Limiting Reactant, Theoretical Yield, and Percent Yield

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Objective

Students will be able to distinguish limiting versus excess reactants in a chemical reaction as well as calculate percent yield as demonstrated by doing an activity, taking notes, and performing practice questions.

Big Idea

In chemical reactions a limiting recant causes a reaction to stop, while an excess reactant is leftover. Additionally one can calculate percent yield using the experimental value from performing a lab and the theoretical value from calculations.

Introduction

In this lesson students learn about limiting reactants, excess reactants, theoretical yield, actual yield, and percent yield. 

  • This activity aligns with HS-PS1-7: Use mathematical representations to support the claim that atoms, and therefore mass, are conserved during a chemical reaction.
  • This lesson aligns with NGSS Science and Engineering Practice 5: Using Mathematics and Computational Thinking because students are performing calculations using stoichiometry and the percent yield equation.  

For this lesson there is an activity in the engage section for which students need a ziploc bag filled with nuts and bolts.  I use 18 nuts and 7 bolts.

 

Engage

10 minutes

To begin the lesson I have students perform an activity with their groups where they have a chance to figure out what a limiting reactant is using everyday materials.

  • The activity is found on the top section of the student notes graphic organizer.  
  • I give each table group a plastic Ziploc bag filled with 18 nuts and 7 bolts.
  • I prompt students to follow the steps of the activity.
  • As students are working I walk around to make sure that they are working on the activity together as a group.
  • This video gives an explanation of how I have the students do the activity.
  • This is a copy of one student's filled in notes with the engage activity filled in.
  • The most common thing that I need to help students with is their not knowing what a nut and bolt are so when I pass out the bags to the groups I tell them the bolts are the ones that look like nails or screws and the nuts are the circles.

Explain/Elaborate

60 minutes

This section is the main part of the lesson where I do notes with my students.  I present the notes using the Unit 5 lecture 6 PowerPoint and students fill in the notes on their Unit 5 Lecture 6 Notes Graphic Organizer.  The lesson is set up where I first explain the material and then students perform examples.

  • The first section of the notes (slides 1-4) is where I explain what a limiting and an excess reactant are.  I also go over how to calculate a limiting reactant.  I have students convert all the way from grams to grams of each reactant (even though they could stop at moles) because it is less confusing in the long run for students.
  • In the next section of the notes (slides 5-10) I go through an example with students to figure out the limiting reactant and excess reactant.
  • In the next section of the notes (slides 11-13) I review with students the definitions of theoretical, actual, and percent yield.
  • In the final section of the notes (slides 14-20) I have students perform example problems using what they have learned in the lesson.  Students have a fairly easy time with the percent yield examples (1 and 2), but the 3rd example is very tricky and I have to lead students through the problem.
  • This is a copy of one student's filled in notes.

Evaluate

40 minutes

In this final section of the lesson students perform practice questions.

The questions are found on the Unit 5 lec 6 homework- limiting reactant paper.

I give students about 40 minutes of class time to start the questions so that they can get help from their partners and so that I can walk around and help them out.  If students do not complete the questions in class I expect them to complete the questions for homework. 

There was a typo on number 4 in the homework so that they theoretical is 0.37 grams and the actual is 2.50 grams which makes a very high percent yield.  I changed the problem so that next  year the actual yield with be 0.25 grams.  Here is a copy of the fixed practice questions.

This is one student's filled in homework.

I check the homework for completion the following day by giving students a stamp and then go over the answers with the answer key.  I make sure to take time to go over HOW to answer the questions, especially 3 and 4 which are tricky (and #4 which had the mistake).