Evaluating Chemical Expressions

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Objective

Students will be able to determine how many atoms of each element are in a chemical expression.

Big Idea

Subscripts indicate how many atoms for an element are in a compound, and coefficients indicate the quantity of molecules or formula units. Multiply the coefficient by the subscripts to know how many atoms of each element are contained in a chemical express

Introduction

In this lesson students will calculate how many atoms of each element are in a chemical expression. By chemical expression I am referring to chemical formulas for formula units for ionic compounds molecules for covalent compounds or elements and any accompanying coefficients.

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 because being able to evaluate chemical expressions is an important prerequisite skill for students who will need to balance chemical equations.

It aligns to the NGSS Practice of the Scientist#5, Using mathematics and computational thinking, in that students will be required to use arithmetic in order to name how many atoms of each compound are in the chemical expression.

It loosely aligns to the NGSS Crosscutting Concept of Energy and Matter in that balanced chemical equations are how we can symbolize conservation of mass.

In terms of prior knowledge or skills, students should already be able to tell what a chemical symbol is. If they do not have this knowledge, it is suggested that they get comfortable with this understanding before trying to interpret formulas of compounds.

There are no special materials needed for this lesson.

 

 

Do Now/Activator

10 minutes

Do Now: I start class by asking students to read a section of their text (which is similar to this reading) that explains what a chemical formula is.

I reason that this is a good way to start class because the text reading is quite accessible and I want to start a new unit by making students feel comfortable with the basic content.

Activator: I explain where we have been in our course to date.  I note that in our first unit we used chemical and physical properties to solve a crime. In the second unit we looked at atoms and how they form compounds. Now, I explain, we will look at chemical reactions. A chemical reaction involves using chemicals to create new chemicals by rearranging atoms.

I then ask for a student to explain what a chemical formula is. A student relates to the class that a chemical formula tells how many atoms of each element are in a compound. I ask for some examples and students note that H2O and C6H12O6 are examples.

 

Mini-lesson and Guided Practice

15 minutes

Mini-lesson: I begin the lesson by handing out notes called How to Interpret Chemical Formulas. I then review each of the examples, pointing to the subscripts and exponents to make more clear where the interpretation of the chemical expression is coming from.

Students take notes for the following three examples, which are typical of the kinds of practice problems I will subsequently have them perform. I project them using a document projector and point to numbers and symbols as I describe them.

I use these examples: H2O, Ca(NO3)2, 2 Ca(NO3)2. For the expression H2O I emphasize that the 2 means that there are two hydrogen atoms, and that it only applies to the H, not the O.

For the expression Ca(NO3)2, I note that the 2 needs to be distributed to everything inside the parentheses. This means that there are 6 O atoms because you have to multiply. It also means that there are 2 N atoms; I note that when there is no subscript, the subscript is one. I also note that there is 1 Ca atom, because there is no subscript.

For the expression 2 Ca(NO3)2 I note that it is similar to the expression Ca(NO3)2, but this time there is a coefficient of 2 in front of the expression. I note that all the other rules apply. The 2 still needs to be distributed to everything inside the parentheses, for example. However, once the distribution happens, the coefficient has to be distributed. This means that there are double the number of atoms for each element as in the previous example. To calculate out how many, multiply the 1 calcium atom, the 2 nitrogen atoms, and the 6 oxygen atoms by 2, for a total of 2 calcium atoms, 4introgen atoms, and 12 oxygen atoms.

Guided Practice

After the lesson I ask students to do the first three problems on the Interpreting Chemical Formulas Practice problems. After students have had a chance to work these problems we discuss the answers to make sure that everyone has a basic understanding of what they will practice today.

 

Application

25 minutes

Student Activity: Students continue to work on the practice problems. I circulate around the room answering questions and observing student work. The first 9 problems, which do not have coefficients, are quite easy for students, and for many the parentheses do not pose much of a challenge. However, when parentheses and coefficients are combined in one expression in subsequent problems, this gives students more of a challenge.

I want students doing this work because this assignment is scaffolding. They will need to be able to evaluate chemical expressions if they are going to be able to balance chemical equations, and balanced chemical equations are necessary for doing the types of stoichiometry problems students will ultimately have to do in this unit.

Catch and Release Opportunities: I remind students to look at their notes and compare how I did the different types of problems to how they are doing them.

Stopping class to remind students to look at their notes before consulting me is important because students need to gain more confidence in using their resources to solve problems. Too often, they turn to me as their first resource, instead of after they have exhausted other avenues.

 

Debrief

10 minutes

To wrap this lesson up I ask students to discuss these three questions at their tables, and then I randomly call on three students to report what they discussed:

1.            What is a chemical formula? What does it tell you? (It is chemical symbols and numbers; it tells you how many atoms of each element are in a molecule)

2.            What do the subscripts in a chemical formula tell you about the chemical? (how many atoms of each element are in it)

3.            What do coefficients tell you? (how many molecules you have)

We then look at student work. As this typical sample shows, most students feel comfortable with the learning objective at the end of class. However, knowing that many did not at the beginning, I am glad that we spent time building towards balancing chemical equations, which will be the students' next lesson.

Finally, a student discusses how to evaluate chemical expressions , as shown in this video. Most students have this understanding, as evidenced by my observations of individual students. I feel like they are ready to  move on to the next step in the stoichiometry unit, which is balancing chemical equations.