014 Hydraulic Elevator

Expt 014 -- Hydraulic Elevator

In an age-old demonstration, a water-filled bottle with a card over its mouth is inverted, and atmospheric pressure is shown to be greater than the water pressure, for the card is held suspended in place. The following activity serves as a logical extension of that demonstration. The question is posed: if atmospheric pressure is greater than the water pressure, then the net force on the card should be upward; why, then, does the card not move upward?

Chemical Concepts

Atmospheric pressure is strong.
Atmospheric pressure may be stronger than hydrostatic pressure if the water column is less than 33 ft.
If a reaction takes place in a series of steps, the rate of the reaction is determined by the rate of the slowest step.

Safety

Use ordinary laboratory safety procedures.

Procedure

Insert the stopper securely into one end of the pipe.
Fill the pipe completely with colored water as you would a tall glass. Have the basin beneath to catch any overflow. You may wish to use a funnel.
Float the test tube in the pipe, and then push it down so that its lip is level with the lip of the pipe. (Some water is pushed out.)
Ask students what will happen if you invert the pipe and let go of the test tube. Then do it.

!!!Click here to See Movie.
!!!Click here to See Movie. Movie is accelerated 5 times the actual speed by time-lapse techniques.
Rate Determining Step Variation
As soon as one test tube rises 10 cm add the test tube with the largest diameter. After that one rises 10 cm add a third test tube. Observe the relative rate of travel. Add additional test tubes if desired. Which test tube is the rate determining step?

!!!Click here to See Movie. Note this movie is NOT accelerated.

Other Variations:

With the elevator gradually making its way to the top, push the stopper slightly to the side, allowing some air to enter. Result: the tube moves downward (along with the column of water. Explanation: Now atmospheric pressure is allowed to "help out" the water pressure, and certainly atmospheric pressure + water pressure > atmospheric pressure alone.
With the elevator climbing, cup your hand over the open end of the pipe, trapping the dripping water. Now the demonstration appears simply as a floating test tube catching a ride with a rising bubble. [But what is the tube floating on, water or air?] Remove your hand, and the same demonstration as before can now be seen as the test tube catching a ride on a rising bottomless bubble. [But what is a "bottomless bubble?"]

Questions

List all the forces acting on the test tube, and indicate the direction in which each force is acting.
What happens when the test tube is released? Why?
Is atmospheric pressure always greater than water (hydrostatic) pressure? Explain.
If the series of ascending test tubes can be likened to the successive steps in a chemical reaction, which test tube would represent the rate determining step?
The following sequence of reactions is proposed:

(All substances are gases.)

NO₂ + NO₂ --> NO₃ + NO
NO₃ + CO --> NO₂ + CO₂

The rate of the reaction is measured and found to be:

Rate = k [NO₂]²

Which step is the rate determining step?

Handout Makeup

Name ___________________________ Class _______

Teacher __________________________

BeckerDemos 014 Hydraulic Elevator

Watch the movies.

Answer the questions.

Curriculum-

This demonstration may be used to illustrate the power of atmospheric pressure.
If multiple tubes of different sizes are used it is a powerful illustration of a rate determining step. Use when discussing the kinetics of reaction sequences.

Activity-

Demonstration, Teacher or Student

The following activity is best suited for a demonstration and is most appropriate immediately after the bottle/card demonstration mentioned above. Once the audience agrees that the card cannot move upward because it is too big and the mouth of the bottle is in the way, then show them this demonstration.

Safety-

Use ordinary laboratory safety procedures.

Time-

Teacher Preparation: 5-10 minutes

Class Time: 5-10 minutes

Materials-

a few drops of food coloring
a long glass (or plastic) pipe (D = 2-4 cm) with solid rubber stopper to fit
1-3 test tubes or vials (See Lab Hints.)
water
a 1- to 2-L beaker or basin
a few permanent marker or colored electrician's tape

Disposal-

Dispose of solutions at the sink. Save the apparatus for reuse.

Lab Hints-

Make sure the test tubes or vials are able to pass through the pipe freely but with a minimum of clearance space. Electrician's tape may be wrapped around the test tube to increase the diameter to just the right size. Colored tape also makes the test tube more visible from a distance.
If the test tubes are the right size without tape, mark them with a permanent marker to make them more visible.
For the rate determining step variation, you may wrap tape around one tube to make it slower than the others if the tubes are the same size. You must have at least 3 tubes for this demonstration but more is even better.

Observations-

Surprisingly, the test tubes move upward, seeming to defy gravity, yet proving that the atmospheric pressure is indeed greater than the water pressure in the pipe. [It is worth pointing out that neither the card/bottle demonstration nor this hydraulic elevator would have the same outcome were the container of water taller than 33 ft or so. That large a column of water would exert a pressure greater than 1 atm.*]
The "confrontation" between the water pressure and the atmospheric pressure, with the card/test tube in the middle can be likened to an arm-wrestling contest. In the card/bottle demonstration, though, the stronger of the two is not able to show its superiority because there is something blocking the way (namely, the neck of the bottle). With the hydraulic elevator, however, this obstacle is removed and the atmospheric pressure is shown to be stronger.

* While doing demonstrations that illustrate the strength of our atmospheric pressure, it is important to emphasize that although atmospheric pressure is powerful, it is not infinitely powerful. The Magdeburg sphere provides a good example: a softball-sized sphere made of two metal bowls, has a vacuum drawn on it. This is usually passed around the room, with the students convincing themselves that it is impossible to pull apart the two halves -- or that if it were possible, it would be due to the incompleteness of the vacuum inside. This is not the case. The vacuum, by definition, is nothing and is therefore capable of doing nothing. It is the atmospheric pressure holding the sphere together, and since atmospheric pressure is finite, it must be possible to pull the sphere apart -- even with a perfect vacuum inside. Note: it becomes a very good exercise for the students to determine how many pounds of force are required to pull apart a Magdeburg sphere, given its diameter and the atmospheric pressure in the room.

Rate Determining Step

The second larger tube rises more slowly than the first tube. A column of water forms between the two tubes and increases in length while the column of air between the 2nd and 3rd tube remains nearly constant. The largest diameter tube is the slowest. All tubes, which follow the largest diameter tube, must ascend as slowly as it does. This demonstration gives a clear picture of a rate determining step. The water building up above the slowest test tube is analogous to the build-up of the particles involved in the rate-determining step. It is an easy jump to discussions of rate determining steps in reaction sequences.

Answers-

Q1. List all the forces acting on the test tube, and indicate the direction in which each force is acting. A1. Gravity (downward), water pressure (downward), atmospheric pressure (upward), also frictional forces begin to work downward once the tube starts rising.
Q2. What happens when the test tube is released? Why? A2. The tube accelerates upward, then reaches a constant velocity. This is due to the strength of the atmospheric pressure, greater than the sum of the water pressure and the gravitational force acting on the test tube.
Q3. Is atmospheric pressure always greater than water (hydrostatic) pressure? Explain. A3. No. Atmospheric pressure (at sea level) averages around 14.7 psi. In other words, a column of air with a base of one square inch and extending to the top of our atmosphere weighs approximately 14.7 pounds. A column of water with a base of one square inch would only have to be about 33 feet tall to have this same weight. Thus, 33 feet of water exerts the same 14.7 psi pressure as our atmosphere. Had the pipe of water been taller than 33 feet, the test tube would have accelerated downward!
Q4. If the series of ascending test tubes can be likened to the successive steps in a chemical reaction, which test tube would represent the rate determining step? A4. In the movies, the second tube is slowest. In your demonstration, it may be a different one.
Q5. The following sequence of reactions is proposed:
(All substances are gases.) NO₂ + NO₂ --> NO₃ + NO NO₃ + CO --> NO₂ + CO₂
The rate of the reaction is measured and found to be: Rate = k [NO₂]²
Which step is the rate determining step? A5. Step "a" is the rate determining step. No dependence on CO concentration is observed. If b were the rate determining step, then the CO concentration would be part of the reaction's rate law.

References-

Thanks to John Mauch, Pasco WA, for the Rate-Determining Step Variation.

Key Words 1-

atmospheric pressure, rate determining step, kinetics