Introduction to Acid Base Theory

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Objective

Students will be able to differentiate between different definitions of acids and bases.

Big Idea

Definitions for acids and bases evolved as our understanding of how acids and bases are formed and how they interact became more refined.

Introduction

In the last lesson students got to experience acids and bases through hands-on activities. In this lesson students begin to study the structure, roles, and nomenclature of acids and bases through guided reading questions and the use of flash cards.

This lesson does not align to a NGSS Disciplinary Core Idea; the NGSS do not have a unit dedicated to acids and bases. Nonetheless, I do believe this unit is important for a number of reasons. First, it corresponds to Massachusetts Chemistry Framework 8.1, which asks students to “define the Arrhenius theory of acids and bases in terms of the presence of hydronium and hydroxide ions in water and the Bronsted-Lowry theory of acids and bases in terms of proton donors and acceptors.” In addition to complying with my state standards, I also believe that it is important to teach students this material because acid-base theory is an integral part of chemistry. Students will see this material in college and so for these reasons I include this unit in my introductory chemistry course.  

This lesson aligns to the NGSS Practices of the Scientist of Obtaining, evaluating, and communicating information because this is a day in which students will gather background knowledge about acids and bases.

It aligns to the NGSS Crosscutting Concept of Structure and Function because students will look at the molecular structure of acids and bases in order to better understand them.

In terms of prior knowledge or skills, students have very little understanding of what acids and bases are, and they have no understanding of them at the molecular level. This is an introductory lesson.

There are no special materials needed for this lesson; students used our class chemistry textbook (A Natural Approach to Chemistry) to answer reading questions.

Do Now/Activator

10 minutes

Do Now: I start today’s class by asking students to make flash cards for the acids and bases on the tables on the left on page 415 of their text. On one side I ask students to record the acid or base name, and on the other side I ask students to record the chemical formula. I reason that this is a good activity that requires very little teacher guidance, which frees me up to take attendance. I do not want them doing anything else with acids and bases because today is day that I am focusing on literacy; students will be using their textbook to record information. I want to introduce the topic so that students who are lower-level readers will have some context for the text they will be reading.

Mini-lesson

15 minutes

Mini-lesson: I begin this lesson by holding up a package of Sour Patch Kids, which many of my students are familiar. I point out that two of the ingredients are tartaric acid and citric acid; I remind students that they need to move beyond the cartoon idea that acids are something akin to “the dip” in the Roger Rabbit movie. In fact, acids and bases are a class of chemicals. Some will react quickly and violently, while others react slowly and indiscernibly.

I then note that this begs the question of what an acid and a base are. I tell the class that they will read more about this in today’s class, and that my mini-lesson today is about giving them an overview of what they will be reading and introducing some vocabulary that might be confusing. I first note that many chemicals do something called dissociation, in which the chemical splits into ions. Water does this, for example; it splits into an H+ and an OH-. These ions are the hydrogen ion and the hydroxide ion. I note that pure water has an equal amounts of these ions in a given sample. However, some chemicals tend to produce more of one or the other type of ion in water. Acids produce more H+ ions and bases often produce more OH- ions. This is a good summary of the Arrhenius definition of acids and bases.

I then explain that sometimes H+ ions are called protons. I remind students that the hydrogen atom only has a proton and an electron, and if it loses an electron, all that remains is the proton. This becomes important in the Brønsted-Lowry definition of acids and bases because acids are chemicals that donate protons and bases are ones that accept protons.

I then pass out the Acids and Bases Reading Questions which guide students in finding the background knowledge I want students to learn for this unit from their text. I note that they should work on questions 1-19 and also work with a partner to study their acid and base names.

This instructional choice reflects my desire to give an introduction to the vocabulary that students will encounter in their reading.

Application

25 minutes

Student Activity: During the work portion of the class students are busy reading the assigned pages and answering the reading questions. I walk around the classroom helping students who have questions. Most of the questions are easily accessed by students, but the two that were challenging for students were these:

9. Can something be an acid and a base? Why or why not?

10. Water can act as an acid and a base. Explain what water does when it acts as each of these.

The wording of these questions confused students. When this happened I simply noted that what I should have added in question 9 was the phrase “at the same time.” This was the point I was trying to make. While water is amphoteric, it either acts like an acid or a base depending on what is dissolved in it.

I want students doing this work because I believe it is more educationally valubale for them to wrestle with getting the information than it would be for them to sit and listen to me lecture for the whole class.

Catch and Release Opportunities: As students begin to finish the reading questions I explain that we will go over the questions as a whole class and that students should be practicing memorizing the acid and base names that they made flash cards for at the beginning of class.

Stopping class to discuss this is important because some students will finish sooner than others and I want them to have a meaningful learning activity to turn to so that the learning environment stays productive.

Debrief

10 minutes

To wrap this lesson up I ask if any students have learned the names of the acids and bases. This video of students studying acids and bases shows that some students have made significant progress toward this learning objective, and serves as a model to the rest of the class that this is a manageable task. I then randomly call on students to provide answers for the reading questions. Most students have the correct answers, and students who do not are given the chance to change their answers.

In this example of acid base student notes the student not only recorded her answers, but she also subsequently began to organize her notes with the goal of studying for the acid-base test.

Ending class this way allows me to make sure students got out of the reading what I hoped they would, and it allows weaker readers the chance to correct their work and increase their confidence from the work that was correct.