CHEMISTRY EXPERIMENTS
For Advanced & Honors Programs
by
Book Cover & Preview Text
This week, the lab exercise had us investigating a double replacement reaction between two solutions. The first solution was 0.2 molar Lead Nitrate. The other solution was 0.2 molar Sodium Iodide. The determination of how and why they reacted, and as a result, the prediction of the Theoretical Yield requires several logical assumptions. We begin by analyzing the Lead ions generated by the first solution, the Lead Nitrate. The element Lead has two possible oxidation states. A complete Mass-Balance Analysis allows us to predict the outcome of the reaction. These states are (+2) and (+4). When you determine the electron configuration for the element Lead, we find that it has four valence electrons in the 6th energy level. These four electrons are found in two sub-levels. A pair of electrons are in the 6s sub-level, and there are an additional two unpaired electrons in the 6p sub-level. The two unpaired electrons are easily removed in a chemical reaction run at or near room temperature. The Lead atom would express a (+2) oxidation state. However, if the reaction were heated, then the two paired electrons in the 6s sub-level could also be driven off, giving the Lead atom a (+4) oxidation state. Since we used chilled solutions from the refrigerator, for reasons to be explained later, it is assumed that the Lead would express the (+2) oxidation state. Only the two unpaired electrons from the 6p sublevel would be exchanged in this reaction. The two possible outcomes for the product can be tested. The PbI2 product, formed with chilled solutions, is yellow in color. The PbI4 product, formed during heating the solutions, is orange in color. The Iodine atom had several possible oxidation states. They are (-1), (+1), (+3), (+5), and (+7). Since the Lead atom has a positive charge, it is logical to assume that the Iodine must have the (-1) oxidation state in this reaction. With these two assumptions, one derives the combining ratio between Lead and Iodine to be 1:2 in this reaction. This lab will combine 50 ml of the lead solution with 100 ml of the iodine solution. The product expected (PbI2) is slightly soluble in water above 15 Celsius. For this reason, both solutions and all water used for washing the product are kept refrigerated. An effort must be made to not exceed this 15 Celsius threshold. Further, by keeping the volumes at 150 ml in this experiment, we eliminate the total water volume as a factor in the percentage yield of the product. If volumes for all lab groups are equal, then the losses to solubility (Ksp) are under control. These losses would be the same in each group. After all of the lab groups samples were made and were filtered, the product was dried overnight. Rapid heating in an oven to dry the samples was to be avoided, as this could cause oxidation of the Lead compound, thereby changing our results. All of the filters were washed sufficiently, so all of the excess Lead or Iodine was removed from the precipitate before drying. The sodium nitrate by-product was also washed free of the precipitate. This reaction relies on Collision Theory to produce a product. Anything which can guarantee a collision of the reactants will improve the outcome of the reacton. Heating the reaction would improve the collision of the reactants, but it would also make the precipitate soluble. In addition, the combining ratio for heated lead iodide is greater than the colder solution ratio. In trying to improve the reaction outcome by heating, you would in fact alter the hypothetical yield. In closing, let us summarize the concepts covered by this exercise. All atoms rely on Collision Theory to react. Anything that supports these collisions will increase the amount of final product. Materials that appear insoluble are still slightly soluble, unless certain precautions with temperature and total volumes are taken. Atoms join in specific combinations. Robert Dalton called this the Law of Definite Proportions. Any deviation from this exact ratio creates a Limiting Reactant, which limits the amount of product formed by that reaction. This ratio is determined by the oxidation states of the atoms reacting. Temperature can influence both the oxidation state expressed by an atom, and the solubility of the reaction product. And finally, by comparing the Mass-Balance of the reaction, we can see if a law is at work, and how much of the reaction has actually run to completion.
Formats
Book Details
About the Book
Gifted and talented students and any student interested in pursuing a science major in college needs a rigorous program to prepare them while they are still in high school. This book utilizes a format where the application of several disciplines—science, math, and language arts principles—are mandated. Each lab concludes with either an essay or a detailed analysis of what happened and why it happened. This format is based on the expectations of joining a university program or becoming an industrial science professional. The ideal student lab report would be written in a lab research notebook, and then the essay or final analysis is done on a word processor to allow for repeat editing and corrections. The research notebook has all graph pages, a title section, and a place for the students and their assistants to sign and witness that exercise. The basic mechanics of the lab report—title, purpose, procedure, diagrams, data table, math and calculations, observations, and graphs—are handwritten into the book. The conclusion is done on a word processor (MS Word), which allows the instructor to guide the student in writing and editing a complete essay using the MLA format. When the final copy is completed, the essay is printed and inserted into the lab notebook for grading. At the end of the term, the student has all their labs in one place for future reference. These lab notebooks can be obtained for as little as $ 3.00 per book. This is money well-spent. In our district, the Board of Education buys the books for each student. The BOE sees these books as expendable but necessary materials for all science and engineering instruction.
About the Author
James Signorelli has taught advanced placement chemistry, honors chemistry and honors physics for over 38 years in a northern New Jersey public school district. This school became a state magnet school for gifted and talented students in 2001. At that school, has served as the Science Department Chairman overseeing the implementation and application of high rigor classes in all of the sciences for over seventeen years, while still carrying a full teaching schedule. Mr. Signorelli is on staff at two local universities working on both teacher outreach and middle college programs. He serves on the board of directors of the New Jersey section / American Association of Physics Teachers. He was a senior research flavor chemist for over eight years. This allowed him to write and give advice on curriculum which prepared students for both an industrial and educational course of study. Mr. Signorelli believes that a trend in schools to replace hands on lab experiences with a virtual lab done on the computer is a mistake. He has experienced ill-prepared students in college who had never worked with real chemicals or equipment. His colleagues also have commented on this growing problem. It appears that the business office has taken over the classroom. Decisions on how or what to teach is effected by the bottom line….cost! This lab manual brings the student back into the lab to perform real experiments on diverse topics within the framework of an honors and advanced chemistry program. Throughout his years of teaching and industry, Mr. Signorelli has been honored for his research on various essential oils; an invited speaker at several chemists symposiums; as the chemistry teacher of the year by the American Chemical Society; cited as an outstanding teacher by the Bergen County and New Jersey State legislature; recognized by Princeton University for his teaching; and has been elected to Who's Who Among American Teachers.