Two Lecture Experiments Demonstrating Limiting Quantities
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Submitted by: Clyde R. Dillard, Brooklyn College, Brooklyn, N. Y. 11210 Checked by: Linda Shepard, Hiram College, Hiram, Ohio 44234
EXPERIMENT II
EXPERIMENT l PREPARATION
PREPARATION
Provide 1.00 M HCI, identical rubber balloons with large necks, 3 Erlcnmeyer flasks (125 ml), and 0.6, 1.2, and 2.4 g portions of magnesium metal ribbon.
Have available 1.0 M HC1, 0.4, 2.0, and 4.0 g portions of calcium metal, thymol blue indicator, and three 1-1demonstration cylinders.
DEMONSTRATION
DEMONSTRATION
Place exactly 100 ml of 1.00 M HCl in each Erlenmeyer flask, taking care not to wet the necks of the flasks. Coil the weighed sections of ribbon and insert a coil in the neck of each flask so that they are held above the solution by the spring action of the coil. Fit the rubber balloons over the necks of the flasks and fasten with masking tape. Because magnesium metal reacts with hot water, the flasks should be set in a tray of cool water. Push the 1.2-g coil into the acid and observe that the balloon inflates as hydrogen is evolved and that a clear solution results. Next push the 0.6-g coil into the acid. This balloon inflates to about one-half the size of the first balloon. Finally push the 2.4-g coil into the flask and, with occasional swirling, let stand until no further reaction ensues. Despite the presence of twice as much magnesium as the first sample, the balloon inflates only to the same size as that of the first sample. Indeed, after the reaction rxcess magnesium is observed in the flask.
Measure out exactly three portions of 100 ml of 1.0 A+' HCl and place in the 1-1 demonstration cylinders. Add the 0.4-g portion of calcium metal t o one cylinder. After rcaction has subsided, show that a clear solution is obtained. Demonstrate that excess acid is present by adding a few drops of thymol blue indicator to the cylinder. Next add the 2.00-g portion of calcium metal to another cylinder. After reaction, add a few drops of thymol blue indicator. Point out that the observed color indicates that the HCI is completely used up, while the clcar solution shows that the calcium metal has also been consumed. Finally add the 4.00-g portion (0.1 mole) of calcium metal to the remaining. cylinder. Add gradually to keep the reaction from becoming too vigorous. After no further reaction ensues, point out the white precipitate, Ca(OH),, obtained in this case. Add thymol blue indicator to show the presence of hydroxide ion.
REMARKS
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The volumr of hydrogcn librratrd in the reaction Mg 2HC1 RIgCl, Hz is limited, therefore, by the amount of either magnesium or hydrochloric acid present.
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A694
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Journal of Chemical Educofion
REMARKS
In some cases a reagent present in excess may undergo further reaction yielding products other than those predicted by the original chemical equation.
