...... edited by: GERARDBARUCH Lackey High School Indian Head. Maryland 20640
.................. ................ .......................... ............ ........... ............. An Experimental Introduction to Stoichiometry
t h e solution t o splash out of t h e test tube). Continue adding hase until the indicator chanees " from colorless to a oermanent oink color. shawine" that the base has eomnletelv .renet.ed with ............ .......the . mid^ Record the total number of drops of base required. 4) Repeat steps (1)to (3)using a fresh 2.0-em%mple of hydrochloric acid. Good technique should lead to a difference of two drops or Less between the two determinations. Be sure to use the same medicine dropper throughout the experiment. 5) Determine the average number of drops of hase required t o neutralize 2.0 em3 of 1.00 m01.dm-~ hvdroehloric acid. 6 ) A.. HI aleps 1) to ( 5 , . find the m e r n a e ~ ~ u m h ~f e r drops of hase requtrd to neutralize: (a1 4. )rm'$.I 1dtI mc~ldm~'hydrochlorre acid ibo 2.0 cm'of 1.00 mol4n>:'.ulf~.r1~ .acid ~
Mlchael J. W e b b Memial Universily of Newfoundland Sir Wilfred OrenfellCollege Cmer Brook. Newfoundland Canada A2H 6P9 CHECKED BY: George Radciiffe Cathedral High School Springfield. MA 011 18
Purpose
To show by experiment that t h e quantities of materials used in chemical reactions a r e related to balanced chemical e a u a tions. Equipment
It is intended that students work individually. Each student requires one 10-cm3 graduated cylinder, six (or less) 20 X 150 mm t e s t tubes, o n e medicine dropper, a n d one 50-cm3 beaker. Procedure 1) Using a 10-cm3graduated cylinder, obtain 2.0 cm3 of 1.00m ~ l . d m - ~ hydrochloric acid and pour into a test tube. Add two drops of phenolphthalein indicator. 2) Obtain approximately 10 em3 of 3.0 mol.dm-3sadium hydroxide solution in a 50-cm3 beaker. (This is sufficient sodium hydroxide for the wholeoftheexp~riment 3) Using the medicine d r u p p r , add s d i u m hydroxide solution one d r o at ~ a time lo the acid in the test tube. (Hold the medicine drokoer verticallv throuehout to ensure that the d r o size ~ is uniform') Shake the k t t& frum %decos~deafter eachhropuf ba* IS added \Do no1 shake u p a n d down:this may causesame of
of Chemical
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1) S.I. units of concentration are used throughout. The symbol M may be substituted for mobdm-3and ml for em3, if necessary. 2) I t is essential that the concentrationsof the two acids be eaual but not necessarily a t 1.00 m ~ l . d m - ~If. approximately 1 mol.dm-3 acids are available the concentration of one. of . . them . . . .e . .m.he . .adjustrd on the basisof a trial r u n thruugh thcexprr~ment. 2 ) 'I'hr roncentratmn of hydrochloric arid decrease* wtth time. A
Raymond Walten College Cincinnati. OH 45236
Journal
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Teacher Notes
Richard Barnes
192
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Education
New expermems and dlrectnons m teachlng chemorlv through the use 01 the $baraton are orovioed n thm leatse Experiment; will be fully detailed and will be field tested before they are published. Contributions shmld be sent to the feature editor. Gerard Baruch received his BS in chemistry from Raosevelt University in 1965 and his MEd and Ed0 from the University of Massachueens in 1971 and 1973. respectively. He began his teaching career in 1966 at Marshall High School in Chicago. He is presently a chemistry tcvcner at 1 achy n gh School Charles C o . Maryland, and a cnernmy instructor at h nce Georw Cammumtv Colleoe SClence currlculbm and programs for the secandav school level He also c o - a d b e d the Teachers Gunde' and Laboralmy Guda lor Prlnclple~olScience "Books 1 and 11. pbblorhcd Oy Chdrles E Merrll
fresh batch of the acid should be prepared a t the beginning of each academic year. 4) If the sodium hydroxide solution is approximately three times more concentrated than the acids, then the number of drops of base required will be neither too large nor too small. 5) Dataobtained by two different students will not necessarily agree because of variations in the medicine droooen. For a given student consistent data should be obtained from one run to another, as long as the same medicine dropper is used throughout.
Sample Data and Calculation Table Number of drops of base used Run 1 Run 2 Average
Acid sample 2.0 em3 HCI 4.0 em3 HCI 2.0 em3 H&On
10 21 21
11
19 20
10.5 20 20.5
Pre-Laboratory Assignment Balance a n y of t h e following equations which a r e n o t already balanced:
-
+ HnS04 Ba(0H)z + HCI
Ca(0Hh
NaOH NaOH
+ HC1-
+ HzSOl
-
CaSOa + Hz0 BaClp + Hz0 NaCl
+ HzO
NarSOa
+ Hz0
What type of reaction does each of the above equations represent? Post Laboratory Assignment Remember, when answering these questions, t h a t numerical data always contain some uncertainties. In this case, even with good experimental technique, t h e r e is likely to h e a n uncert a i n t y of o n e d r o p in t h e average of two runs. 1) When the volume of hydrochlorieaeid was doubled, what change was observed in the number of drops of sodium hydroxide solution required to neutralize the acid? 2) What was the relationship between the number of drops of base required to neutralize equal volumes of the two different acids? 3) Write balanced equations far the reaction of sodium hydroxide with: (a) hydrochlorieacid, (b) sulfuric acid. (This was part of your pre-laboratory assignment.) 4) Try to relate your answer to question (2) to thk balanced equations in question (3). By inspecting the balanced equations can you see why one acid required more base to neutralize it than the other acid did? Explain.
volve fairly simple practical work, t h e interpretation of t h e
data requires t h e use of mole calculations. Special Notes 1) The concept of stoichiometry is usually introduced in an arithmetical way using balanced equations and the mole concept. Many students have difficulties with this approach. The oresent experiment is intended to provide an exp&imental introduction to the topic before the more mathematical work begins. 2) This exoeriment also introduces a simolified tvoe of titration. calculations. 3) Before attempting this experiment, it is necessary that students have some understanding of solution concentrations (they must realize the significance of the equal acid concentrations), are familiar with simple reaction types (such as the neutralizations carried out here), and can balance equations by inspection. 4) A short follow-uo discussion is sueeested to reinforce the new "" concepts and to assist weaker students. The more mathematical treatment of stoichiometry can then commence.
Field Testing T h i s experiment was tested by a group of 21 students in our summer immersion chemistry course, described earlier in THIS JOURNAL. Students taking this course have completed grade eleven h u t have n o previous exposure t o chemistry. T h e average age of t h e group was 17 years a n d 2 months. Acknowledgment I a m grateful t o Dr. G. W. Rayner-Canham a n d Mr. J. M. Brisson of t h i s d e p a r t m e n t for assistance with field testing, a n d to Dr. D. W. Russell of t h e D e p a r t m e n t of Education, Dalhousie University, for helpful suggestions. Supplementary Material I l>etabl~~.~,!~t,?it,sn ~~ump l l~~ itctn 'l'llc 1lh.i s l w ~ ~ . ihv mtmher d h p s
