and molecules affected diffusion. When the temperature was changed the diffusion rated changed because either they got excited in the heat or slowed down in the cold. Now for mass in did not change but it did affect how a certain gas was affected by the temperature change and compared to another gas that was of less or greater mass. We were told that we should already no the out come of the lab because of common sense that we placed in freezing the particles should slow down and when placed under a hot lamp the particles should get excited. After experimenting and recording the data Graham’s Law stays strong that “the relative rates at which 2 gases under identical conditions of temperature and pressure will diffuse, varies inversely as the square roots of the molecular masses of the gasses.” (Notes) As for error in the lab, there was a couple that could not be prevented. One the time of insertion could have been slightly off, the timing until the ring appeared could have gone wrong, and the reaction to smoke ring could have been delay as well. For the rest of the lab everything seemed to go smoothly without a problem. I can now see how all the notes and formulas are coming together and they all make sense as we go through them.


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