In this lesson students will review how to write formulas for ionic compounds and how to balance chemical equations. These topics are essential to the new material in this lesson, which is learning how to write balanced chemical equations for neutralization reactions.
This lesson aligns to the NGSS Practices of the Scientist of Developing and Using Models because the balanced chemical reaction is a written depiction of a chemical reaction. The process of using the equations to show what is happening in the test tube introduces a level of complexity beyond what students would observe from seeing color changes or even pH changes.
It aligns to the NGSS Crosscutting Concept of Cause and Effect in the sense that balanced chemical equations for neutralization reactions are best understood by examining the smaller scale mechanisms within the system—in this case the formation of a salt and water.
In terms of prior knowledge or skills, students have seen in a previous lesson that mixing an acid and a base together produces a chemical reaction. They have also learned about differences between acids and bases in this lesson and in this lesson.
There are no special materials needed for this lesson.
Do Now: Students begin class by reading page 430 in their textbook, and taking notes based on the following prompt that is on the projector screen when they walk into the classroom:
Record the following in your notes:
I reason that this is a good way to begin class because it introduces students to the material for today. The textbook is set up so that every student can extract this information, even if they do not completely understand it. By getting this early exposure, students will have begun thinking about neutralization reactions before I give my lesson on the subject; it is my hope that they will have a little momentum going into the lesson. After I take attendance I walk around the room to see how students are doing, and they confirm my expectations.
Activator: After students have had a chance to do this assignment, I ask a student to show their answers to the class.
The example of an acid base reaction is HCl + NaOH --> NaCl + H2O. The acid is the HCl and the base is the NaOH. The salt is the NaCl. I then point out that there is a mistake in the textbook, which notes that a salt is “an ionic compound in which the positive ion comes from an acid and the negative ion comes from a base.” In fact, it is the opposite: a salt gets its negative ion from the acid, and the positive ion from the base.
Mini-lesson: My first challenge is to help students remember how to write salts. I ask them to look at pages 233-235 in their textbook. The first thing I ask them to look at is the idea of balancing the charge, as shown in the middle of page 233. The total positive charge has to be equal and opposite to the total negative charge. I point out that the way to achieve this balancing of charge is to add subscripts. I then remind students that individual atoms can have a charge, and I remind them that the different groups on the periodic table provide us with a hint about what the charge is. I then point out that page 234 shows how to deal with polyatomic ions. I note that these are groups of atoms that have a charge when they are bonded together, and that there is a list on page 235. They do not have to memorize the list, but they do need to know how to use the list to find the charge of polyatomic ions.
This instructional choice reflects my desire to slowly and methodically build up to the skill of writing neutralization reactions. Being able to write the chemical formulas for salts seems like a good first step.
Guided Practice: I ask students to write the salts for problems 2-4 from the Ionic Bonding Practice problems. I then show the class the answers using the Ionic Bonding Practice answer key. This balancing charges in salts video shows students explaining how to do this skill.
Most students met with success on this task, and so I release them to finish these practice problems.
Student Activity: Students work on the remainder of the Ionic Bonding Practice problems. While they are working I walk around and answer students' questions, help them stay on task, and look for common mistakes.
I want students doing this work so that they have a chance to practice the skill that they have just learned.
Catch and Release Opportunities: The one common mistake I see from walking around is that students have forgotten how to use the periodic table to determine the charge of a monatomic ion. I remind them that Groups 1 and 2 have charges of 1+ and 2+, and Groups 17, 16, and 15 have charges of 1-, 2-, and 3-, respectively.
Stopping class to discuss this is important because if students are to be able to write balanced chemical equations they will need to start with correct formula units for their salts
Mini-lesson, Part 2: Once students have had a chance to write chemical formulas for salts I turn back to what we did at the beginning of class. I point out that a neutralization reaction produces a salt and water, and so if you can write the salt, you can write the reaction. I remind them that the definition of a salt is “an ionic compound in which the negative ion comes from a base and the positive ion comes from an acid.” I remind students that the other product in a neutralization reaction is water. I then model how this works using the neutralization reaction between HCl and NaOH. I note that the base gives up the positive Na+ and the acid gives off the negative Cl-. I note that these charges balance each other out, and so the final reaction would look like this:
HCl + NaOH --> NaCl + H2O
I note that the coefficients do not change because the subscripts did not change.
Guided Practice, Part 2: I ask students to do the first practice problem in the Neutralization Practice Problems I sheet. After they have had a chance to try it, I ask a student to show their work and explain how they derived the chemical formula for the salt, as well as the coefficients.
Student Activity, Part 2: I then release students to finish the Neutralization Practice Problems I sheet. I walk around and answer questions and point out mistakes when I see them. I use the Neutralization Practice Problems I Answer sheet as my guide. The balancing the whole equation video is a good example of how I support student thinking during this part of the lesson.
The big challenge is around writing the chemical formulas for salts, especially for salts containing polyatomic ions. I remind students to use the table on page 235 of their text book.
To wrap this lesson up I briefly review the second neutralization practice problem. I remind students to get the salt’s positive ion from the base, and the salt’s negative ion from the acid. I note that students need to then balance the charge for the salt before they balance the entire chemical equation.
I then give each student a ticket to leave that they complete without using any notes. They must write the neutralization reaction for H3PO4 and NaOH.
Ending class this way allows me to review the objectives for today’s class and see how students did with these objectives.
Most students show confusion in the ticket to leave, and so I know I will need to modify the pace of this week’s lesson so that they can get extra time working on this skill. In student work #1, the student did not balance the chemical equation. In student work #2, the student was able to write the correct answer. In student work #3, there is no evidence of student understanding, as is the case in student work #4.