Learning Objectives: What should students know and be able to do as a result of this lesson?
Calculate the concentrations of the hydronium ion and hydroxide ion of solutions
Use pH measurements to predict the strength of acids and bases (Molarity will be held constant)
Create and analyze particulate representations for strong and weak acids or bases
Distinguish between strong and weak acids or bases by analyzing experimental data
Prior Knowledge: What prior knowledge should students have for this lesson?
This lesson is focused on applying the concept of strong and weak acids or bases; therefore, prior knowledge of the following is expected:
Hydronium ion is measured for acids and bases,
Calculate pH, pOH, H+, or OH- from when given one of these values,
Water is polar, the hydrogen is partially positive and the oxygen is partially negative (most electronegative); therefore, when substances dissociate in water the partially positive hydrogen end of water surrounds the negative ion being dissolved, and the partially negative oxygen end of water surrounds the positive ion that is being dissolved (solvation).
Guiding Questions: What are the guiding questions for this lesson?
What is the relationship between a high acid pH measurement andthehydronium ion?
Strong acids and weak acids are determined by the extent to which they ionize, strong acids ionize completely and weak acids partially ionize; therefore, when comparing acidic solutions with the same molarity, pH measurements can be used to predict whether an acid is considered strong or weak. A low pH measurement (~1,2,3) indicates that there are a lot of hydronium ions in the solution; therefore, we can predict that it is a strong acid. A high pH measurement (~4,5,6) indicates that there are fewer hydronium ions in the acidic solution; therefore, we can predict that it is a weak acid.
What is the relationship between a high (or low) basic pH measurement andthehydroixde ion?
Strong bases and weak bases are determined by the extent to which they ionize, strong bases ionize completely and weak bases partially ionize; therefore, when comparing basic solutions with the same molarity, pH measurements can be used to predict whether a base is considered strong or weak. A low pH measurement (~8, 9, 10) indicates that there are few hydroxide ions in the basic solution; therefore, we can predict the base is weak. A high pH measurement (~12, 13, 14) indicates that there are a lot of hydroixde ions in the basic solution; therefore, we can predict that the solution is a strong base.
What property of water preventsthehydronium ion from recombining with its anion, and the hydroxide ion from recombining with itscation?
polarity (see prior knowledge)
On the molecular level, what does a solution containing a strong acid or base look like?
Strong acids and bases ionize completely; therefore all molecules or ionic compounds should be broken into their ions and solvated by water with the appropriate end of water facing the ion.
On the molecular level, what does a solution containing a weak acid or base look like?
Weak acids and bases partially dissociate; therefore, some of the molecules will be shown held together.
Can you illustrate the ionization of acids and bases using particulate representations?
Be sure all illustrations demonstrate the law of definite proportions. This goes for strong and weak acids or bases.
Teaching Phase: How will the teacher present the concept or skill to students?
(10 minutes) The teacher will begin the lesson with aKWL chart (seeattachements).
Using the KWL chart, the students will list everything that they know (K) about acids, bases, and pH measurements.
Once the students have completed the K portion, the students will share their knowledge. (How the teacher chooses who will share is to their discretion; however, I recommend pulling sticks from a cup prepared in advance. To ensure everyone gets a chance to respond, do not replace the sticks in the cup until every student has had a chance.)
Upon completing the K, have students list what they want to learn (W) about acids, bases, and pH measurements. Share answers using the same procedure in step 3.
At this point the teacher will pause the KWL activity and move to the POE activity (give students the POE handout; see attachments).
(10 minutes) The teacher should ask the students to make a prediction from the information provided on the POE handout.
The teacher should ask students to share their predictions using the method from step 3.
The teacher should then check the pH of the solutions in the data table. Have the students record their observations in their data tables.
POE Teacher Instructions: Place 20 mL of 0.01M HCl in a 50-mL beaker and 20 mL of 0.01M CH3COOH in another 50 mL beaker.
Use the pH meter or pH strips to measure the pH of the 0.01M HCl solution. Allow students to observe the pH meter reading and record their data in the table provided. Repeat for the 0.1M CH3COOH solution.
The teacher should then have the students explain their observations. Have students share with the class using the method in step 3. At this time the teacher should probe the students using the guiding questions to guide them to the correct answer.
The teacher should now refer the students back to the KWL chart where they will fill in what they learned from the POE (L). Allow students to share with the class using the method in step 3.
(15 minutes) Next, the teacher should use the Formative Assessment 1 to evaluate the students (see attachments). Have student to rate themselves by writing the level of understanding ontheKWL chart. At this time the teacher should roam around the classroom checking the student rates.
The teacher should provide one-on-one time to provide clarification/remediation to those students that rated themselves as a 1 or 2. Meanwhile, students that rated themselves as 4 should peer tutor the level 3 student so that they will become a 4. Once the teacher has completed providing feedback to the level 1 and 2 students, he/she should follow-up with the level 3 students. The goal is for the students to leave the lesson as a level 3 or 4.
(15 minutes) Give each student a copy of the Laboratory: Strong verse Weak. Allow time for each student to read the background and write their hypothesis. Before students leave check their hypotheses for misconceptions. Address as needed on a one-on-one basis.
Inform students of the Summative Assessment that will take place on Day 4.
Day 2: Materials needed: Laboratory handout and all materials, solutions, and instruments listed in the handout
The teacher should reiterate the learning objectives, provide students with the opportunity to share their hypotheses.
Students should began and end their experiment today. The students should follow the procedure as written in the laboratory handout.
Homework: Analysis and Conclusion of the laboratory. The students must complete this assignment at home because no time will be allotted in class.
Remind students of the Summative Assessment on Day 4.
(25 minutes) The students will break into their lab groups and discuss their responses to the Analysis and Conclusion portions of the laboratory.
(10 minutes) The teacher debriefs (go over the lab and the expected results) the class by leading a discussion using the Analysis and Conclusion of the laboratory as a guide. Use the sticks to choose students to respond.
(10 minutes) Have students to rate their understanding (record on front page of lab handout). The teacher should follow-up with those that are rating themselves as levels 1 or 2, and the level 4 students should peer tutor the level 3 students. Again, the teacher should follow-up with the level 3 students as well.
Remind students of the Summative Assessment that will take place tomorrow.
Day 4: Allow students the entire class period to complete the attached summative assessment.
Guided Practice: What activities or exercises will the students complete with teacher guidance?
KWL (see Teaching Phase)
POE (see Teaching Phase)
Independent Practice: What activities or exercises will students complete to reinforce the concepts and skills developed in the lesson?
The student will complete the Analysis and Conclusion portions of the Strong verses Weak lab for homework. This activity must be completed as homework because a debrief will be performed in class the next day (Day 3 of this lesson).
Closure: How will the teacher assist students in organizing the knowledge gained in the lesson?
The closing element of this lesson is the debrief of the Strong verses Weak lab. The teacher debriefs (goes over the lab and the expected results) the class by leading a discussion using the Analysis and Conclusion of the laboratory as a guide.
The teacher will determine if the students have reached the learning targets of this resource at the end of Day 4 when the students take the Summative Assessment. All students scoring a C or higher will be considered proficient.
Formative Assessment 1:
The teacher will gather information about the students' understanding of strong verses weak acids and bases through the Know, Want to Know, Learned (KWL) and Predict-Observe-Explain (POE) activities (see attachments). The teacher will pose the guiding questions throughout the POE activity while using a rating system (see attachments) to determine student understanding.
The teacher will use one-on-one instruction to guide students that rate themselves levels 1 or 2. The goal is for each student is to be either a level 3 or 4 at the end of Day 1.
Formative Assessment 2:
This Formative Assessment is the Strong verses Weak lab (see attachments). It will begin on Day 2 and end with a debrief on Day 3.
Feedback to Students
Formative Assessment 1: The teacher will use one-on-one instruction to guide students that rate themselves 1 or 2. The goal is for each student is to be either a level 3 or 4 at the end of Day 1 (see attachments). The students will have the opportunity to use the teacher feedback at the end of day 1 when creating a hypothesis for the Strong verses Weak lab (see attachments).
Formative Assessment 2: The students will receive feedback about this assessment on Day 4 during a 15 minute laboratory debrief. The feedback from the debrief will be used to clarify any misconceptions that may have occurred when the students completed the Analysis and Conclusion portions of the laboratory for homework.
Accommodations & Recommendations
Since the Analysis and Conclusion of the homework is very lengthy, I recommend that the teacher assigns portions of the homework to students with accommodations. The assigned Analysis portion may vary per student. Choose what correlates to the students' strengths. For example, if they are strong in math then assign the problems. If they are strong in particulate drawings then assign this portion, and so on. Or you may mix it up a bit, give them two questions you feel correlate to their strengths and one that will really challenge them. The remainder of the Analysis should be completed in their group during the first 25 minutes of class on Day 3.
Use the following links to provide the students with extra help:
Robert Ayton: "Acids and Bases" (extra help on calculating pH, pOH, H+, or OH-)
Suggested Technology: Probes for Data Collection, Scientific Calculator
Special Materials Needed:
Popsicle Sticks: Write student names on them and place them in a cup. Draw student names out to determine who will respond to guiding questions.
pH meter or pH strips
Two 50 mL beakers
Laboratory: Strong verses Weak
Four large test tubes
test tube rack
50 mL graduated cylinder
pH meter or pH strips
0.1M HCl, 0.1M CH3COOH, 0.1M NaOH, 0.1M NH3
Common misconception: Be sure to address that low pH does not always means the acid is strong. It depends on the concentration of the solution (how many H+ ions are present). A weak acid could read as a 1 on the pH scale if it is very concentrated, for example, 13 M CH3COOH. However, the solutions used in this lesson are very low in concentration (0.01M and 0.1 M); therefore, the trend of strong acids having a pH around 1, 2, and 3 and weak acids having a pH around 4, 5, 6 should be consistent. Emphasize that this can only be seen when the acids are in equal concentrations, as in this activity.
Calculations of weak acids and bases involves acid-base equilbiria and are beyond the scope of this curriculum; therefore, these types of problems are not included in this lesson.
It may be a good idea to have the students to memorize the most common strong acids and bases when the pH is first taught.