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"Education is not the filling of a pail, but the lighting of a fire." W. B. Yeats
Contact: elizabeth@levien.net      Classroom website: http://erlevien.blogspot.com/


Introduction to Stoichiometry — Lesson Plan #4

Grade level(s): 10th, 11th

Content area: Chemistry

Describe the learners : The learners are students interested in the sciences or those who are seeking to fulfill either their physical science credit to graduate (according to Maryland State Requirements). This is an on-level class of students, aged 15-17 years old. Their SES background is diverse, with about 10% of our class on FARMs.   Typical behavior is orderly, though some students need to be reminded to stay on task.   In terms of interests, this is also a diverse classroom: some are taking chemistry because they are very interested in science and pursuing a science career; other students are only in here to complete their science graduation requirements.   A few students have IEP and 504 plans.

Describe your physical teaching environment : This lesson will take place in the laboratory half of the classroom, where students work at lab benches that have faucets and drains (and a gas line, though it is not necessary for this lab).

Lesson context : Students will begin to now consider the quantities of chemical reactions, as well as their qualities.   They have been successfully writing balanced equations and classifying the equation.

Goal(s) : The goal of this lesson is to begin looking at stoichiometry—or the quantities pertaining to a chemical reaction.

Maryland Standard(s) Addressed :

Expectation 4.4

The student will explain and demonstrate that matter undergoes transformations in such a way that the products have properties that are very different from those of the starting materials.

Science Indicator 4.4.A

(5) The student will demonstrate that adjusting quantities of reactants will affect the amounts of products formed. At least --

  • use the coefficients of a balanced equation to predict amounts of reactants and products

Objective :   Students will continue practicing writing balanced equations and classifying the type of reaction, but now, students will focus on the stoichiometry of chemical reactions.

Estimated time : 1 class period

Materials needed : This lesson requires:

  • Whiteboard, blackboard, overhead projector
  • Student sets of worksheets

Prerequisite skills : Students need to be able to:

  • Have a firm understanding of the mole, mass, and Avogadro’s number.
  • Write a chemical formula and a balanced chemical reaction
  • Classify a chemical reaction according to the pattern it follows

Introduction – (5 minutes)

MotivationRemember those cookies we were talking about baking a few days ago? Well, I did everything I was told to do but I only ended up with 12 cookies.   I needed to bake enough cookies for all of my classes, plus some for the cookie-mongering teachers.

ConnectionLuckily, because the ratios of those ingredients are set, I could easily ‘scale’ up the recipe to bake for over 100 hundred people!   And no matter how many cookies I baked, they always tasted the same, because they were the same!   Each cookie had the same proportions of ingredients.     

Information Well, the same is true of a chemical reaction.   Because the ratios of reactants and products are set, chemists can differ the amounts of products by differing the amounts of reactants.   The math behind this is termed: stoichiometry.   Inform the students of the objectives: today, we are going to be predicting the amounts of products yielded based on the given information about the amounts of reactants, and vice versa!

Presentation: (25 minutes)

  • In an effort to reduce the initial anxiety students often feel towards stoichiometry, both because of its name and the fact that it is all math-based, take the time to ease these fears:
  • Before we look at chemical reactions, let’s prove to ourselves that we are already familiar with stoichiometry.   Here’s a question, if I told you that you had 298 quarters, is there a way to figure out how many dimes or nickels that is equal to, without actually having to count? (Yes, someone offers a mathematical shortcut involving multiplication). Great! That saves me a lot of time.   What about with cooking?   If I realize I have 4 bags of chocolate chips, can I figure out how many cookies I can make? (Again, someone intuitively knows that this is possible and should be able to provide the math).
  • In chemistry, we have the same accurate and predictive powers in stoichiometry.   Let’s look at an example:
  • Lead will react with hydrochloric acid to produce lead (II) chloride and hydrogen. How many moles of hydrochloric acid are needed to react with 0.36 moles of lead?
  • First, write the balanced equation:   Pb(s) + 2HCl(aq) -->PbCl2(s) + H2(g)
  • Second, write the given: 0.36 moles of lead
  • Third, write what we are looking for: moles of HCl?
  • Create a series of conversion factors that use the set ratios of the chemical species based on the balanced equation and their molar masses:
  • ? moles of HCl =   0.36 moles of lead (2 mole HCl/ 1 mole Pb)
  • First, perform dimensional analysis to make sure that my units reduce to the unknown units, in this case, moles HCl.
  • Second, multiply the appropriate numbers to calculate the amount of the species in question.
  • Sometimes, we can work backwards.   For instance, if you know you want to make 300 cookies, you can figure out the amounts of the ingredients you’ll need to purchase at a grocery store.   Same goes for chemistry!
  • Let’s try one together:
    • How many moles of nitrogen gas are needed to produce 0.48 moles of ammonia?
    • N2 + 3H2 --> 2NH3
    • Given: 0.48 moles NH3
    • Unknown: moles N2 gas
    • Set-up: ? moles N2 gas = (0.48 moles NH3) (1 mol N2 / (2 moles NH3)

Guided Practice: (15 minutes)

    Provide students with worksheets.   They will begin working on the problems with their partners.   Reassure students that with stoichiometry, practice makes perfect.   It is important that they continue trying these problems to begin to develop an ease with solving these problems.

Independent Practice:

    The work should be finished for homework.

Closure(5 minutes)

  • Review what stoichiometry means: if you know how much of one reactant or product you have, you can figure out the amounts of the other reactants and products involved.
  • This makes writing correct formulas and balanced equations doubly important—imagine calculating that a recipe only requires 3 chocolate chips instead of 300!

Assessment

  • Formative assessment - gauge students' responses and continually check for understanding prior to moving on to the next example.   If students have trouble with either the recipe or money analogy, explain in detail.
  • Summative assessment – Students will have to complete the guided practice questions and turn them in for grading.

Application of Bloom’s Taxonomy: This lesson is mainly focused at the knowledge level.   As the lesson proceeds, students will have to exhibit comprehension in order to solve these problems.   It is important to emphasize what the math means by using real examples.

Extensions:

      • For their week-long assessment, have students implement stoichiometry into the reactions.
      • Create a situation where they realize ignorance of stoichiometry leads to a catastrophe!

Differentiating Instruction :

  • As this lesson is teacher-centered, there is a risk that students will not pay attention.   Be sure to ask open-ended questions throughout the presentation, making sure to call on different students everytime.
  • For more advanced learners, have students try to solve problems before you do.

Accommodations:

Provide handouts and guided notes that allow students to watch and listen without having to frantically try to take notes.

 

Student Handout

Name
Date
Period
    

Stoichiometry Practice Problems

 

  • Carbon will react with zinc oxide to produce zinc and carbon dioxide. How many moles of CO2 will be produced if 0.38 moles of ZnO is completely reacted?

 

 

  •   How many moles of NaBr will be produced when 0.69 mol of bromine reacts according to the equation:        Br2 + 2NaI --> 2NaBr + I2?

 

 

  • Phosphorous will react with bromine to produce phosphorous tribromide. How many moles of PBr3 will be produced if 0.78 mol of Br2 is reacted?

 

 

  • How many moles of hydrogen will be produced if 0.44 mole of CaH 2 reacts according to the equation: CaH2 + 2H2O --> Ca(OH)2 + 2H2?

 

 

 

  • How many moles of oxygen will be needed to react with 0.38 moles of C3H8 to cause complete combustion?

 

 

 

  • Iron and oxygen react to make Fe2O3. How many moles of Fe2O3 will be produced if 0.18 mol of Fe reacts?

 

 

  • How many moles of water will be produced in the complete combustion of 2.35 moles of benzene, C6H6?