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Unformatted text preview: Chapter 4: Quantities of Reactants and Products 113 Chapter 4: Quantities of Reactants and Products Teaching for Conceptual Understanding Continue to reinforce the three levels for representing matter by showing reactions, drawing particulate diagrams, showing models, and writing chemical equations. Balancing chemical equations is another one of those skills best learned as an algorithm. Warn them that random trial-and-error methods can cause more errors and much frustration. Once students master balancing chemical equations, do not assume they fully comprehend the meaning of the notation. Research has shown a common misconception that the coefficient refers only to the first element in the formula. For example, 2 NH 3 (g) means there are 2 N atoms and 3 H atoms Many misconceptions from Chapter 3 will carry through to this chapter, such as assuming that H 2 O 2 is really a H 2 molecule and a O 2 molecule, or Mg(NO 2 ) 2 contains an Mg atom, and two molecules of NO 2 . One way to improve the students’ comprehension of balanced chemical equations is for them to write corresponding word statements on both a particulate and molar level. For example: O 2 (g) + 2 H 2 (g) 2 H 2 O( l ) This equation can be stated with words as: • One molecule of oxygen reacts with two molecules of hydrogen to produce two molecules of water. • One mol of oxygen gas reacts with two mol of hydrogen gas to produce two mol of liquid water. In addition to assessing student comprehension of the balanced equation, this exercise reinforces nomenclature and builds communication skill. Expand on the idea of interpreting balanced equations on a molar level by introducing the mass relationships among and between the reactants and products. For example: O 2 (g) + 2 H 2 (g) 2H 2 O( l ) 1 mole 2 mol 2 mol 32 g 4 g 36 g When figuring the mass relationship, some students will forget to multiply the molar mass by the coefficient, e.g., 32 grams of oxygen react with 2 grams of hydrogen to form 18 grams of water. Other students that are still confused by the mole concept may think 1 gram of oxygen reacts with 2 grams of hydrogen to produce 2 grams of water. Stoichiometry is one of those topics where students “can’t see the forest for the trees.” Then give a sense for why stoichiometry works using the Active Figure 4.5 showing the decomposition of water and Figure 4.6 showing a displacement reaction. Figure 4.8 shows a generic sequence of stoichiometric calculations from Chapter 4. You may want to use this as an overview and then highlight each individual portion as you introduce it or work sample problems. A common misconception of limiting reactants is that the reactant with the smallest mass is the limiting reactant....
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This note was uploaded on 04/20/2008 for the course CHE 131 taught by Professor Kerber during the Spring '08 term at SUNY Stony Brook.
- Spring '08