H. Eugene LeMay, Jr. and Kenneth C. K e m p University of Nevada Reno, 89507
I I
Writing Chemical Equations An introductory experiment
Finding meaningful experiments for the first several weeks of the freshman chemistry course is a recurring problem. Very commonly, initial experiments involve quantitative physical measurements intended partially to emphasize the importance of the quantitative aspects of chemistry. However, this laboratory experience does not meet with many students' expectations of chemistry, and they frequently express a desire to conduct experiments placing greater stress on chemical reactions. Indeed, chemical reactions can he colorful and exciting while numbers can seem "cold." Additionally, emphasis on the purely quantitative aspects of chemistry may leave the impression that qualitative observations are somehow less significant. It therefore seems worthwhile to capitalize on this interest and give an early exposure to some descriptive chemistry. This can also provide a means of introducing students in the laboratory to describing chemical changes using chemical formulas and balanced equations. The type of experiment described below clearly illustrates to the student the basis of an equation, uiz. that before a chemical equation can he written must he identified. Too often students expect products for reactions to he those which balance t h e reaction (e.g., many expect the thermal decomposition of KC103 to yield KC10 or alternatively 0 3 ) . We have developed and successfully used an experiment in which possible products of a series of reactions are tabulated together with properties which may he useful in identifying each substance (e.g., whether the material is a gas, liquid, or solid, its color, its odor, and its solubility). The student carries out a series of reactions, carefully ohserving what happens during each. From a consideration of the elements in the reactants and the properties of the products together with the information provided, the student deduces the products. Finally he writes a balanced chemical equation for the reaction. The common oxidation states of the elements and charees of oolvatomic ions are also made available to assist ;he st;d;nt. Without such information students are easily frustrated in writing equations for chemical reactions in the laboratory because of their insufficient background. Two reactions which have been used in this experiment are the thermal decomposition of KBr03 and the reaction between Na2C03 and HCI. In the first case the student is instructed to heat a pea-sized quantity of KBr03 in a cool Bunsen-burner flame.' The student obsewes the melting of the salt accompanied by bubbling which indicates the evolution of a gas. The gas is tested with a glowing splint and found to support combustion. The gas is identified as Oz using information such as that found in the table, which is a representative portion of the type of information provided the student. The student is next instructed to dissolve in water the residue remaining from the reaction. Portions of the resulting solution are tested in three ways: by flame test, by addition of AgNO3, and by addition of chlorine water. It is found that the solution produces a purple flame, forms a cream-colored precipitate with AgNOs, and forms a brown substance, soluble in CCln, when treated with chlorine water. Using the table the product is identified as KBr. The following equation is then completed and balanced
Formulas and Properties of Possible Reaction Products" Physical State Compound
1 H2 Gas
Liauid
I Solid (all water soluble)
I:
NaOH
1L
KOH
Properties
...
Colorless, odorless, "pops" with &win# rnli"'. .....
Colorless, odorless, does not support comhurtion Pale yellow-green, pun~e-ent odor Colorless, odorless, supports combustion Volatile, brown-oranpe. soluble in CCb Yellow flame test, forms white ppt with AeNOa Yellow flame teat, reacts with water to form NaOH Yellow flame test, reacts with seida to produce C 0 2 Yellow flame test, forms dark-brown ppt with AKNOI, turns phenolthalein pink Purple flame teat, react. with chlorine water to form Br?, forms cream-colored ppt with &NO8 Purple flame test, reacts with H a to form KOH Purple flame teat, forms dark-brown ppt with AgNOz, turne phenolthalein pink
' Experimental details on bow to eonduct these test are given elsewhere in the experiment.
In the case of the reaction between NazC03 and HCI the student notes the evolution of a gas and tests this with a glowing splint. By reference to information such as that in the table the gas is identified as COz. If there is uncertainty expressed about this identification, the instructor may suggest the limewater test for COz. The compound remaining in solution is identified with use of the flame test and by treatment with AgN03. This permits identification of NaCl from the limited possibilities. The following equation is then completed and balanced
Other reactions which are included in this experiment are mainly ionic metathesis and redox reactions which are accompanied by color changes or the formation of a gas or precipitate. Some further examples are the reactions between zinc and hydrochloric acid, silver nitrate and sodium chloride, silver nitrate and copper, and potassium bromide and chlorine water. In a sense the experiment is a take-off on qualitative analysis and can use many of the same reactions. Furthermore, household chemicals of familiarity to students with no prior chemistry background can be utilized. The student response towards this experiment has been very favorable. The students not only have an opportunity to carry out many chemical reactions, hut they are also confronted with a solvable and challenging puzzle in a scientific style. The better students, in particular, appreciate the departure from the more common "cookbook" nature of early laboratory experiments. Furthermore, the experiment can be adapted to any level freshman chemistry course and does not require special supplies or facilities. Finally, the experiment is susceptible to the same grading techniques as experiments involving unknowns. 'A cool flame is used to avoid "spattering" since the reaction is vigorous and proceeds more readily than the corresponding decomposition of KCIOI. Use of a small test tube also helps restrict the quantity of KBrOa used and keeps the student from inserting the glowing splint completely into the tube. Volume 52, Number 2. February 1975
/
121
