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Teaching Stoichiometry

Teaching Stoichiometry. Paul Daye Oksana Hrycyk. Curriculum Expectations. D2.1 use appropriate terminology related to quantities in chemical reactions, including, but not limited to: stoichiometry, percentage yield, limiting reagent, mole, and atomic mass [C ]

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Teaching Stoichiometry

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  1. Teaching Stoichiometry Paul Daye Oksana Hrycyk

  2. Curriculum Expectations • D2.1 use appropriate terminology related to quantities in chemical reactions, including, but not limited to: stoichiometry, percentage yield, limiting reagent, mole, and atomic mass [C] • D2.3 solve problems related to quantities in chemical reactions by performing calculations involving quantities in moles, number of particles, and atomic mass [AI] • D2.5 calculate the corresponding mass, or quantity in moles or molecules, for any given reactant or product in a balanced chemical equation as well as for any other reactant or product in the chemical reaction [AI] • D2.6 solve problems related to quantities in chemical reactions by performing calculations involving percentage yield and limiting reagents [AI] • D2.7 conduct an inquiry to determine the actual yield, theoretical yield, and percentage yield of the products of a chemical reaction (e.g., a chemical reaction between steel wool and copper(II) sulfate solution), assess the effectiveness of the procedure, and suggest sources of experimental error [PR, AI]

  3. Lesson Sequence • Lesson 1: Introduction of the Mole Concept • Avogadro’s number • Lesson 2: Balancing Chemical Reactions • Lesson 3: Types of Stoichiometric Problems • mass-mass, mass-volume, volume-volume • Lesson 4: The Limiting Reactant • Lesson 5: Percent Yield • Calculate Actual and Theoretical Yields

  4. Stoichiometry • Students will be able to : • Understand moles, mass, representative particles (atoms, molecules, formula units), molar mass, and Avogadro’s number. • Determine the percent composition of an element in a compound. • Write balanced chemical equations: for example, for a given mass of a reactant, calculate the amount of product produced. • Determine the limiting reactants: calculate the amount of product formed when given the amounts of all the reactants present. • Calculate the percent yield of a reaction.

  5. Introduction to Stoichiometry • Stoichiometry is the chemical term used to describe calculations that allow us to find the amounts of chemicals involved in a given reaction. MINDS ON - Making S’mores • In this “Minds On” activity students will be introduced to simple stoichiometry. • Each student will be given a small bag containing either gram crackers, marshmallows or chocolate pieces • Students will form groups of three and determine how many s’mores can be made when they combine their bags of ingredients, which ingredient is the limiting reactant, and which are in excess. • Students will use abalanced recipe (equation): 2 Gc + 1 M + 4 Cp = Gc2MCp4 ( or 1Sm) Where: Gc = gram cracker Cp = chocolate pieces M = marshmallow Sm = S’more

  6. 2Gc+ 1 M + 4Cp Gc2MCp4 Balancing Equations and the Limiting Reactant • MINDS ON - Making S’mores continued: • Students will complete this exercise that provides an additional example of a balanced equation and a limiting reagent using common materials. • Students will be required to make-up their own example of a balance equation and share it with a partner. A balanced equation for making a S’more is: excess Gc and excess Cp ? Gc2MCp4 30 M excess M and excess Cp ? Gc2MCp4 30 Gc 30 Gc and excess M 12 Cp ? Gc2MCp4

  7. Cl H H H Cl Cl Cl H reactants products reactants products H H Cl Cl Unbalanced and Balanced Equations • Students then apply the concept of balanced reactions to real chemical equations. H2 + Cl2 HCl H2 + Cl2 2 HCl H Cl Cl Cl H H (unbalanced) (balanced) 2 1 2 2 1 2 2 2

  8. Avogadro’sNumber • A MOLE of any substance contains as many elementary units (atoms and molecules) as the number of atoms in 12 g of the isotope of carbon-12. • This number is called AVOGADRO’s number NA = 6.02 x 1023particles/mol • The mass of one mole of a substance is called MOLAR MASS symbolized by MM, which can be found on the periodic table. • Units of MM are g/mol • Examples H2 hydrogen 2.02 g/mol He helium 4.0 g/mol N2 nitrogen 28.0 g/mol O2 oxygen 32.0 g/mol CO2 carbon dioxide 44.0 g/mol Consider a mole to be a collection of atoms/molecules. Here are some common collections we use everyday and the numbers of items in them.

  9. How Big is a Mole? One mole of marbles would cover the entire Earth (oceans included) for a depth of three miles. One mole of $100 bills stacked one on top of another would reach from the Sun to Pluto and back 7.5 million times. ? quadrillions thousands trillions billions millions 1 mole = 602213673600000000000000 or 6.022 x 1023 It would take light 9500 years to travel from the bottom to the top of a stack of 1 mole of $10 bills.

  10. Stoichiometry Steps 1. Write a balanced equation. 2. Identify known & unknown. 3. Line up conversion factors and determine the: Mole ratio - moles  moles Molar mass - moles  grams Molarity - moles  liters soln Molar volume - moles  liters gas Core step in all stoichiometry problems!! 4. Use the mole ratio to calculate moles 5. Use molar mass to calculate grams 6. Check answer.

  11. Stoichiometry - Problem Solving 2KClO3 2KCl + 3O2 ? mol 9 mol • Students will apply their knowledge while working in groups to solve a variety of questions involving mole ratios and molar mass. How many moles of KClO3 must decompose in order to produce 9 moles of oxygen gas? 2 mol KClO3 x mol KClO3 = 9 mol O2 = 6 mol KClO3 3 mol O2 Mole Ratio

  12. Stoichiometry Experiment • Labs Experiment • Students will have the opportunity to perform a small experiment involving Stoichiometry and Baking Soda(NaHCO3) Students will: • calculate theoretical mass of NaCl based on a known mass of NaHCO3. • Experimentally determine the actual mass of NaCl produced. • Calculate the percent yield of the product. Assessment • Students will write-up and submit lab report • Students will complete a worksheet with follow-up exercises designed to assess their understanding of the concepts. • Assessed according to K/U, T/I, C achievement chart • Safety Consideration • Remember to always wear safety goggles. • Tie back long hair • Wear gloves when dispensing acid • Follow established guidelines when working with Bunsen burners

  13. Stoichiometry using Gizmo Activities • These interactive activities will allow students to learn how to balancing chemical equations and determine the limiting reactant. • Students will simulate different types of chemical reactions. i.e. combination, double displacement, decomposition. • Students will complete worksheet and submit for assessment

  14. Students will solve a variety of stoichiometric problems that deal with real world examples. • Students will understand the importance of stoichiometry in related areas such as medicine, agriculture and space exploration. Stoichiometry in the Real World • Airbags • For an airbag to deploy an exact quantity of nitrogen gas must be produced in an instant • 2 NaN3(s)  2 Na(s) + 3 N2(g) • Fuels • How many grams of liquid oxygen would a rocket have to carry to burn 10 kg of rocket fuel diborane completely?

  15. Differentiated Assessment • Prior to introducing stoichiometry, a diagnostic quiz will be administered testing student knowledge on chemical bonding (both ionic and molecular) and chemical reactions, as well as calculating percent, using ratios and applying algebra (rearranging formulas, manipulating variables, solving for unknowns etc.) • The students will be expected to hand in the Student Exploration Worksheet associated with the Stoichiometry/Balancing Equations Gizmos Lab, which will be marked for Knowledge and Understanding, and Thinking and Inquiry. • Students will be assessed during the Stoichiometry and Baking Soda lab based on their performance for communication. The follow-up worksheets based on this lab will be marked for Knowledge and Understanding, as well as Application. • Students will write one or two quizzes assessing the progress of their understanding, including definitions, balancing equations and solving stoichiometric problems. • Students will be assessed daily on their oral responses during in class discussions. • For summative assessment, students will write a unit test which will assess all the learning skills and will include various definitions, lab results and all types of stoichiometric problems (some with already balanced chemical equations, some without), including determining empirical formulas, percent yield etc.

  16. Potential Student Difficulties and Solutions • Students may experience difficulty with the mathematics involved, specifically with balancing equations and knowing how and when to use stoichiometric formulas. Students will need to apply knowledge of ratios to balance equations and algebra to solve stoichiometric formulas. • Time should be spent reviewing these math concepts, as well as working through various applicable problems to clarify the methodology, including tips and tricks. Another method, particularly with solving stoichiometric problems, is to teach students to not rely on formulas, but rather use logic. The method of dimensional analysis can be taught to students • Students may have problems understanding the concept of a mole and Avogadro’s number. • Explain the concept through the baker’s dozen analogy. A dozen is to 12 units the same way a mole is to 6.022 x 1023 units, except that a dozen is often a numerical term used to describe quantities of food, such as muffins or eggs, while a mole is a numerical term used to describe quantities in chemistry, such as atoms or molecules.

  17. Potential Student Difficulties and Solutions cont. • Students may also find the law of conservation of mass and balancing equations challenging. • Students may have difficulty with assigning coefficients to the chemical compounds particularly when compound have more than one of the same element (i.e. use subscripts). • The Balancing Equations Gizmos Lab and the S’Mores and weight-balance analogies along with clear definitions and examples may help clarify any misunderstandings as well as clarify various other questions that may arise. Various methods for solving balancing equation problems may also be introduced like trial and error, or mathematical ratios and charts.

  18. Annotated References • Mortimer, C.E. (1978). Chemistry: A Conceptual Approach. D. Van Nostrand Company, New York. This book was used primarily as a source of background information on stoichiometry, the law of conservation of mass, moles, chemical equations etc. Although the material presented in this textbook is a brief and not well-explained, it is an excellent resource to refresh a teacher’s understanding of general chemistry, as well as offers some excellent, systematically and clearly solved examples, as well as an abundance of practice questions for students. • We found these websites useful: • Balancing Chemical Equations Gizmo. http://www.explorelearning.com/index.cfm?method=cResource.dspView&ResourceID=408 • Chemistry: Stoichiometry lesson plans/ideas. http://www.unit5.org/chemistry/stoichiometry.html

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