Mole Ratios

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Objective

Students will be able to calculate the number of moles produced or needed in a chemical reaction using a balanced chemical equation.

Big Idea

The mole provides chemists with a bridge between the microscopic and the macroscopic world. Mole ratios are used to predict the amount of product formed or reactants needed.

Introduction

In this lesson students will be able to calculate the number of moles produced or needed in a chemical reaction using a balanced chemical equation. They start by reviewing balancing chemical equations. They then take notes on what the coefficients mean in a balanced chemical equation and on how to use mole ratios. Students spend the bulk of class time is practicing using mole ratios.

This lesson aligns to the NGSS Disciplinary Core Idea of HS-PS1-7: Use mathematical representations to support the claim that atoms, and therefore mass, are conserved during a chemical reaction by introducing the idea of the mole as the conversion from the atomic to the macroscopic scale. It also requires that students balance chemical equations before they can determine the amount of moles needed or produced.

It aligns to the NGSS Practice of the Scientist of Using Mathematics and Computational Thinking because the use of mole ratios requires students to use basic algebraic thinking and analysis.

It aligns to the NGSS Crosscutting Concept of Stability and Change because chemical reactions are about constructing explanations of how things change.

In terms of prior knowledge or skills, students should have a solid understanding of how to balance chemical equations, and an understanding of what a chemical equation means.

There are no special materials needed for this lesson outside of what I provide.

 

 

 

Do Now/Activator

10 minutes

Do Now: Students begin class by balancing 3 chemical equations.

H2O + O2 --> H2O2

NaBr + CaF2 --> NaF + CaBr2

HNO3 + NaHCO3 --> NaNO3 + H2O + CO2

I reason that this is a good way to start class because students are just returning from vacation and I would like them to start class with a subject in which most students showed proficiency. I also know that balanced chemical equations are necessary for the proper use of mole ratios.

Activator: I ask a student to show their work or share their coefficients. They should be:

2 H2O + O2 --> 2 H2O2

2 NaBr + CaF2 --> 2 NaF + CaBr2

HNO3 + NaHCO3 --> NaNO3 + H2O + CO2

Note the last one is already balanced. I remind students that the first thing they should always do with a chemical reaction is to check to see if it is already balanced.

 

Mini-lesson and Guided Practice

15 minutes

Mini-lesson: Students will take notes using the Mole ratio notes organizer. I begin by interpreting one of the balanced chemical equations from the Do Now. I note that in the expression 2 H2O + O2 --> 2 H2O2 there is a ratio of 2:1:2. At the microscale, this reaction is mixing 2 molecules of water with 1 molecule of oxygen to produce 2 molecules of hydrogen peroxide. As long as the ratio stays the same, we could have more than 2:1:2. We could have 2 dozen: 1 dozen: 2 dozen, for example. However, both of those examples deal with masses that are so small that they are not practical to work with. Luckily for us, we have the mole, which you may recall is a large quantity: 6.022 x 1023.  Another way to interpret a balanced chemical equation is to say 2 moles of water with 1 mole of oxygen to produce 2 moles of hydrogen peroxide. The ratio stays the same, but now we are dealing with so many molecules that we can actually see and weigh them; in other words, we are now dealing with chemicals at the macroscale.

At this point I pause and ask students to pair with a partner to explain what this expression (2 NaBr + CaF2 --> 2 NaF + CaBr2) means using the words mole, molecule, and ratio. This pause in the mini-lesson is important because while I do not feel that practice around this concept is necessary, I do want students to stop and take a moment to process and work with what I said.

I then explain that we are going to learn how to use mole ratios. I note that mole ratios are a ratio comparison between substances in a balanced equation, and I explain how to use mole ratios to determine how many moles of product you can make or how many mole of reactant you need to make a certain amount of product. This video shows me teaching how to use mole ratios.

Guided Practice: Students are given a similar problem—How many moles of H2O2 will I produce if I start with 7. 2 moles of O2? The answer for this problem is 14.4 mol of H2O2.

I chose this particular focus so that students would see the importance of putting “asked” over “given.” I also know that students can watch me solve problems and it will make sense to them, but once they start working the problems themselves that is when the questions arise, and when learning occurs.

 

Application

25 minutes

Student Activity: During this portion of class students work on clarifying their confusion and then practicing how to use mole ratios using the Mole Ratio Practice problems. I circulate around the room observing student work, answering questions, and determining if there is a common sticking point for students that I can address during a catch and release moment.

I know that students will learn this skill if they practice, ask questions, and spend some time at toward the end of class articulating what they learned. 

 

Debrief

10 minutes

To wrap this lesson up I ask students to articulate answers to the following questions with a partner, and to record their answers:

  1. What 2 things can coefficients tell us in a chemical equation?
  2. Why do chemists use moles?
  3. What is a mole ratio used for?
  4. How do you use a mole ratio?

The answers I am looking for are:

  1. How many molecules and how many moles are involved in the reaction.
  2. To bridge the gap between the nanoscale and the macroscale.
  3. To compare two items in a chemical balanced chemical reaction.
  4. You multiply what is given by a ratio, written as a fraction, that compares what is being asked about on top, and what is given on the bottom. The numbers come from the coefficients in the balanced chemical equation.

After students have had a chance to work on this, I ask for partners to share their answers. Question 2 was the most difficult for students to understand. Next year I would like to spend more time on the topic of the mole. It was clear that students only have a vague understanding of what the mole is and why it is important. I would like to try this lesson next year.

Ending class this affords students the opportunity to reflect on what they learned in class. This synthesis is a key part of the learning process. It also gives students the chance to fill in gaps in their understanding. Discussion allows students to discover points of confusion, or to gain confidence if they clearly understand the points made in today’s class.

I ask students to continue working the practice problems for homework, and note that we will start with reviewing these problems in the next class. This student work is typical of the kind of work that came back, and so I was pleased about how students reacted to this lesson.