Hazards and Disposal Proper precautions should be observed when handling hydrochloric acid.
experience a buoyancy force, made evident by an apparent loss of weight (or mass) which is equal to the weight (or mass) of the fluid displaced." In this experiment the expanding balloon displaces half a gram of air and, therefore, the system experiences a similar "loss of mass" during the reaction and the conservation of matter is, of course, still observed.
CAUTION
Rubber gloves and eye protection should be worn. The reaction products may be flushed down the drain with plenty of water.
Demonstration3: The Conservation of Matter Revisited To demonstrate the law of conservation of matter without resurrecting Archimedes, demonstration 1is modified as follows:
There is no pressure build up in demonstration 3, despite the system being closed, provided the filtering flask has been evacuated adequately.
'A stopcock is fitted to the side-arm of a 500-mL Erlenmeyer filtering flask via a 5-cm piece of rubber tubing (a Teflon buret tip works well in place of the stopcock). T h e acid and carbonate are carefully placed in the flask that is then stoppered and evacuated using a water aspirator pump that has a strong suction. AR& about 3-4 min the stopcock can beclosed, the aspirator how removed, and the flask carefully placed on the balance that is tared to zero. .As before the reaction is initiated by gently shaldng the flask.
Simple and Inexpensive Kinetics A Student Laboratory Experiment and Demonstration Submeed by: David K. Erwin Rose-Hulman Institute of Technology Terre Haute, IN 47803 Checked by: Wavne L. Smith ~ o 6 ycollege Waterville, ME 04901
This time there are no surprises, no mass loss is observed, and the conservation of matter law is clearly evident.
Too often, chemical kinetics experiments are thought to be complicated, expensive, and time-consuming. Laboratory experimentation, as well as an in-class demonstration, involving the study of chemical kinetics can be performed using a safe, simple, and inexpensive apparatus. This apparatus requires only that systems to be studied must produce a gaseous product that can displace water. Both qualitative and quantitative information can be obtained.
Other Concepts Illustrated In addition to demonstrating the law of conservation of matter, these three experiments illustrate some important concepts to students including acid-base chemistry, a little basic stoichiometry, gas density and solubility, and the Archimedes principle. In addition, the surprising results of demonstration 2 also provide a valuable lesson: not to "expect the exoeded."
3 Plastic Tubing
Rubber stoppers
.
Hose-clamp assembly
Wash bottle with reactants Wash bottle filled
with water
Ti
h
-
Water bath
. Magnetic Stirrer (optional)
1 Assembly of the apparatus forstudying chemical kinetics. 926
Journal of Chemical Education
Graduated cylinder
Simplicity of the Apparatus The a ~ o a r a t u sshown in the firmre consists of two 125mL wid&outh one-piece polyeth;lene wash bottles. Plastic tubing is used to connect the two rubber stopwrs (size: #2) and ;an be made from the stem of a 9-in.-disposable plastic pipet. The hose-clamp assembly wnsists of a pinch clamp fastened to a piece of rubber tubing. The graduated cylinder, small beaker, and container for the water bath can be plastic. The magnetic stirrer and stir bar are optional. After this apparatus is assembled, one wash bottle (the one on the right) is filled with water to the "fill line" mark. Next, all of the reactants (solids, catalysts, water) except one are added to the other wash b o t t l e t h e reaction wash bottle (the one on the left). Using a water bath, the contents of the wash bottle with the reactants are allowed to equilibrate to the desired temperature. The final reactant (which should be in the liauid phase and should be the one most reactive or the one whose rate data is to be collccted~is measured into the small beaker. A 9-in. plastic pipet may be used to measure and transfer the reactant to the small beaker. The hose-clamp assembly is removed from the spout of the reaction wash bottle. With the rubber stoppers securely in the wash bottles, the wash bottle with the reactants is squeezed until water begins to drip from the other wash bottle's spout. Squeezing is maintained a t a constant nressure. and the hose-clamu assemblv is then placed on the spout of the water-filleri wash bokle. The contents of the small beaker (wntaining the fmal reactant) are brought into contact with the spout of the squeezed wash bottle. Saueezina of the wash bottle is stopped. and the contents of'the small beaker are allowed to'de &awn into the wash bottle with the other reactants. Immediately the timing is begun and the hose-clamp assembly is removed from the water-filled wash bottle and placed on the spout of the wash bottle with all of the reactants. Agraduated cylinder is placed under the spout of the water-filled wash bottle or a count of the drops of displaced water is initiated. The wash bottle with the reactants is shaken or stirred and the data concerning the time and volume of water displaced are collected. ~
~
..
Versatilihr of the Aooaratus This apparatus may be used to demonstrate factors that affect the rates of chemical reactions. Comparisons may be done individually or with two complete setups. For a comparison of different reactants that produce the same prod"ct, the reaction of zinc with hydrochloric acid and m&esium with hydrochloric acid can he compared. (This can he done using either 0.65 g of zinc or 0 . 2 4 of ~ magnesium in
10 mL of water in the reaction wash bottle and adding 2 mL of 6 M HCl through the spout.) For a comparison of the effect of the surface area reactants, the reaction of either mossy zinc or powdered zinc with hydrochloric acid can be compared. (This can be done using either 0.65 g of mossy zinc or powdered zinc in 10 mL of water in the reaction wash bottle and adding 2 mL of 6 M HC1 through the spout.) In order to show the influence of different concentrations of one reactant, the reaction of zinc with different concentrations of hydrochloric acid can be used and compared. (This can be accomplished using 0.65 g of zinc in 10 mL of water in the reaction wash bottle and adding either 2 mL of 6 M HCI or 4 mL of 6 M HC1 through the spout.) Temperature effects can be followed using the reaction of zinc with hydrochloric acid at two different temperatures. (This can be done using 0.65 9 of mossy zinc in 10 mL of water in the reaction wash bottle a t either room temperature or a t ice water temperature and adding 2 mL of 6 M HCI through the spoutJThe effect of a catalyst can be demonstrated using the reaction of zinc with hydrochloric acid without any catalyst and with a solution of CuS04. (This can be accomplished using 0.65 g of mossy zinc in 10 mL of water with and without five drops of 1M CuS04solution in the reaction wash bottle and adding 2 mL of 6 M HCI through the spout.) Quantitative kinetic information involving rate constants, orders of reactions, and activation energies also can be determined. The dewmposition of hydrogen peroxide into water and oxygen using aqueous potassium iodide solution as a catalyst is a simple reaction to obtain kinetic information. (This can be done by using 20 mL of water and 5.0 mL of 1.0 M KI solution that has been made slightly basic with a few drops of 6 M NaOH per liter of ICI solution in the reaction wash bottle and by adding 2 mL of 3% Hz02 (obtained from a drug store) through the spout.) For a comparison of various catalysts, the decomposition of hydrogen peroxide into water and oxygen can be studied using aqueous potassium iodide solution and solid manganese dioxide. (This can be done by repeating the details above for the potassium iodide catalyst as one part and by using 0.1 g of powdered MnOz in 25 mL of water in the reaction wash bottle and adding 2 mL of 3% HzOz (ohtained from a drug store) through the spout for the other part.) In order to obtain activation energies, the above reactions may be used a t temperatures of 15 'C, 20 'C, 25 'C, , 40 'C. Thus, both qualitative and quanti30 T , 35 ~ Cand tative chemical kinetics experiments and demonstrations may be followed with the use of this apparatus and approach.
Volume 69 Number 11 November 1992
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