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MSJ2e_Ch06_ISM1_June26_p258

MSJ2e_Ch06_ISM1_June26_p258 - Chapter 6 Energy and Chemical...

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Chapter 6: Energy and Chemical Reactions Chapter 6: Energy and Chemical Reactions Teaching for Conceptual Understanding The confusion between heat and temperature you may have observed with your students in Chapter 1 will become more prevalent when covering the topic of thermochemistry. Have students imagine a bowl, a bucket, and a barrel of boiling water. If a thermometer were placed in each container, which one would cause the mercury to raise the highest? Which container would be the better heat source on a cold winter night? Students have a personal knowledge of heat and temperature that matches scientific knowledge: however, they confuse the two when scientific language enters the picture. Use language carefully and precisely when discussing the concepts of this chapter, Questions 126 and 127 in the Questions for Review and Thought also address this issue. Unlike reactants and products which can be seen on a macroscopic level and diagrammed on a particulate level, the heat of a reaction is an abstract idea to most students. They may know that heat is absorbed or released during chemical reactions and often think of it as a reactant or product. However, they don’t always see the stoichiometric relationship between the enthalpy change and the reactants and products. Have students write all the possible stoichiometric relationships for the enthalpy change and molar amounts of reactants and products. Consider the reaction: 2H 2 (g) + O 2 (g) + 2 H 2 O( ) H = –571.60 kJ Some of the stoichiometric relationships are: 2 μολ Η 2 571.6 κϑρελεασεδ 571.6 κϑρελεασεδ 2 μολ. Η 2 Ο φορμεδ Suggestions for Effective Learning Reinforce the meaning of exothermic and endothermic by keying in on the prefixes ex- or exo-as in the word exit meaning to leave or go out and end- or endo-as in endomorphic, endoderm, and endocrine, words familiar to students from their life science classes. Some people like to use the similar-sounding words “inhale” (air goes in) and “exhale” (air goes out). Enthalpy change is another topic where students miss the general idea because we confuse them with the specifics. Most students grasp the concept of heat released or absorbed in a chemical reaction and that this heat change is expressed by a H value. But by the time we introduce the enthalpy of formation, enthalpy of combustion, enthalpy of sublimation, enthalpy of reaction, standard molar enthalpy of formation, enthalpy of decomposition, enthalpy of fusion, etc., the students lose sight of the concept. Every time you use a specific enthalpy term, clearly indicate that it is still referring to the heat released or absorbed, but it is for a specific named process. Make connections to consumer applications of thermochemistry by discussing chemical handwarmers used by winter campers and instant cold packs used by injured athletes. Pass samples of each around the room.
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