• Assess students’ understanding orally by asking questions as you work through the examples. Provide additional examples and practice as needed.
• Collect and assess the worksheets and the lab questions and conclusion.
• Have students choose a recipe and write a paragraph describing it, using the analogy of the baker using the recipe compared to a chemist using moles.

To prepare, read through the Limiting Reagents Lab and gather all materials needed before Day 2.

Note: It is a common misunderstanding for students to think that chemical reactions show mass relationships instead of mole relationships. For example, a student may interpret the chemical reaction Zn + 2HCl → H₂ + ZnCl₂, as “one gram of zinc plus two grams of hydrochloric acid yields one gram of hydrogen plus one gram of zinc chloride.” Instead, the reaction shows that “one mole of zinc reacts with two moles of hydrochloric acid to yield one mole of hydrogen and one mole of zinc chloride.”

Day 1: Stoichiometry Instruction and Practice

Begin the lesson by having students answer the questions on the Chocolate Chip Cookies handout (project with a document camera or make copies) (S-C-8-2_Chocolate Chip Cookies.doc). Explain that in a similar way to how bakers use recipes, chemists use “recipes” too, but they are called chemical equations. Instead of cups and teaspoons, chemists use moles; chemical compounds are the ingredients and products. Looking at a chemical equation shows a chemist how much of a substance is needed to react with another substance to get a certain amount of product. Define stoichiometry as calculating quantities in chemical reactions. For example,

2Na + Cl₂ → 2NaCl

Explain the reaction in terms of moles, and ask, “How many moles of sodium and chlorine would you need to get 10 moles of NaCl?” (You would need 10 moles of sodium and 5 moles of chlorine.) “If you had 3 moles of chlorine, how many moles of sodium would you need? How much sodium chloride would be produced?” (You would need 6 moles of sodium, and 6 moles of sodium chloride would be produced.)

Explain that we can use mole ratios to calculate the moles of one substance if we know the moles of another substance in the reaction. Show students the following example:

2Na + Cl₂ → 2NaCl

“How many moles of chlorine are needed to react with 5 moles of sodium, without any sodium left over?” (2.5 moles of chlorine; calculations shown below)

Ask, “How many moles of sodium chloride will be produced if you react 3.1 mol chlorine gas with an excess (more than you need) of sodium?”

Tell students that given a balanced chemical equation, we can also convert between moles and mass, volume, and number of representative particles. Hand out copies of the Mole Map (S-C-8-2_Mole Map.pdf).

Go over each of the Conversion Examples step-by-step, showing students how to use the Mole Map to solve the problems (S-C-8-2_Conversion Examples.doc).

Optionally, explain how to convert between moles and volume of a gas at STP and ask the following two questions. Explain that 1 mole of any gas at STP has a volume of 22.4 Liters. Use the Mole Map to explain how to perform the conversions.

Ask, “What is the volume of 3 mol of oxygen at STP?”

Ask, “What is the volume of 3 mol of chlorine gas at STP?” (67.2 L)

Show students how to apply the mole concept and molar conversions to balanced chemical reactions, using the examples below. Have students write the examples in their notes. Be sure to reinforce the concept that balanced chemical reactions show molar relationships, not mass relationships.

Example 1

Zn + 2HCl → H₂ + ZnCl₂

In the balanced reaction above, when zinc metal reacts with hydrochloric acid, hydrogen gas and zinc chloride are the products.

• How many moles of HCl are needed to produce 1 mole of ZnCl₂?

(2 moles)

• If ½ mol of Zn is available, how many moles of ZnCl₂ can be produced?

(½ mol ZnCl₂)

• If ½ mol of Zn is available, how many moles of HCl would be used up?

(1 mol HCl)

Example 2

2Fe(s) + 3O₂ → Fe₂O₃

The balanced reaction above shows the reaction when an iron nail reacts with oxygen in the air to produce rust, Fe₂O₃.

If an iron nail with a mass of 30.7 g rusts away completely, what is the mass of the rust?

Step 1: Calculate the number of moles of iron in the nail.

Step 2: Convert from moles of iron to moles of rust.

Step 3: Convert from moles of rust to mass of rust.

Example 3 (Use this example only if you have included conversions to gases at STP in your lesson.)

2Fe + 3Cl₂ → 2FeCl₃

The balanced reaction above shows how iron and chlorine gas combine to produce iron(III) chloride, FeCl₃. What volume of chlorine gas will produce 4.0 moles of FeCl₃? The mole ratio of moles of FeCl₃ to moles of Cl₂ is 2:3. Note: Define and explain the term mole ratio at this point.

Therefore, 6 moles of Cl₂ are required to produce 4 moles of FeCl_3.

Hand out the Stoichiometry Practice worksheet (S-C-8-2_Stoichiometry Practice and KEY.doc). Have students work independently to solve each of the problems during class or for homework. Remind them to show their work and label the units.

Day 2: Limiting Reagents Instruction and Lab

Explain the role of limiting reagents in chemical reactions. (Note: The term “limiting reagent” is sometimes used interchangeably with “limiting reactant.”) Use the following example. When a car burns one gallon of gasoline, it reacts with oxygen from the air and produces products such as CO (carbon monoxide), H₂O, and others. After the gallon of gasoline is used up, there is still oxygen in the air, but the reaction stops because there is no more gasoline. The limiting reagent is the gasoline, because it is completely consumed in the chemical reaction. Have students copy the example below into their notes.

Example

C + O₂ → CO₂

Tell students, “In this reaction, only 3 grams of carbon are available but there is plenty of oxygen.”

Ask, “Which is the limiting reagent?” (carbon)

Ask, “How many grams of CO₂ would be produced?” (11.00 g CO₂

1 mol C will produce 1 mol CO₂, because the mole ratio is 1:1.

Therefore, 0.25 mol C will produce 0.25 mol CO₂.

Hand out copies of Limiting Reagents Practice (S-C-8-2_Limiting Reagents Practice and KEY.doc). Have students work on the handout independently.

Explain that limiting reagents play an important role in carrying out chemical reactions in the laboratory. For example, a pharmaceutical manufacturer knows that the amount of product that can be produced depends on the amount of the limiting reagent that is available. The amounts of reactants and products are very important in pharmaceutical manufacturing because the human body requires a precise amount of medication for it to be effective but not toxic. Tell students that in the lab activity, they will be determining which compound is the limiting reagent in a chemical reaction.

Give students copies of the Limiting Reagents Lab handouts (S-C-8-2_Limiting Reagents Lab and KEY.docx). Read through the procedure for Day 1 with students and answer any questions they have. Divide students into groups of four and have them complete the Day 1 lab procedure and answer questions 1 and 2. Have them clean up all materials.

During the next lesson, have students complete the Day 2 procedure for the lab and answer the remaining Data and Analysis questions, as well as write a conclusion for the lab. After students have finished the lab, go over the calculations for questions 9 and 10 step-by-step with the class. Discuss the role of the limiting reagent in this chemical reaction.

Extension:

• In addition to the Mole Map resource, you can provide students who might need an opportunity for additional learning with the Conversion Factors resource to aid them as they solve stoichiometry problems (S-C-8-2_Conversion Factors.doc).
• As an alternative conclusion for the lab, students who may be going beyond the standards can write an explanation of how it would be possible for CaCl₂ to be the limiting reagent in the chemical reaction that they carried out.