Thinking about Chemicals

This introductory lesson will provide you with a pre-unit assessment of your students current knowledge of chemicals and questions about them. Through class discussion and their observations of a common chemical, students will provide you with Information you can use to assess both their initial ideas about chemicals and their observation and recording skills. This lesson Introduces students to the concept of properties and sets the stage for the activity in the next lesson observing the properties of common classroom objects. 

Chemistry is the study of chemicals and how they interact. Chemicals consist of the 106 known elements and any of their combinations (the chemical water, for example, consists of the elements hydrogen and oxygen). Everything is a chemical, whether it is made by nature or by humans. Our bodies are made of chemicals, and we produce chemicals such as carbon dioxide. We need chemicals from food and the air we breathe. 

In this first lesson, students observe and describe an unknown or mystery chemical. This chemical—cornstarch—Is one of the five unknowns the class will Investigate in Lessons 2 through 11. (While students will not know the chemical s Identity at this time, they will apply their knowledge and skills to discover it in Lesson 13.) This activity introduces the class to the concept of properties, those characteristics used to describe an object. Here, students use their senses to examine physical properties such as color, shape, texture, and odor. To ensure safety, they do not use their sense of taste at any time in this unit. 

Note Many teachers find that their students have had little experience observing and describing the properties of objects. Before you begin the unit, have students try one or more of the observing and describing activities in Appendix E. 

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When thinking about chemical reactivity, chemists usually focus their attention on bonds, the covalent interactions between atoms within individual molecules. Also important, hotvever, particularly in large biomolecules like proteins and nucleic acids, are a variety of interactions between molecules that strongly affect molecular properties. Collectively called either intermolecular forces, van der Waals forces, or noncovalent interactions, they are of several different types dipole-dipole forces, dispersion forces, and hydrogen bonds. 

The conclusion that should be drawn from this discussion is that there are two kinds of acidity that must not be confused (1) an intrinsic acidity, which is best approximated by gas-phase measurements and which reflects the properties of the ions and molecules in isolation, and (2) a practical liquid-phase acidity in which solvation effects may play the dominant role. In interpretation of structure-reactivity relationships, the liquid-phase acidity will probably be misleading unless the structures being compared are very similar for thinking about chemical behavior in solution, however, the liquid-phase acidities are clearly the important ones. 

The unit begins with a pre-unit assessment lesson in which students share what they think about chemicals and what they would like to learn and gain from their first experience observing and describing an unknown material. In Lesson 2, students encounter the mystery of their five unknown solids (sugar, alum, talc, baking soda, and cornstarch), and assemble the tools they will... 

Students share their present thinking about chemicals and discuss what they would like to learn about them. 

At this time, students also produce class lists of what they now think about chemicals and what they would like to learn (see Figure 1-1) as well as Individual notebook entries that reveal some of their current thinking on chemicals. Both products are used for pre- and post-unit assessment. (See Post-Unit Assessment on pg. 171 for more information.)... 

Label two sheets of newsprint or poster board "What We Think about Chemicals and What We Would Like to Know about Chemicals, respectively. You may need to add extra sheets. 

Display the sheet What We Think about Chemicals." As each group representative makes his or her report, record the responses. Put a check next to duplicates to acknowledge all students ideas. 

In this lesson, students use a third chemical liquid, red cabbage juice, to explore further the chemical properties unique to each unknown. Following the completion of this test, students analyze all the data they have collected over the past seven lessons and summarize what they now know about the physical and chemical properties of each of the five unknown solids. This is a good time for the class to add to the What We Think about Chemicals list from Lesson 1. 

How We Are Finding Out about the Unknowns 1 list (from Lesson 3) What We Think about Chemicals list (from Lesson 1)... 

Focus students attention on the What We Think about Chemicals" list. Using a new color of marker, add students latest thoughts to the list. If students have any new questions, add them to the What We Would Like to Know about Chemicals" list. 

Display the newsprint and title it What We Think about Chemicals. Conduct a class brainstorming session and record students thoughts. 

A rate law is a statement about how the rate of a reaction depends on the concentrations of the participating species. If one thinks about chemical reactions as something that happens at the molecular level when molecules collide with one another, it makes sense that the number of collisions, and hence the rate of reaction, should depend on how many molecules of each type there are that is, their concentrations.1... 

Chapter 9. The fundamental reactor modeling principles covered in Chapters 2-8 provide the framework in which we think about chemical reactors. We understand which phenomena cause which observed reactor behaviors, and which design variables should be changed if We wish to alter the reactor performance. But when we want to make quantitative predictions of reactor performance, we require values for the model parameters. It is a simple fact that most of the parameters needed for the chemistries and reactor configurations of interest are Uot available in the literature. To make these models useful in standard industrial practice, therefore, we must be able to conveniently determine or estimate these parameters from experimental data collected on the system of interest. Chapter 9 covers this important topic iof parameter estimation, which is not usually addressed in a systematic manner in introductory treatments of reactor analysis and design. 

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