In this lesson, students will learn how to follow a step-by-step problem solving method for limiting reactant stoichiometry problems. This method can be particularly beneficial for students who struggle with completing these calculations.
By the end of this lesson, students should be able to
- Apply a specific problem solving method to successfully calculate the limiting reactant in a given problem.
- Use dimensional analysis to complete a calculation.
- Determine which reactant is limiting and which reactant is in excess.
This lesson supports students’ understanding of
- Limiting Reactant
- Dimensional Analysis
Teacher Preparation: None
Lesson: 60 minutes
- Student handouts available via download
- Periodic table
- No safety considerations are needed for this activity.
- The Limiting Reactant Map is very valuable for students who struggle to complete multi-step calculations, and dimensional analysis.
- The downloadable student handouts should be used as guides when introducing limiting reactant stoichiometry problems.
- For more information about the map, read about it in the November 2016 issue of Chemistry Solutions.
- If you are interested in learning about a similar problem solving method for all stoichiometry problems, read about the Stoichiometry Set-up method in the September 2015 issue of Chemistry Solutions, or refer to this lesson.
For the Student
Steps to follow in order to determine the limiting reactant:
- Write a balanced chemical equation.
- Write what is given and what needs to be calculated on the equation as follows:
- Data in moles go under the equation.
- Data in other units go above the equation.
- Draw a box containing two lines underneath any one of the reactants. This will serve as the chosen reactant for future calculations.
- Label the top line “Have.”
- Label bottom line “Needed” and put an “X” on the line.
- Draw a path, using arrows, from the given data to the box under your chemical equation.
- Separate vertical and horizontal arrows.
- Horizontal arrows must go under the equation.
- Using the given quantity of the chosen reactant, calculate its number of moles and write the answer on the line labelled “Have.”
- Using the given quantity from the other reactant, calculate the number of moles needed for the chosen reactant and write the answer on the line labelled “Needed.”
- Determining the Limiting Reactant: If there is more reactant on the “Have” line than what is identified on the “Needed” line, that reactant is in excess. If there is less of the chosen reactant than the quantity needed to completely react, then the chosen reactant is the limiting reactant.
The same map technique can be used for converting any type of unit in a stoichiometry problem as well as limiting reactant problem. The following table shows how to use many different units in this same technique.
This technique provides a road map for solving any type of stoichiometric problem involving a limiting reactant calculation. If the:
- vertical arrow goes through a material measured in grams, the molar mass is used in the conversion
- unit given is particles (atoms, molecules, or formula units), Avogadro’s number will be used in the conversion
- problem deals with gas, either molar volume (at STP) with be used or the ideal gas law (solving for n = PV/RT)
- vertical arrow goes through a solution, the solution’s molarity will need to be used
- vertical arrow goes through an electron in an electrolysis problem, the amperage and the faraday constant will be used