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Lab-Effects of pH on the Structure of a Polyelectrolyte

Grades: 11-12 (AP Level Chemistry)
Author: Nello Cotugno
Source: Original - This material is based upon work supported by the National Science Foundation under Grant No. EEC- 1542358.

Abstract

Students will explore how the viscosity of a polyelectrolyte (polyacrylic acid) changes with changing pH. They will explain viscosity changes in terms of the intermolecular forces and the equilibrium of the ionization reaction. Lab activities include determining the Ka and pKa of PAA after measuring the pH of the PAA solution, titrating the weak acid (PAA) with a strong base (sodium hydroxide) and graphing the titration curve, measuring the efflux times at different pH values then calculating the relative viscosity of a PAA solution using a U-tube viscometer and graphing the results. As a conclusion, students will describe how what was learned in the lesson could be applied to create a useful product such as a wound covering.

The lesson is designed as a review of equilibrium, properties of acids, titrations, and intermolecular forces that can be given leading up to the AP exam.

Objectives

What should students know as a result of this lesson?

  • Intermolecular forces determine the shape and properties of polymer chains
  • Types and strengths of intermolecular forces (IMFs) can be manipulated by making changes (such as pH) that affect the equilibrium of a reaction according to LeChatelier’s principle
  • Changing the equilibrium of a reaction can alter the physical and chemical properties of materials such as polymers

What should the students be able to do as a result of this lesson?

  • Determine the types and relative strengths of intermolecular forces associated with functional groups within a polymer
  • Determine the Ka and pKa of a weak acid polymer by making pH measurements
  • Design and perform an experiment to perform a weak acid polymer titration and graph the data
  • Calculate percent ionization of PAA at different pH values
  • Design and perform an experiment to determine the viscosity of a weak acid polymer at different pH values and graph the data
  • Compare and contrast the graph obtained for a weak acid polymer with the graph of a non-polymeric weak acid (such as acetic acid)
  • Relate changes in pH to changes in the equilibrium of a reaction in terms of LeChatelier’s principle
  • Explain changes in polymer structure in terms of changes in types and strengths of IMFs
  • Apply the principles learned in this lesson to useful polymer properties (Ex: properties of films used for wound coverings etc.)

Materials

  • pH meter (Ex: Vernier Accuquest probe system)
  • Biuret
  • 40 mM polyacrylic acid solution
  • 0.10 M sodium hydroxide solution
  • U-tube viscometer
  • 50 mL beakers-8
  • Suction bulb
  • Pipets
  • Stopwatch

Procedures

Engagement

  1. Review equilibrium, intermolecular forces, weak acids and related topics before the lab
  2. Ask the question “Why are these topics important in everyday life and how are these principles used to make useful materials?”
  3. Discussion and completion of pre-lab questions-formative assessment

Exploration

  1. Students complete a WebQuest on the properties and uses of plastics
  2. Quiz on information learned in the WebQuest and the pre-lab questions before starting the lab activities

Explanation

  1. Review main points of WebQuest and pre-lab questions, discuss how principles relate to products presented in the WebQuest
  2. Explain lab activities

Elaboration

  1. Discuss how to set up data tables, graph results and control variables
  2. Assessment-conclusion and post-lab questions

Prerequisites

  • Equilibrium and equilibrium calculations
  • Meaning/calculations involving Ka and pKa
  • Weak acid titrations and calculations
  • Intermolecular forces and functional groups

Best Teaching Practices

  • Acting as facilitators rather than directors of students' learning
  • Providing a variety of materials and resources to facilitate students' investigations
  • Modeling inquiry behaviors and skills
  • Posing thoughtful, open-ended questions and helping students do the same
  • Encouraging dialogue among students and with the teacher
  • Keeping children's natural curiosity alive and as a teacher, remaining a curious, life-long learner

Alignment with Standards

NGSS Standards:

  • HS-PS1-2: Construct and revise an explanation for the outcome of a simple chemical reaction based on the outermost electron states of atoms, trends in the periodic table, and knowledge of the patterns
  • HS-PS1-3: Plan and conduct an investigation to gather evidence to compare the structure of substances at the bulk scale to infer the strength of electrical forces between particles.
  • HS-PS1-6: Refine the design of a chemical system by specifying a change in conditions that would produce increased amounts of products at equilibrium.
  • HS-ETS1-2: Design a solution to a complex real-world problem by breaking it down into smaller, more manageable problems that can be solved through engineering.
  • HS-PS2-6: Communicate scientific and technical information about why the molecular-level structure is important in the functioning of designed materials.

Ohio Standards:

Science Inquiry and Application

During the years of grades 9 through 12 all students must use the following scientific processes with appropriate laboratory safety techniques to construct their knowledge and understanding in all science content areas:

  • Identify questions and concepts that guide scientific investigations
  • Design and conduct scientific investigations
  • Use technology and mathematics to improve investigations and communications
  • Formulate and revise explanations and models using logic and evidence (critical thinking)
  • Recognize and analyze explanations and models
  • Communicate and support a scientific argument

Content Knowledge

Students should already be familiar with the following topics:

  • Dimensional analysis, significant figures, scientific notation, and statistics
  • Intermolecular forces-dipole-dipole, hydrogen bonding, London dispersion forces
  • Basic lab techniques
  • Constructing and interpreting graphs, calculating slope of a line.
  • Solving for an unknown in an equation
  • Experimental design, including working with variables (independent, dependent, controlled)
  • Equilibrium
  • Properties of strong and weak acids and bases

Safety

Applications

  • Hydrophilic/hydrophobic coatings and films (wound coverings, food containers, fabric coatings)
  • Humidity sensors

Assessment

Completed lab report

Ap Chemistry-style questions (multiple choice and free-response)

Other Considerations

Grouping Suggestions:

  • Students work in pairs but are encouraged to share information with other groups

Pacing/Suggested Time:

  • Can be conducted as a “Polymer project” using the lab developed in RET 2017 “The Molecular Weight of a Polymer” to prepare/review for the AP Chemistry exam.

Printable PDF Worksheets

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