Summary

In this lesson students will write dissociation reactions to make connections between conjugate acid-base pairs. They will use beaker diagrams in a cooperative group activity to better understand why the pH calculation for a weak acid/base is not the same for a strong acid/base. Finally, students will apply these concepts in a lab in which they will identify several unknown, clear, colorless solutions using factors such as pH, conductivity and reactivity. The activities in this lesson can be used in sequence or as standalone activities.

Grade Level

High School (AP Chemistry)

AP Chemistry Curriculum Framework

This laboratory experiment supports the following units, topics, and learning objectives:

• Unit 4: Chemical Reactions
  • Topic 4.2: Net Ionic Equations
    • 4.2.A: Represent changes in matter with a balanced chemical or net ionic equation: a. For physical changes. b. For given information about the identity of the reactants and/or product. c. For ions in a given chemical reaction.
  • Topic 4.5: Stoichiometry
    • 4.5.A: Explain changes in the amounts of reactants and products based on the balanced reaction equation for a chemical process.
  • Topic 4.6: Introduction to Titration
    • 4.6.A: Identify the equivalence point in a titration based on the amounts of the titrant and analyte, assuming the titration reaction goes to completion.
• Unit 7: Equilibrium
  • Topic 7.5: Magnitude of the Equilibrium Constant
    • 7.5.A: Explain the relationship between very large or very small values of K and the relative concentrations of chemical species at equilibrium.
• Unit 8: Acids and Bases
  • Topic 8.2: pH and pOH of Strong Acids and Bases
    • 8.2.A: Calculate pH and pOH based on concentrations of all species in a solution of a strong acid or a strong base.
  • Topic 8.3: Weak Acid and Base Equilibria
    • 8.3.A: Explain the relationship among pH, pOH, and concentrations of all species in a solution of a monoprotic weak acid or weak base.
  • Topic 8.5: Acid-Base Titrations
    • 8.5.A: Explain results from the titration of a mono- or polyprotic acid or base solution, in relation to the properties of the solution and its components.
  • Topic 8.7: pH and pKa
    • 8.7.A: Explain the relationship between the predominant form of a weak acid or base in solution at a given pH and the pKa of the conjugate acid or the pKb of the conjugate base.

Objectives

By the end of this lesson, students should be able to

• Categorize strong and weak acids and bases using chemical formula, pH data, or particulate drawings.
• Classify salts as acidic, basic, or neutral and connect to conjugate acids and bases.
• Relate the dissociation reaction and K value to strength of an acid or base.
• Calculate the pH of strong and weak acids and bases as well as salts.
• Apply theoretical pH values and other chemical tests in order to determine the identity of unknown solutions.

Chemistry Topics

This lesson supports students’ understanding of

• Acids & Bases
• pH
• Strong vs Weak Acids/Bases
• Salts
• Dissociation
• Conjugate Acid-Base Pairs
• Equilibrium Constants

Time

Teacher Preparation: 75minutes

Lesson: 250 - 300minutes

Materials

• Activity 1: Comparing Strong and Weak Acids and Bases
  • Acid/Base Index Cards
  • Printed Sets of Beaker Diagrams (Particulate Drawings)
  • Student Activity & Notes
  • Teacher Answer Key for Activity & Notes
• Activity 2: pH Calculations
  • Student Handout/Notes
  • pH of Everything booklets (contains additional pH calculations from Big Idea 6 such as buffer). Note that this document is designed to be photocopied double sided and folded in half. The pages will be in order with review information and practice problems for each concept when it is copied and folded.
  • Teacher Answer Key for Student Handout & pH of Everything booklet
• Activity 3: Categorizing Salts
  • Student Handout/ Notes and Practice Problems
  • Class Set of Practice Cards. Note that these are designed to be copied double sided and cut into cards. One side has the chemical formula and the other side has information such as classification, conjugate acid/base pair, and dissociation reaction.
  • Kahoot Game Account: LCD Projector, teacher computer, and student cell phones or computers
  • Teacher Answer Key for Practice Problems
• Activity 4: Lab (per group):
  • Student Lab Handout
  • 100mL of 0.1M “Unknown” Solutions (select 3 from: NH ₄Cl, NH ₄NO₃, NaI, Ca(NO₃)₂, NaOH, KCl, NaNO ₃, NaC₂H₃O₂, C₆H₅NH₂, HC ₂H₃O₂)
  • Dropper bottle of 0.1M AgNO₃ (need less than 5mL)
  • Dropper bottle of 1M Na ₂CO₃ (need less than 5mL)
  • Conductivity Tester/meter
  • 3 strips of pH paper (or pH meter)
  • 3 strips of blue litmus paper
  • 3 strips of red litmus paper
  • 3 watch glasses
  • 6, 50mL beakers
  • 3 plastic pipettes
  • Waste beaker
  • Teacher Answer Key

Safety (For Lab Portion/Activity 4 Only):

• Always wear safety goggles when handling chemicals in the lab.
• Students should wash their hands thoroughly before leaving the lab.
• When students complete the lab, instruct them how to clean up their materials and dispose of any chemicals.
• When working with acids, if any solution gets on students’ skin, they should immediately alert you and thoroughly flush their skin with water.
• When diluting acids, always add acid to water.

Teacher Notes

• This is a series of lessons and activities which cover AP levels concepts about weak acids, bases, and salts. These can be used as a complete sequence of lessons or as standalone activities. Since the 2014 updated AP curriculum, it has become especially important for students to be able to analyze lab data for pH and interpret particulate drawings (beaker diagrams) in addition to calculating the pH. Included are notes and practice problems which will help students be able to categorize weak acids and bases as well as acidic, neutral, and basic salts.
• Allow approximately 45minutes for lab set up, and 30minutes for copying and cutting materials.

Lesson Outline:

• Note about placement of these lessons in unit sequence:
  • Students should already be able to classify chemicals as acids or bases (both organic and inorganic) based on the chemical formula. This series of lessons assumes that students already have background on calculating pH of strong acids and bases. This includes a basic understanding of the relationship between pH, pOH, and Kw.
• Note about Ka and Kb values:
  • These can vary from book to book. If you get an answer which is slightly different than what is listed, it may be because your K values are different than what is used here. All of the K values used here are included at the start of the answers.

Activity 1: Comparing Strong and Weak Acids and Bases

• Warm-Up (10minutes): Create a class set of index cards for each student to have one. On each in large, bold print write an acid or a base. Include strong acids, strong bases, weak acids, and weak bases. Include organic and inorganic. Sample set: HCl, HNO ₃, HBr, HNO₂, HF, HCN, CH ₃COOH, C₆H₅COOH, NaOH, KOH, Mg(OH)_2, NH₃, CH₃NH₂, (CH₃)₃N, CH ₃CH₂NH_2, etc.
• As the students walk in, hand them an index card. Create two categories on the board and label: “ACID” and “BASE.” Have the students place their card into the appropriate section as they walk into the classroom using magnets (you can also do this on a wall or table if you don’t have the board space). After everyone has placed a card, ask the students as a class if they notice any misplaced cards. This is a great place to review with everyone how acids and bases are classified based on chemical formulas. Hold onto the cards, they will be used again!
  • Answer: Acids should either begin with H or end in COOH. This is a general rule to help students recognize acids. HCl, HNO ₃, HBr, HNO₂, HF, HCN, CH₃COOH, and C ₆H₅COOH are all acids from the sample above.
  • Bases should contain a metal cation and OH or be an organic molecule containing N (could be N, NH, NH ₂) without a charge. NH₃ is also a base.
• Review (10minutes): Review the difference between dissociation and dissolving. A great example of this is the difference between how sugar (C ₆H₁₂O₆) dissolves versus how table salt (NaCl) dissolves and dissociates. The sugar molecules remain intact when they dissolved -- each C ₆H₁₂O₆ molecule breaks intermolecular forces from other C ₆H₁₂O₆ but no intramolecular bonds are broken. It does NOT break into carbon, oxygen, and hydrogen. That makes sense based on our observations because we don’t see charcoal and gas bubbles when sugar dissolves!! There is no dissociation. When the salt dissolves, it breaks into Na ⁺ and Cl^- because it also dissociates into the ions.
• Class Activity (40-50minutes): Print a set of the provided beaker diagrams for each group (these can be reused) and the student handout. Students work in teams through a series of questions which help them to analyze the beaker diagrams. This activity focuses on the difference in dissociation and pH values between strong/weak acids/bases. At the end, students are asked to follow a model to write dissociation reactions. Throughout this activity, it is important to walk around to each group to ensure they are making clear connections between the pH values, diagrams, and categorizations. Answer key provided.
• Follow-Up Notes (15-20minutes): Follow notes on the weak acids and bases including categorization of strong/weak acids/bases based on chemical formula, dissociation vs. dissolving, and drawing particulate diagrams. Most of these topics are covered in the activity and it is helpful to refer to the activity throughout the notes. Answer key provided.
• Assessment (10minutes): Reuse the cards from the warm-up and pass one out to each student, but this time place four sections on the board: Strong Acid, Weak Acid, Strong Base, and Weak Base. Ask students to place the card in the correct section using a magnet. Ask students if they see any misplaced cards. You can also have the students write the dissociation reaction under each of the cards.

Activity 2: pH Calculations

• Warm-Up (10minutes): Students will review strong base pH calculations and solution calculations by solving the following problem:
  • Calculate the pH of a solution when 4g of NaOH are dissolved into 250mL of water.
  • Answer: 4g x 1mol/40g = 0.1mol
    0.1mol/0.250mL = 0.4M NaOH = 0.4M OH^- because NaOH is a strong base and dissociates completely NaOH à Na⁺ + OH^-
    pOH=-log[OH^-] = -log[0.4] = 13.6
• Notes and Practice Problems (30-40 minutes): Follow the notes provided for calculating the pH of weak acids and bases. Students should complete the practice problems.
• Assessment: This is a great time to hand out the pH of Everything Booklets which contain a review for the pH of everything. Students can work on this a few pages at a time through pH calculations. You can collect and grade this or post the key to allow the students to self-check. This can be used as classwork or homework.

Activity 3: Categorizing Salts

• Warm-Up (10minutes): Write the dissociation reaction for the following acids and bases. Label the conjugate acid/base pairs:
  HClO, ClO^-, CH₃NH₂, CH₃NH₃⁺
• Answers:

• Notes and Practice Problems (20minutes): Use the supplied notes in the student handout to direct students. Practice problems are also included and require students to determine whether a salt is acidic, basic or neutral based on the chemical formula. Students also practice writing dissociation reactions for the salts which are acidic and basic. This connects to the conjugate acids and bases.
• Informal Assessment (20minutes): Play this Kahoot Game (need a free account to access) with students. Students use their phone/tablet/computer to play this quiz game. The quiz covers strong/weak acids/bases and salts. This is also a great informal assessment.
• Assessment: Students can now complete several more pages in the pH of Everything Booklet as either reinforcement for classwork or homework.
• Practice Cards : Students can use these in several ways. They can sort and classify based on the chemical formulas or they can predict the conjugate acid/base and check their answer. Refer to the Materials Section above for preparation instructions.

Activity 4: Determining Unknown Solutions LabActivity 4: Determining Unknown Solutions Lab

• Warm-Up (5minutes): Write the reaction for combining solutions of magnesium nitrate and sodium carbonate. Would this same reaction occur between potassium nitrate and sodium carbonate?
• Answer: The reaction would not occur with potassium nitrate because both potential products are soluble in water and therefor no precipitates would form.

Mg(NO₃)₂ + Na₂CO₃ → MgCO₃(ppt) + 2NaNO ₃

Net Ionic: Mg ²⁺ + CO₃^2- → MgCO₃(s)

Double Replacement Reaction with the formation of solid precipitate

• This is a good review for the lab because the students will need to use pH, conductivity, and reactivity to determine the unknown solutions. This would be an example of reactivity where magnesium nitrate reacts with sodium carbonate but the potassium nitrate is not.
• Pre-Lab (30-40minutes): Students need to complete pre-lab questions and prediction chart (question #9) before starting the lab. This can be assigned as homework if you are short on time. You can also have students complete this in lab teams during class. It is a good idea to do one sample in the chart. If students have incorrect calculations/reactions on their chart, they will have a tough time determining the unknown solutions.
• Lab experiment and Data Analysis (40-50minutes): Students work in groups to collect data for three different unknown solutions. They must use the data they collect from the experiment to determine the unknown solutions.
• Three suggested/good unknowns to use are: Ca(NO ₃)₂, NaOH, and HC₂H₃O_2.
• Post-Lab (5-10minutes): You can have the students discuss their results (identity of the unknowns) and how the theoretical values for pH and conductivity compared to the experimental values.

• Additional Assessments:
  • 2012 AP Free Response Question #1 parts a and e.
  • 2016 AP Free Response Question #6 part a.

• Video Resources:
  • Practical Applications of Acids: Modern Marvels Episode – Acid available for purchase or through history channel site with login.
  • (review) Strong Acids
  • (review) Strong Bases
  • Weak Acids 1 and Weak Acids 2
  • Weak Bases
  • Conjugate Acid-Base Pairs Review
  • Categorizing Overview: first bit has some background noise)
  • pH of Salts

Essential Knowledge from AP Curriculum:
(See Teacher Guide)