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In this lesson, students should be able to use a graphic organizer to help them solve stoichiometry problems. This lesson utilizes the Cornell note format.
By the end of this lesson, students should be able to
- Understand the concept of stoichiometry.
- Understand the importance of stoichiometry in an industrial setting.
- Use a graphic organizer to construct a solution to a stoichiometry problem.
- Calculate theoretical yield using dimensional analysis.
This lesson supports students’ understanding of
- Dimensional Analysis
- Mole Concept
- Theoretical Yield
Teacher Preparation: 10 minutes
Lesson: 45 minutes
- Document camera and projector OR whiteboard and markers
- Notebook paper formatted as Cornell notes. Please see the picture below.
- No safety considerations.
- Have students set up their note paper before the lesson begins. In the top section, students should title their notes “Stoichiometry”. In the next section, Students should write the essential question “How is theoretical yield calculated using stoichiometry?” The next section is divided; the left column is for questions and the right column is for notes (the answers to the questions). The bottom section is reserved for students to summarize their notes. Their summary should be a brief answer to their essential question.
- Students should read the background section and answer questions in their notes before the teacher explains the process of stoichiometry using the example.
- Keep students engaged by asking them to calculate answers as you work through the example. Students should already have an understanding of the concept of the mole and molar mass. Students should also be able to balance equations prior to this lesson.
- You may omit the “Volume of gas” stems on the stoichiometry map if you would rather address gas stoichiometry as part of a gas laws unit. These stems can be added later when you feel it is most appropriate.
- Procedure: This procedure is for the instructor in order to guide the lesson, and should be followed after students read the background section and answer the Getting Started questions. Bold portions of this procedure should be written in to the students’ notes. Italicized portions should be written in to the students’ left column (questions column) and the notes they write in the right hand column should explain the italicized portions. See the example notes below:
- Example Problem: What mass of oxygen is produced when 150.0g of water decomposes in to hydrogen gas and oxygen gas?
- Ask students how they can relate the decomposition of water in to hydrogen gas and oxygen gas. Lead students to the conclusion that this relationship can be written as a balanced chemical equation.
- Step 1: Write a balanced chemical equation.
- Ask students to help you write the equation. They should write the balanced chemical equation in their notes.
- 2H2O → 2H2 + O2
- Step 2: Identify the known and the wanted.
- Have students read the example problem again. The known is the amount of substance they were given; in the example problem, that is 150.0g of water. The wanted amount is the amount they need to calculate; in the example problem, that is the mass of oxygen (O2) produced.
- Known: 150.0 g of H₂O
- Wanted: g of O2 produced
- Ask students to use the chemical equation to related the amount of water to the amount of oxygen. They can think of it as a recipe if they want to- for every two parts water, they will make one part oxygen. Instead of parts, they can think of it as molecules. For every two water molecules, one oxygen molecule will be produced. Since chemists deal with large quantities of molecules, it is easiest to think of the relationship in terms of moles. For every two moles of water, we will produce one mole of oxygen.
- Step 3: Use the balanced equation to determine the mole ratio (relationship between the known and wanted).
- In this example, the mole ratio is 2 mol H₂O:1 mol O₂. Students need to understand that this is the connection between the known substance and the wanted substance, and that the mole ratio comes from the coefficients in the balanced chemical equation.
- Step 4: Determine the path to the solution using the stoichiometry map.
- Have students copy the graphic organizer in to their notes. This is their stoichiometry map. Students should start at the left of the graphic organizer with their known amount and move to the right towards their wanted amount. For this example, they are starting with “known mass”. They will have to use a conversion factor (in the box) to convert mass to moles. Then they will use the mole ratio in the middle of the graphic organizer. They need to calculate a wanted mass, so they will need another conversion factor to get to their answer.
- Step 5: Use the conversion factors to set up dimensional analysis.
- Students should be somewhat familiar with dimensional analysis. They need to set up the dimensional analysis so that the known goes first, then the conversion factors. The units of the conversion factors should cancel out top to bottom. The only conversion factor that does not cancel out is the unit you want to calculate. In our example problem, we will end up with grams of O₂. Feel free to change the conversion factors to proportions if that’s how you teach dimensional analysis.
- Step 6: Calculate the theoretical yield by multiplying across and dividing top by bottom.
- Have students multiply the numbers on top (150.0 x 1 x 1 x 31.998). Ask them if there’s an easier way to calculate the top. Students should understand that the number 1 can be left out of their calculation; therefore, the calculation is simplified to (150.0 x 31.998). The bottom is calculated by multiplying (18.015 x 2). The product of the top is divided by the product of the bottom. Have students do the calculation. They should determine that about 133.2 g of O₂ will be produced in the reaction. Have students understand that this calculated value is considered the theoretical yield- the amount that should be produced if the reaction goes to completion.
For the Student
Chemistry is used outside of the classroom in many different industries such as medicine, food production, agriculture, and manufacturing. Plastics manufacturing is a billion-dollar industry that utilizes chemistry. According to http://www.essentialchemicalindustry.org, over 60 million tons of polyethylene, a component of plastics, is produced every year worldwide. These plastics are used in a wide variety of products from food packaging to clothing to prosthetic limbs. Polyethylene terephthalate, or PETE, is a common type of polymer used in plastics. Bis (2-hydroxyethyl) terephthalate, or BHET, is a component of PETE and is formed by the synthesis of ethylene glycol and terephthalic acid.
The plastics industry must use specific amounts of ethylene glycol and terephthalic acid in order to produce the BHET necessary to make PETE. If a chemical plant needs to make 5000 tons of plastic each month, how do they know how much of the reactants to use? The chemists that figure out these amounts must be specific; since the chemicals used in the production of plastic cost money, any unused reactants will amount to wasted money. Chemists figure out the amount of chemical to use to create a specific amount of product by using the process of stoichiometry. In stoichiometry, known amounts of a product or reactant are used to calculate unknown amounts of a different substance. The chemists in the plastics manufacturing plant can use stoichiometry to calculate the necessary amounts of ethylene glycol and terephthalic acid necessary to produce the 5000 tons of plastic they want to make each month.
In stoichiometry, chemists will use a mathematical process called dimensional analysis to convert the known amount (5000 tons of PETE) in to the wanted amount (tons of ethylene glycol and terephthalic acid). We can use simpler chemical reactions to practice using stoichiometry in chemistry. In this lesson, you will learn how to perform stoichiometric calculations using dimensional analysis. You will be able to calculate the theoretical amount of product (theoretical yield) that should be produced in a chemical reaction when the amounts of reactants are known.
- Organize your notebook paper as shown in the diagram.
- Write the following questions in the left column (the questions) column. Answer the questions using the background information. Write your answers (your notes) in the right column.
- What is stoichiometry?
- How can stoichiometry be used outside of the classroom?
- What mathematical process will be used in stoichiometric calculations?
Write this question at the top section of your notes page:
How is theoretical yield calculated using stoichiometry?
- How many molecules of H2 are produced from the decomposition of 150.0 g of water? (Remember that atoms and molecules are considered particles!)
- What is the theoretical yield (in grams) of water produced when 50 g of H2 gas reacts with oxygen in a synthesis reaction?
- What mass of oxygen is necessary for 25 g of H2 to form water? Assume the reaction goes to completion.
Summarize your notes in the bottom section of the page. Make sure your brief (1-2 sentence) summary answers the essential question at the top of your notes.