Summary

In this activity, students will complete a guided-inquiry activity to investigate the ionization of strong and weak acids. Bead models are used to study acid ionization on a particulate level. Students analyze seven acid models, some representing strong acids and others weak acids, and make generalizations about the relationship between acid strength and dissociation.

Grade Level

High School

NGSS Alignment

This activity will help prepare your students to meet the following scientific and engineering practices:

• Scientific and Engineering Practices:
  • Using Mathematics and Computational Thinking
  • Developing and Using Models
  • Analyzing and Interpreting Data
  • Constructing Explanations and Designing Solutions
  • Engaging in Argument from Evidence

Objectives

By the end of this activity, students should be able to:

• Explain the molecular difference between strong and weak acids based on differences in ionization.
• Accurately describe how diprotic acids ionize.

Chemistry Topics

This activity supports students’ understanding of:

• Acid/base theories
• Weak vs. strong acids and bases

Time

Teacher Preparation: 60 minutes to prepare the acid bead models (but reusable for years with minimal prep in the future)
Lesson: 60 minutes

Materials

• Student handout
• 7 petri dishes with acid bead models (see instructions below for making models)

Safety

• No specific safety precautions need to be observed for this activity.

Teacher Notes

• This activity is being republished with permission from the Journal of Chemical Education, Copyright © 2011 The American Chemical Society and Division of Chemical Education, Inc. The original article can be found at https://pubs.acs.org/doi/full/10.1021/ed100849b.
• This activity can be used to introduce the difference between the ionization of a strong acid and weak acid. It also focuses on the ionization of a diprotic acid versus a monoprotic one. Before using the activity, students need to be familiar with acid naming and formula writing, as well as reversible reactions. Although this activity focuses solely on acid ionization, the concept can also be applied to explain the difference between strong and weak bases.
• This activity uses plastic beads as models for molecular and ionized acids. The models represent seven different acids, some strong and some weak. By noting the differences in the models and counting the number of beads representing molecular acids versus ionized acids, students learn about acid ionization and develop an understanding of the difference between a strong acid and a weak acid. Instructions for model assembly are included at the end of the teacher notes. Two of the models from the activity (acids #2 and #7) are pictured below for reference:

• Many students do not understand the molecular difference between a strong acid and a weak one. Most can identify that a 0.1 M strong acid has a pH around 0 or 1 and a weak acid of the same concentration has a pH around 5; however, students are usually unable to explain why the pH values are different. This activity addresses this discrepancy in understanding by using particulate models made from beads in Petri dishes to help students compare the ionization percentage of strong and weak acids.
• Students often have misconceptions about diprotic acids as well, which can also be addressed using this activity. Two common misconceptions regarding strong or weak acids are that sulfuric acid is a strong acid because each molecule has two hydrogen ions, and that all diprotic acids completely ionize in water. In this activity, students will see that some diprotic acids are weak acids (H₂CO₃) and therefore do not completely ionize in water.

Activity Structure

• Students will read the background information and complete pre-activity questions – this could be assigned for homework the night before completing the activity, done individually as a warmup, or completed in small groups, at the teacher’s discretion.
• As a whole class, go over the answers to the prelab questions, then review the “example acid” diagram and questions on the student handout together. Teachers can use this “example acid” to discuss how to count the whole acid beads and the ionized acid beads, as well as writing the balanced ionization reactions and show students where to include this information in their data table.
• Divide students into 7 groups and give one Petri dish to each group. Allow 5-10 minutes for students to complete the table for that acid on the student handout. Then have groups rotate to the next Petri dish and repeat until they have examined all 7 acid models.
• While students are working in their groups, circulate among them to answer any questions that come up and to make sure they are interpreting the models correctly. Some common mistakes to watch for include:
  • Incorrectly counting the number of intact acid molecules and ionized acids. Students may not recognize that a blue bead (H⁺) and another colored bead (A^–) represent one ionized acid molecule and incorrectly count it as two separate acids.
  • Incorrectly interpreting diprotic acid models. The diprotic acids start out as two blue beads attached to one other color bead. When they dissociate, they will have one blue bead still attached to the other colored bead and one lone blue bead, as only one H⁺ ion dissociates at a time. Use the bead models to help students recognize that the diprotic acids dissociate into H⁺ and HA^–, not H⁺ and A^2–.
• Once students have completed the table, have them label each acid as strong or weak in the final column of the table. Based on the background reading and prelab questions, they should realize that acids with high rates of ionization are strong acids and those with low rates of ionization are weak acids. Confirm that everyone has the identified the strong and weak acids correctly before they move on to the analysis questions.
  • Note that strong acids are known to ionize completely (95–99%) and weak acids only partially (less than 5%). Because each model has only 10 molecules or ions, the strong acids ionize 90–100% and weak acids 10–20%. Though these percentages are incorrect, students can still understand how labeling the acid strength depends on the degree of ionization. This could lead to a good conversation about the limitations of models.
• Students can complete the analysis questions in their groups, individually, or for homework, as time allows.

Model Assembly

• To make the complete set of acid models, you will need 170 pony beads (6×9 mm): 100 beads of a single color to represent hydrogen (blue is used in this guide); 70 other colored pony beads (10 of each of 7 different colors); 7 Petri dishes, and glue. The beads can be purchased at hobby stores and plastic Petri dishes can be ordered from most scientific catalogs. Bead glue or a hot glue gun can be used to “bond” the molecules.
• The easiest method of assembly is to place one bead on the sharpened end of a pencil. Apply the adhesive to the bead and then attach the second bead. Hold about 20 s. Apply a small amount of adhesive to the outside where the two beads are attached. Pop the beads off the pencil onto a piece of cardboard. Rotate the beads periodically to prevent them from sticking to the cardboard while the glue is cooling and hardening.
• Assemble the models in the Petri dish according to the table below. Label each Petri dish with a number. Use tape to seal the Petri dishes shut.
• It takes approximately 1 h to make one complete set of acid models. Once made, the models can be reused for years.
• The table below lists the contents of each Petri dish, distinguishing between bonded and individual beads. The colors used for these 7 models are: B = blue; P = pink; R = red; Or = orange; G = green; Y = yellow. The colors may be changed based on what you have available, but to avoid student confusion, make all the models of H⁺ ions the same color throughout the activity (as described in the table, H⁺ is always blue).