STOICHIOMETRY

OISE Senior Chemistry AQConcept Presentation – November 9, 2011

Presented by Wendy Louis

Expectations Addressed from the ‘Quantities in Chemical Reactions’ Strand of Grade 11 Chemistry – University Preparation

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]

D3.4 Explain the quantitative relationships expressed in a balanced chemical equation, using appropriate units of measure (e.g., moles, grams, atoms, ions, molecules)

Key concept students will learn: The coefficients of a chemical equation are the basis for interpreting and predicting

quantitative relationships in chemical reactions (stoichiometry).

By the end of the unit students should be able to:

i) Determine the amount of any substance in a reaction, given the amount of any other substance;

ii) Determine the mass of any substance in a reaction, given the amount or mass of any other substance.

Expectations & Key Concepts

Background Information

The mole concept is introduced and used in stoichiometric calculations. Students learn to convert the mass of a substance to the number of particles and the number of moles of the substance.

The mole concept is the link that relates the macroscopic measurement of mass or volume with the submicroscopic number of particles.

The following is an overview of the lesson sequences for the unit.

The focus of this presentation will be on Lessons 2- 4. It will be presented in depth since this lesson represents the expectations to be addressed (D 2.5 and D 3.4) and concepts to be learned. The lab, Gizmos lab work period, and Peer Activity ‘Developing Flowcharts to Solve Problems’ also address these expectations.

Hook : ‘Hamburger Sandwiches’ Problem

Introduce the ‘Hamburger Sandwiches’ problem as an Investigation for students to solve without teacher assistance. This problem links their understanding of balanced equations, masses of reactants and products and to the concept of converting moles to mass in chemical equations.

(Lesson courtesy of Journal of Chemical Education Vol. 80 No. 9 September 2003 - see pdf file attached)

‘Hamburger Sandwiches’ Problem

Activity 1: Understanding Formulas and the Use of Equations

The principal has organized a school meeting and you are charged with ordering lunch from a hamburger chain called Burger Chem that makes sandwiches using two or three identical slices of bread.

You ordered 100 MacChem sandwiches, but somebody made a mistake and 100 HambChem and 100 CheeseChem were delivered instead. You need to convert these other types of sandwiches into 100 MacChem before the principal notices the error.

Q: How many HambChem and CheeseChem do you need to obtain 100 MacChem sandwiches if compositions are:

- HambChem: 2 slices of bread (B) and 2 hamburgers (H)- CheeseChem: 2 slices of bread and 2 slices of cheese (Ch)- MacChem: 3 slices of bread, 2 hamburgers, and 1 slice of cheese

Activity 2: Understanding Mass Conservation

Q: What is the mass of 100 MacChem sandwiches? Is there any relationship between the mass of 100 MacChem sandwiches and the masses of 100 HambChem plus 50 CheeseChem?

Data: mass of each slice of bread (B), 25 g; mass of each hamburger (H), 80 g; mass of each cheese slice (Ch), 15 g.

Student Exploration to Discover How to Use Molar Masses in

Stoichiometric Problems

Following the ‘Hamburger Sandwiches’ problem, the teacher can allow the students to work in groups on the following problem to observe their problem solving skills (diagnostic assessment):

If 3.75 grams of nitrogen gas are used in the following reaction, are 3 x 3.75 grams of hydrogen gas required for the reaction to occur completely? Why

or Why not? N2 (g) + 3 H2 (g) → 2 NH3 (g)

The teacher can provide the following hints:

Convert the macroscopic information / numbers (large-scale) to the microscopic numbers (small-scale).

“You need to use the molar mass of a compound (microscopic) to find its actual / real (macroscopic) mass”

Depending on student success and progress with the previous problem, the teacher can present the following steps to help students organize their solution process.

Four steps required to solve stoichiometric problems (Algorithm)

1. Write the balanced equation for the reaction. 2. Convert the given mass or volume of a reactant or product to moles.

3. Use the coefficients in the balanced equation to set up the appropriate mole ratio and multiply to calculate moles of the desired reactant or product.

4. Convert moles of the desired reactant or product to mass or volume required.

OR

Alternatively students could use the GRASS / GRASP (Given, Required, Analysis, Solution,

Sentence /Phrase) method.

Providing Algorithms to Assist with Problem Solving

Topic 1: Predicting the Mass of a Reactant

The students and teacher will now conduct formal ‘scientific’ calculations using the following reaction.

Q: Calculate the mass of the second reactant in the reaction:

N2 (g) + 3 H2 (g) → 2 NH3 (g)

Problem: How many grams of hydrogen gas are required for 3.75 grams of nitrogen gas to react completely?

Solution: Using the Four Steps process:

Step 1: The equation is balanced.

Step 2: Convert the given mass or volume of a reactant or product to moles.

Calculate the number of moles of nitrogen gas in 3.75 grams of nitrogen.The atomic mass of nitrogen is 14.007 u. There are 2 nitrogen atoms, therefore 28.014 u = 28.014 g / mol.

Set up a ratio to calculate the number of moles in 3.75 g of nitrogen: 28.014 g = 3.75 g 1 mol x x = 0.13 moles of nitrogen

Lesson 2: Using Molar Masses in Stoichiometric Problems

Step 3: Use the coefficients in the balanced equation to set up the appropriate mole ratio and multiply to calculate moles of the desired reactant or product.

1 mole of N2 (g) reacts with 3 moles of H2 (g) So the number of moles of hydrogen required = 0.13 x 3 = 0.39 moles

Step 4: Convert moles of the desired reactant or product to mass or volume required.

Molar mass of hydrogen = 2 x 1.008 u = 2.016 u = 2.016 g / mol

Set up a ratio to calculate the mass of hydrogen in 0.39 moles:

2.016 g = x1 mol 0.39 mol

X = 0.79 g of hydrogen

Concluding statement: 3.75 g of nitrogen requires 0.79 g of hydrogen to react completely.

Lesson 2: Using Molar Masses in Stoichiometric Problems cont’d

Topic 2: Predicting the Mass of a Product

The students and teacher will attempt to calculate the mass of the product given questions such as...

N2 (g) + 3 H2 (g) → 2 NH3 (g)

Problem: What mass of ammonia is formed when 3.75 g of nitrogen gas reacts with hydrogen gas?

Solution: Using the Four Steps process:

Step 1: The equation is balanced.

Step 2: Convert the given mass or volume of a reactant or product to moles.

Earlier we calculated that 3.75 grams of nitrogen contain 0.133 moles.

Lesson 2: Using Molar Masses in Stoichiometric Problems cont’d

Step 3: Use the coefficients in the balanced equation to set up the appropriate mole ratio and multiply to calculate moles of the desired reactant or product.

Use the balanced equation to set up a molar ratio: 1 mole of N2 (g) = 2 moles of NH3 (g) 0.133 moles x x = 0.266 moles of ammonia produced

Step 4: Convert moles of the desired product to mass required.

Calculate the mass of ammonia in 0.266 moles of NH3 (g).

The atomic mass of NH3 = 14.007 u + 3 (1.008 u) = 17.031 u = 17.031 g / mol

Set up a ratio to convert moles to mass: 17.013 g = x grams 1 mol 0.266 moles

x = 4.53 g

Concluding Statement: From 3.75 g of nitrogen, 4.53 g of ammonia are produced.

Lesson 2: Using Molar Masses in Stoichiometric Problems cont’d

Student Demonstration: Explaining the Quantitative Relationships Expressed in

Balanced EquationsTo summarize the information learned about

calculations and to assist students make the connection between macroscopic and microscopic levels of reactions have them develop a table similar to the following using the previous reaction or they can use another reaction.

The materials they will have available to demonstrate and teach their learning to the class about their tables :

- jellybeans of 3 different colours (one for each element; to represent particles in the reaction

- mini ‘mole’ animal toys;- scales; and- atoms from molecular kits.

They can then present their tables to the class to explain the relationship between number of :

- Moles;- Grams;- Atoms;-Molecules; and- Particles … in the reaction

N2 (g) 3 H2 (g) 2 NH3 (g)

1 mole 3 moles 2 moles

3.75 g 0.79 g 4.53 g

0.13 moles 0.39 moles 0.26 moles

2 nitrogen atoms

6 hydrogen atoms

2 nitrogen atoms & 6 hydrogen atoms

1 molecule 3 molecules 2 molecules

7.86 x 1022

particles2.35 x 1023

particles1.60 x 1023

particles

Lesson 3: Take up homework problems on Using Molar masses in Stoichiometric Problems

Lesson 4: Handouts, Mole Ratio, Review definitions and concepts by preparing Concept Map

Lesson 5: Using GIZMOs© computer program to Solve Stoichiometric Problems

Lesson 6: Peer Activity ‘Developing Flowcharts to Solve Problems’

Lesson 7: Quiz - ‘Definitions in Stoichiometry’ and Calculations

Lesson 7 & 8: Introduce ‘Actual &Theoretical Yield - Limiting Reactant’

Lesson 9: Lab - Reaction between aqueous copper (II) sulfate and iron. (see the File ‘Stoichiometry Lab’). Students will conduct lab to determine the amount of product produced and the percent yield

Lesson 10: Using Molar Volumes In Stoichiometric Problems

Lesson 11: i) Unit Review (Groups prepare a Concept Attainment Chart on ‘Stoichiometry’) and

ii) Start Culminating Task: Stoichiometric Applications in Industry

Additional Lessons Within the Unit

Student Difficulties & Assessment

Difficulties (see summary handout for potential solutions the teacher can use):

A) Due to the multi-step calculations required to solve mole-to-mass conversion word problems students may experience difficulty organizing their information.

B) Students believe the coefficients in the balanced equation relate the masses of the substances.

C) Students believe that changing the coefficient for a substance changes the molar mass.

D) The chemical vocabulary associated with these operations may prevent the students from applying their mathematical skills to chemical phenomena

Assessment:

Diagnostic Assessment: pre-unit quiz, Hamburger Sandwiches problem, and ammonia reaction problem

Formative Assessment: Homework, Gizmos work, Concept Attainment Strategy and class participation, Lab Report results

Summative Assessment: Unit test, quiz, Culminating Task