Chapter 4Stoichiometry: Quantitative Information about Chemical ReactionsChapter 4Stoichiometry: Quantitative Information about Chemical ReactionsINSTRUCTOR’SNOTESIn this chapter the mathematical concepts of chemical substances are expanded from Chapter 2 to chemical equations. The chapter also includes stoichiometric calculations for reactions occurring in aqueous solution, concentration units (molarity), and the pH scale. This book always stresses the notion of the stoichiometric factorin chemical calculations. We constantly remind our students that everything funnels through this step because reactions occur in a ratio of the number of atoms or molecules of one substance to the number of atoms or molecules in another substance, such as 1 to 2 or 3 to 3. But, in the lab, we measure mass, not numbers of moles, so a conversion between these levels must be done.Students often have difficulties with limiting reagent problems. For this reason we have incorporated a large number of solved examples in the text. Correctly calculating the two ratios in Step 2 of the procedure shown in the text is the key to using the method of this book. The summary table given at the end of each example should help clarify the concept of a limiting reagent. In class, we present alternate ways of determining the limiting reagent such as calculating the amount of product that can be produced from each reactant, or the new method introduced in this edition (page 167), using moles of reaction. We also emphasize percent yield calculations because we find that in percent yield problems some students reverse the identification of actual and theoretical yields.Chemical analysis of mixtures and combustion analysis can be challenging. For the latter it may be useful to stressthat the reason for producing CO2and H2O is to “count” C and H atoms present in the original sample. Of course it is always a good idea to do as many examples as possible. New to this edition is a section on chemical analysis using spectrophotometry. This section relates well to the typical laboratory experiments that can be performed at this point in the course.Chapter 4 requires approximately four to five lectures.SUGGESTEDDEMONSTRATIONS1. Stoichiometry As an introduction to stoichiometry we have used the decomposition of ammonium nitrate: NH4NO3(s) N2O(g) + 2 H2O(g) This reaction is part of suggested demonstration found in Shakhashiri (Volume 1, page 51). The overall reaction is spectacular. However, one of the reaction products (ZnO) is irritating, so the reaction must be done in a very well ventilated room.65
Chapter 4Stoichiometry: Quantitative Information about Chemical ReactionsThe reaction of Mg ribbon with O22 Mg(s) + O2(g) 2 MgO(s) is also useful as a demonstration reaction that can be tied to an example of a simple stoichiometry reaction.