STOICHIOMETRY:
‘Quantities in Chemical Reactions’
OISE Senior Chemistry AQ
Concept Presentation – November 9, 2011
Presented by Wendy Louis
Expectations
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 Concepts to be Learned
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.
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)
3.75 g
??? grams
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”
Providing Algorithms to Assist with
Problem Solving
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.
Lesson 2: Using Molar Masses in
Stoichiometric Problems
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?
Concluding statement: 3.75 g of nitrogen requires 0.79 g of hydrogen to react completely.
Use the Four steps required to solve stoichiometric problems (Algorithm) to solve.
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 completely?
Concluding Statement: From 3.75 g of nitrogen, 4.53 g of ammonia are produced.
Use the Four steps required to solve stoichiometric problems (Algorithm) to solve.
Student Demonstration: Explaining the
Quantitative Relationships Expressed in Balanced
Equations
To 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)
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
particles
2.35 x 1023
particles
1.60 x 1023
particles
2 NH3 (g)
Additional Lessons Within the Unit

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
Student Difficulties & Potential
Solutions
Mathematical Difficulties:
Problem: Due to the multi-step calculations required to solve mole-to-mass conversion word
problems, students may experience difficulty understanding how to start their solution, how to
proceed once started, and how to keep track of what the values they have calculated represent.
Solution: Use The Four steps required to solve stoichiometric problems algorithm. Then, as a
cooperative learning strategy, students will work in groups of 3 -4 developing a flowchart(s) as
an algorithm.
Chemistry Difficulties:
Problem: Another misconception students will have is they might believe that ‘The coefficients
in the balanced equation relate the masses of the substances’.
Solution: A practical example to clarify : The teacher can write an equation on the board for the
formation of water such as, ‘Two moles of hydrogen plus one mole of oxygen react to form two
moles of water’. Emphasize that the coefficients reveal the relationship between the reactants
and the product and provide the conversion factors needed to solve stoichiometric problems,
not their masses.
Student Difficulties & Potential
Solutions Cont’d
Problem: Another misconception students will have is that they might
believe that “Changing the coefficient for a substance changes the molar
mass.”
Solution: Students may use the product of the molar mass and the
coefficient as the molar mass. When the coefficient is also used in the mole
ratio, the stoichiometry is incorrect.
Problem: The chemical vocabulary associated with these operations may
prevent the students from applying their mathematical skills to chemical
phenomena.
Solution: Prepare a word wall with the vocabulary. Then have students
prepare a ‘Definitions’ sheet.
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