$\require{mhchem}$This lesson illustrates how a series of components/resources can be sequenced to create an active learning session for your students. Less emphasis is placed on having the instructor present information to students. More emphasis is placed on having students think, make decisions, and answer a series of questions based on a model. The model is presented in the Student Activity Book and within the Interactive Lecture Demonstration: Students compare water and three aqueous solutions - hydrochloric acid, HCl(aq), acetic acid(aq) and sodium acetate(aq). The first part of the instructional activities focuses on providing information to students about what occurs in four solutions. A definition of a buffer solution and the concept of a buffer solution is delayed until students have explored what occurs in four solutions. A guided-inquiry approach is used with a simple learning cycle: exploration, concept introduction, application. There is a Power Point presentation designed to facilitate active learning by incorporating computer animations, interactive demonstration (or a video of the demonstration), In-Class activities from the General Chemistry Student Activity Book, quiz questions, particles diagrams or "molecular scenes", etc. Instructors can use the lesson as is, or instructors can pick the components they want and use the components in any order they want. However, we believe there is an optimal sequence of instructional activities.
Web Page Author: T. Greenbowe, University of Oregon. This page is under construction.
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Starting with the equilibrium equation for an acetic acid solution representing this weak acid system can be written:
$\ce{CH3COOH(aq) + H2O(l) <=> CH3COO-(aq) + H3O+(aq)}$
*****the Big Idea or Main Concept of a Buffer ******
Starting with the equilibrium equation for an acetic acid solution representing this weak acid system can be written:
$\ce{CH3COOH(aq) + H2O <=> H3O+(aq) + CH3COO(aq)}$ Ka = 1.8 x 10-5
Starting with the equilibrium equation for an acetic acid solution representing this weak acid system can be written:
CH3COOH(aq) + H2O(l) <=> H3O+(aq) + CH3COO-(aq)
When sodium acetate is added to the system, the acetate ion is the common ion. The acetate ion will react with some of the hydronium ions, causing the equilibrium to a shift to the left. Since the hydronium ion concentration decreases, the pH should increase (become less acidic). There is now considerable quantities of both the weak acid (acetic acid) and its conjugate base pair (acetate ion) in the solution.
Small quantities of 010 M HCl and 0.10 M NaOH are added to the solution.
The acetic acid in the buffer solution will react with the newly added sodium hydroxide, NaOH:
CH3CO2H(aq) + OH-(aq) -> CH3CO2-(aq) + H2O(l) K = 1.8 x 109
The acetate anion in the buffer solution will react with newly added hydrochloric acid, HCl:
CH3CO2-(aq) + H2O+(aq) -> CH3CO2H(aq) + H2O(l) K = 5.6 x 104
This mechanism prevents the pH of a solution from changing drastically through the action of each component with incoming acid or base.
An acidic buffer solution consists of relatively high concentrations of a weak acid (i.e. acetic acid) and its conjugate base (i.e. acetate ion). A buffer solution resists changes to pH when small amounts of strong acid or strong base are added to the solution.
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One day of lead time is required for this project.
Materials {to be aadded}
Outline of a possible instructional sequence
1. Model 1 – Comparing the pH of Solutions. Water, 0.01 M Acetic Acid, 0.01 M sodium acetate, 0.01M HCl.
a. Prior to the demonstration, Have students do the Student Activity. Identify each solution as: acidic, basic, or neutral. Write chemical equations to represent what occurs in each solution.
b. Identify each molecule or ion in your equations as: strong acid, strong base, weak acid, weak base, conjugate acid, or conjugate base.
2. Model 2 – the pH of a solution of acetic acid -acetate.
a. Prior to the demonstration, identify the solution as: acidic, basic, or neutral. Write a chemical equation to represent what occurs in the solution.
b. Identify each molecule or ion in your equation as: strong acid, strong base, weak acid, weak base, conjugate acid, or conjugate base.
c. Show the animation of an acetic acid -acetate solution.
3 Interactive Demonstration – Comparing the pH of two solutions.Water vs. Acetic Acid-Sodium acetate when 0.01 M sodium hydroxide and, 0.01M HCl are added.
a. Prior to the demonstration, predict what will happen to the pH of the solution (and water) when acid is added to the buffer solution. Write a chemical equation to represent what occurs.
b. Do the demonstration - pause and show students what occurs at the molecular level using particle diagrams and computer animations, chemical equations. Show the animation of the reactions that occur when hydronium ions are added to an acetic acid -acetate solution.
c. d. Prior to the demonstration, predict what will happen to the pH of the solution (and water) when base is added. Write a chemical equation to represent what occurs.
d. Do the demonstration - pause and show students what occurs at the molecular level using particle diagrams and computer animations, chemical equations. Show the animation of the reactions that occur when hydroxide ions are added to an acetic acid -acetate solution.
4. Calculations a. Students calculate the pH of 0.01 M Acetic Acid- 0.01 M Sodium acetate solution, 0.01 M Acetic Acid, 0.01 M sodium acetate, 0.01M HCl.
b. Students calculate the pH of 0.01 M Acetic Acid- 0.01 M Sodium acetate solution, 0.01 M Acetic Acid, 0.01 M sodium acetate,when 0.01 M HCl is added.
c. Calculate the pH of acetic Acid- 0.01 M Sodium acetate solution, 0.01 M Acetic Acid, 0.01 M sodium acetate, when 0.01M NaOH is added.
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Possible Animations
The following animations present a simplified representation of a molecular view of what occurs when acid or base is added to a buffer system.
http://www.chembio.uoguelph.ca/educmat/chm19104/chemtoons/chemtoons7.htm
Curriculum Notes
A Class activity accompanies this interactive demonstration. A computer animation showing a dynamic representation of the interactions of weak and and conjugate base in the the acid-base reactions at the particle level can accompany this activity. When instructors expect their students to think of buffers in microscopic or symbolic terms and to relate the different representations of buffers with each other, they need to show those representations and their connections with the students explicitly. Students do not generally make these connections on their own or think in terms of "molecular scenes".
Clicker questions asking students to predict what will occur can accompany this activity. Quiz questions assess students understanding of buffer solutions are available. Sample lecture notes accompany this activity.
Learning Objectives
1. Identify two components of an acidic buffer solution and explain the function of each component. Identify the criteria of a classic buffer system: acidic buffer or alkaline buffer.
2. Write the chemical equilibrium equation representing and acetic acid-sodium acetate buffer system.
3. Write appropriate chemical equations and explain how one component of a buffer system reacts when acid is added, and the other component reacts when base is added. Show that these reactions only slightly increase or decrease the pH of the solution.
4. Write appropriate chemical equations and explain how water reacts when acid is added, and when base is added. Show that these reactions increase or decrease the pH of the solution.
5. Write appropriate chemical equations and explain why the concentrations of the two buffer components must be high to minimize the change in pH due to the addition of small quantities of acid,H3O+, or base OH-.
6. Explain why the best pH of an acidic buffer system is ±1 pH of the pKa of the weak acid.
7. Given the initial concentrations of weak acid and the conjugate based and the Ka of the weak acid, calculate th pH of an acidic buffer system. Students are encouraged to use ICE (Initial Change Equilibrium) Tables and the Henderson-Hasselbach equation.
8. Given the initial concentrations of weak acid and the conjugate based and the Ka of the weak acid, calculate th pH of an acidic buffer system after a small amount of strong acid or strong base is added.
9. Given a set of reagents with known concentrations, prepare a buffer solution with a specific pH.
AP Chem Learning Objective
The student can identify a solution as being a buffer solution and explain the buffer mechanism in terms of the reactions that would occur on addition of acid or base.
Pre-requisites
1. Identify acids, bases, salts.
2. Know the name, chemical formula and charge of the polyatomic anions and cations.
3. Write the Ka expression and chemical equilibrium equation for a weak acid system.
4. Given the initial concentration of a weak acid and the Ka, calculate the pH of the weak acid solution. Calculate the pH for a 0.010M acetic acid solution.
5. Given an equilibrium system, predict what will happen when a common ion is added or removed.
6. Identify acid-conjugate base and conjugate acid-base pairs.
7. Write the Kb expression and chemical equilibrium equation for a salt that reacts with water and results in an alkaline solution.
8. Given the initial concentration of a salt solution and the Kb or Ka value, calculate the pH of the salt solution. Calculate the pH for a 0.010M sodium acetate solution.
POGIL Activities (Active Learning, Guided-Inquiry)
Activity 17-1: Buffer Solutions. Hanson, D. M. Foundations of Chemistry: Applying the POGIL Principles,4th edition. (2010) Pacific Crest, IL.
Buffers: How can a solution neutralize both acids and bases? Trout, L. POGILTM Activities for AP Chemistry (2015) Flinn Scientific, Batavia, IL. ISBN 978-1-933709-50-5
https://www.flinnsci.com/pogil-activities-for-ap-chemistry/ap7925/
https://pogil.org/educators/become-a-pogil-practitioner/curricular-mater...
Student Difficulties Involving Buffer Solutions
1. Buffer solutions are commonly viewed by students as static systems instead of dynamic equilibria systems.
2. Students have a difficult time interpreting chemical formulas confidently. Students have difficulty with identifying if chemical formula represents a weak or strong acid, weak or strong base, acidic or basic ionic salt. The chemical formulas for salts of conjugate bases are particularly hard for students to interpret.
3. Students have a difficult time understanding and predicting if a soluble ionic salt will generate an acidic, basic, or neutral solution when dissolved in water.
4. Students have a difficult time writing an equilibrium chemical equation representing a buffer system. Students not understand the relationship between a weak acid and its conjugate base. Students assume that any two chemicals that are mixed will react together and students will write an equation for the chemical reaction between the weak acid and its conjugate base.
HA + A- <=>
5. Student difficulties in understanding buffer conceptually are related to their inability to visualize buffers on the microscopic scale. Students have a difficult representing (drawing) a buffer solution using a "picture diagram" or "molecular scene" and have difficulty interpreting a "molecular scene" representing of a buffer. Students have difficulty relating the macroscopic, microscopic and symbolic representations of buffers. Students who can draw and interpret "molecular scenes" of buffer solutions exhibit and demonstrate a better conceptual understanding of buffers compared to students who cannot do so.
6. Students confuse the concentration of hydronium ion in the buffer solution (used to calculate the pH) with the initial concentration of weak acid (i.e. 1.0 M) - used in the Henderson-Hasselbalch equation. Students are not able to distinguish between or relate the weak acid component of the buffer and the hydrogen ions that determine the pH of the solution.
7. Students believe that all buffers have a pH = 7, neutral.
8. Students do not conceptual relate that fact that pH is a logarithmic scale and the log of the concentration of the H3O+ ions in solution (with acknowledgement of the role that an activity coefficient plays) determines the acidity or alkalinity of the solution. Students’ inability to understand logarithmic functions (base 10) has consequences for their understanding of buffers.
9. Students have difficulty differentiating between Ka and pKa or pH and [H+].
10. Because students do not have a good conceptual understanding of buffers and because they try to approach buffer problems from a purely mathematical perspective, some students believe that there is only one way to solve a particular type of buffer problem.
11. Some students believe the strength of the buffer is determined by the strength of its component acid and base: a buffer made from a strong acid and a strong base would be stronger (have a higher buffer capacity) than a buffer made from a weak acid and a weak base.
12. Some students have the idea that buffers have an unlimited ability to resist pH changes. In fact, buffer solutions have a finite capacity to resist pH changes. In an acidic buffer solution it is the number of moles of weak acid and number of moles of its conjugate base that determine the extent to which a buffer can neutralize added acid or base.
13. Students have difficulty explaining what occurs when strong acid or a strong base is added to a buffer system. Students need to incorporate in their explanation, chemical equations, particle diagrams, and written explanations.
References
Orgill, M.K., & Sutherland, A. (2008). Undergraduate chemistry students’ perceptions of and misconceptions about buffers and buffer problems. Chemistry Education Research and Practice, 9, 131–143.
Drechsler, M., & Schmidt, H. J. (2005), Textbooks’ and Teachers’ Understanding of Acid-Base Models Used in Chemistry Teaching, Chemistry Education Research and Practice, 6 (1), 19-35.
Summerlin, L.; Borgford, C.; Ealy, J. Chemical Demonstrations: A Sourcebook for Teachers; Volume 2; 1987; p. 172-173.
Paul G. Hobe Jr. (1979). Buffer effect demonstration on the overhead projector. J. Chem. Educ., 56 (1), p 47.
James C. Chang (1976). A buffer solution and its action. J. Chem. Educ., 1976, 53 (4), p 228.