edlted by TOM TIPTON
learning activitie~ Nightmares and Igors; Keeping Students Interested Susan C. Warrington Hancock Central High School Hancock. MI 49930
Several vears ago, I noticed that mv students were not retaining several ofthe key concepts inchemistry. They would learn these concepts, but if the concepts were not often used the knowledge hegan to be lost. In order to prevent this, a series of prohlems, called nightmares, was devised so that these concepts would he used repeatedly. Our course is begun . with atomic structure, this is followed by bonding, ions, formulae, percent composition, and balancing equations. Balancing of oxidation-reduction reactions is taught at the same time as the balancing of other types of reactions. After the students learn to balance equations, they learn stoichiometry, including the identification and meaning of limiting reagents. At this point, the first nightmare is given to the students. It covers percent composition, limiting reagent stoichiometry, and balancing an oxidation-reduction reaction (see examnle 1below). .
"
covers percent composition, limiting reagent stoichiometry, balancine an oxidation-reduction reaction, and the gas laws (see example 2 below). This format is followed for the rest of the year. As a topic is finished in class, it is added to a nightmare problem. Included are molecular weight . determination by colligative properties, concentration units and pH, until the final nirhtmare of the year covers most of the concepts taught (see example 3 helowj. At the beginning, the students dislike the nightmares, hut most of them soon begin to enjoy the challenge and see them as an excellent way to maintain a working knowledge of chemistry problem solving. In fact, the students often request more nightmares. I have begun instituting this idea in my second-year chemistry course. These problems are called "Igors" and involve more complex topics: bond order, partial pressure, competing reactions, conversions between molality and molaritv, kinetics. eauilihrium constants, mole fractions, and buff& (see example 4 below).
-
Example 1 If 200 g of compound A (12.3% nitrogen, 3.5% hydrogen, 28.1% sulfur, and 56.1% oxygen; molecular weight = 228; oxidation numher of nitrogen = -3) and 95.0 g of manganese (11) nitrate react in the presence of excess water, how many grams of nitricacid are formed? The other oroducts are sulfuric aeid.. ~ermaneanicacid and B (21.2% nitrogen, 6.1% hydrogen, 24.2% sulfur, and 48.5% oxygen; molecular weight = 132; oxidation number of nitrogen = 3 ) ~
~
.
Answer: 44.2 grams HNOs
Unlverslty of Nebraska
Lincoln NE 68508
Example 2 If 140 g of compound A (41.4% carbon, 3.5% hydrogen, and 55.2% oxygen; molecular weight = 116) react in the presence of 182 g of patassium permanganate and excess sulfuric acid, how many liters of dry carbon dioxide gas are produced? (Assume that carbon dioxide is not soluble in water.) The other products of the reaction are manganese (11) sulfate, potassium sulfate, and water. The gas is collected over water a t 745 t o n and 23'C. Answer: 49.0 liters CO?
Example 3 If compound X, potassium dichromate, and sulfuric acid are reacted, the products formed are chromium (111) sulfate, carbon dioxide, potassium sulfate, and water. Exactly 2,355 ml of a sulfuric acid solution of pH -0.535 were used in the above reaction. An amount of ootassiurn dichromate. enactlv eaual to that used in
curs. Compound X is 2.239% hydrogen, 26.681% carbon, and 71.080% oxygen with amalecular weight of 90.034. In the above reaction, 125 g of it were used. If the reaction is run a t 22.0°C and 741.5 tarr, how many liters of carban dioxide gas are produced? Answer: 23.0 liters COz
Example 4 If concentrated sulfuric aeid drops a n sodium chloride, hydrogen chloride gas is formed. When the hydrogen chloride gas has replaced half of the air in a 10.00 1flask, what is the mole fraction of each gas in the flask (air is 21% oxygen)? The reaction was run until the flask contained only hydrogen chloride gas and the conditions in the collection flask were found to be 77n torr and 18.0DC.To this gas, exactly 250.0 g of water a t 20°C were added forming a solution of hydrochloric aeid. The resulting solution had a density of 1.115 g per milliliter. What are the mole fraction, molality, and rnolarity of the acid? What is the pH of the
-chlorine and water. When the manganese (11) chloride formed is dissolved in 75.0 g of water, what is the freezing point of the solution? Assume that the salt dissociates completely and that no interaction between ions occurs. Use three significant figures for this calculation. Answers: XHa = 0.50 Xo, = 0.10 X N =~0.40 XHa = 0.0297 m = 1.70 M = 1.78 pH = -0.250 13.4 g MnC12 formed freeaine m i n t = -7.91°C
.
I
1
the Feature Editor.
Volume 60 Number 4
A ~ r i 1983 l
357
