Categorizing, Calculating and Applying Concepts from Weak Acids, Weak Bases and Salts (13 Favorites)
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.
High School (AP Chemistry)
AP Chemistry Curriculum Framework
- Big Idea 6: Any bond or intermolecular attraction
that can be formed can be broken. These two processes are in a dynamic
competition, sensitive to initial conditions and external perturbations.
- 6.7 The student is able, for a reversible reaction that has a large or small K, to determine which chemical species will have very large versus very small concentrations at equilibrium.
- 6.11 The student can generate or use a particulate representation of an acid (strong or weak or polyprotic) and a strong base to explain the species that will have large versus small concentrations at equilibrium.
- 6.16 The student can identify a given solution as being the solution of a monoprotic weak acid or base (including salts in which one ion is a weak acid or base), calculate the pH and concentration of all species in the solution, and/or infer the relative strengths of the weak acids or bases from given equilibrium concentrations.
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.
This lesson supports students’ understanding of
- Acids & Bases
- Strong vs Weak Acids/Bases
- Conjugate Acid-Base Pairs
- Equilibrium Constants
Teacher Preparation: 75minutes
Lesson: 250 - 300minutes
- 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.
- 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.
- 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 3CH2NH2, 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
- 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+
- 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+ + CO32- → 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 HC2H3O2.
- 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:
- 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)