Modeling Acids and Bases

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Objective

Students will use modeling software to observe properties of acids and bases.

Big Idea

Acids and bases can be characterized by different pH values, which correspond to hydronium and hydroxide concentrations.

Introduction

In this lesson students use online software to explore differences between strong and weak acids.

This lesson does not align to an NGSS Disciplinary Core Idea; NGSS does 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 8.2, which asks students to “relate hydrogen ion concentrations to the pH scale and to acidic, basic, and neutral solutions.” 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 Developing and using models by giving students the chance to use a model to explore differences at the molecular level between strong and weak acids.

It aligns to the NGSS Crosscutting Concept of Structure and Function because students will look at the molecular substructures of acids and bases in order to help identify whether a solution is an acid or base and whether it is strong or weak.

In terms of prior knowledge or skills, students should have a basic understanding of the Arrhenius and the Brønsted-Lowry definitions of acids and bases as described in this lesson.

The materials needed for this lesson include computers or tablets with online access.

 

Do Now and Activator

10 minutes

Do Now: To start this lesson I ask students to read p.age 416 in its entirety and page 421 through the litmus paper paragraph in their Chemistry text book, and then review Acids and Bases Reading Questions 13-19. I reason that this is a good way to start class because it connects back to the last lesson, and because the model uses pH and litmus paper. I reason that if students can refresh their understanding of these ideas then the model will be more meaningful for them.

Activator: After students have had a chance to individually review the material I randomly call on students to answer each of the following questions. I do this to make sure that students understand what pH is and what the pH paper is

13. Name 2 ways that pH influences living organisms. (p. 416) it can kill fish or affect how plants grow

14. What does pH tell you? (p. 416) pH tells you whether a solution is acid, base, or neutral

15. How does pH relate to acids and bases? (p. 416) acids have a pH <7, bases have a pH > 7

16. In terms of pH, which acids are considered dangerous? Which bases? (p. 416) acids with a pH <2 and bases with a pH >12 are considered dangerous

17. How do you measure pH? (p. 421) indicators, chemical reactions, pH meters, measuring electrical properties

18. How do indicators work? (p. 421) they change color based on the pH

19. What is litmus paper? (p. 421) a paper that has an indicator on it

 

Mini-lesson

10 minutes

Mini-lesson: For the brief mini-lesson I ask students to look at my projected computer screen so that I can show them the features of the modeling software. I first show them that there are two tabs—the Introduction Tab, and the Custom Solutions tab.

I note that they will use the first tab to complete the top half of the Phet Acids Base Lab Data page, and the second tab to complete the bottom half. For the top half of the data page students will measure the conductivity of different acid and base solutions, and their pH's. For the second half of the data page students will measure pH's for acids and bases using various concentrations for each.

I then model how to measure pH using a meter and using the litmus paper. Finally, I show them that the conductivity meter is just a light bulb that has electrodes that you can dip into the solution to see how well the solution conducts electricity.

I note that you can modify the solutions in terms of acid or base by clicking on the different solutions.

This instructional choice reflects my desire to give everyone a good start in conducting the simulator. 

Application

25 minutes

Student Activity: I first instruct all students to log into a laptop and go to the webpage that contains the PhET modeling activity. Different students have different levels of comfort with this task, so I first act as a resource for them, making sure they can get to the right page.

I then walk around and make sure students are correctly filling in the data page I have provided for them. The activity is easily accessed by all of my students, and so I play a pretty limited role as they experiment with the different tools and solutions. 

Debrief

10 minutes

To wrap this lesson up I ask a student to help review their work as shown in this debrief video.

While the data was fairly easy to extract from the model, I suspect that students do not really understand the reasons for why stronger acids conduct electricity better than weak ones, or why there is such a pattern between pH and concentration.

That is OK with me. As I show in this video that ends with a question, ending the class with a question is actually a great way to model the scientific endeavor. So often we learn one thing, but it raises further questions. Future lessons are designed to explore the questions that this lesson ends with.

With that said, students were able to see a number of things from this lesson. 

They were able to see a relationship between conductivity and pH. They were able to see a relationship between ion concentration and pH. They were able to quickly measure the pH of several solutions using a meter and litmus paper. (They will do these things at the lab bench as well in a future lab). I will refer back to each of these experiences in future lessons.