Investigating Rates of Reactions, pt 2
Lesson 4 of 17
Objective: SWBAT design an investigation to speed up or slow down the rate of reaction of an alka seltzer tablet with water.
Day 1 of the investigation went pretty well. Students completed their baseline tests of dissolving alka seltzer in water and thought through their planned changes to speed or slow the reaction. The main changes proposed were:
- Hot water to increase kinetic energy and speed up the reaction
- Ice water to decrease kinetic energy and slow down the reaction
- More water to increase the number of collisions and speed up the reaction
- Less water to decrease the number of collisions and slow down the reaction
- Break the tablet apart to increase collisions and speed up the reaction
- React in Coca-Cola or seltzer water to increase rate of reaction.
I was thrilled with all but the last choice. When I explained to students we were testing the reaction of the Alka-Seltzer with water, not other chemicals, I got a slight push back. I allowed the last two to take place also since it was the first year of me running the lab. The students were intently curious as to the effect, and I was personally curious what the results would be, and how students would tie their results to the collision theory.
The difficult part to the investigation planning is getting students to be specific in their procedures. Many students said they wanted boiling water, but never provided a means to boil the water and had to revise. Likewise with ice water, or measuring out 200mL of water using only a 100mL graduated cylinder. I would have liked to have had an extra day to have students execute a different group's procedure as written and provide feedback. Their feedback is invaluable to my own lab writing, so I think they would benefit from this time in future years.
Additional materials for this lab include hot plates (preferred over bunsen burners due to safety concerns) and ice.
This lesson is aligned to:
- HS-PS1-5: Apply scientific principles and evidence to provide an explanation about the effects of changing the temperature or concentration of the reacting particles on the rate at which a reaction occurs.
- Science and Engineering Practice 1: Asking questions and defining problems
- Science and Engineering Practice 3: Planning and carrying out investigations.
- Cause and Effect Cross Cutting Concept: Systems can be designed to cause a desired effect.
Whole Class Check In
When the period begins I pass out the Alka Seltzer Investigation Rubric to all students. I give them three minutes to look over the rubric before I review it with them. When the three minutes are up, I remind them that the poster is their final product in this project. We focus on what needs to be present on the poster to count for full credit.
In particular, I point out the differences between a 3 and a 4 in various rows of the rubric.
- Lab safety requires proper clean up.
- Molecular Explanations require models of molecular behavior.
- Materials list requires quantities of each piece of equipment
I ask students which rows are the most important, drawing their attention to the data analysis, conclusion and molecular explanations. Since these are worth double points, students need to pay the most attention to doing them well on the final poster.
I remind students that this project is going into their Tests grade book category, which is worth 65% of their overall semester grade.
I then dismiss them to the lab with their partner to finish their lab procedures and data tables. Students are to check their final procedure with me to obtain the alka seltzer tablets to perform their investigation. This check-in permits me to ensure lab safety will be followed prior to students performing the lab, and also to provide feedback on their procedure and materials list in light of the rubric.
When students are finalizing their procedures, they are beginning from one of three places:
- Completed procedure and materials list checked off by me the night before
- Incomplete or inadequate procedure or materials list with feedback from me
- An incomplete procedure/materials list that has not been proofread by me.
Students in scenario 1 generally just need to make their data tables prior to beginning the lab. As they will be assessed on their final data tables on the poster, I remind them to take more data than they need, just as we did in the baseline testing.
Students in scenario 2 can read my feedback and act upon it to get approval, or may need to engage me in a further dialogue about the changes needed. In many cases, there are simple fixes for missing steps. For example, a student may state "Measure 100mL of hot water" without ever having heated water in previous steps. These students quickly correct their errors and omissions and are ready to move on to creating their data tables and beginning the experiment.
Students in scenario 3 need the most attention. I call them my "I'm Just Not Sure" group and discuss their lack of confidence in this reflection. They are stuck for one of three reasons:
- Lack of time to complete work
- Failure to isolate a single variable
- Disconnect between micro-scale collision theory and macro-scale lab parameters
The students in the first camp are the easiest to work with. I set them in the front of the room so they can focus and let them get to work, with the understanding they need to complete their tests today.
The students in the second group need some coaching. I prompt them to explain how they will know which change worked if the reaction rate is changed according to their prediction. This simple prompt generally helps them understand why they need to focus on a single change. If they remain stuck, I give the analogy of asking someone to a school dance. You can dress up, bring flowers, and chocolates, but you don't know which factor was most important in the other person agreeing to go as your date. You know they agreed to go with you, but not whether they are a flower person, a chocolate fan, or thought you looked great.
The last group is the toughest to get up to speed. I have them get out their notes from two days before. I have them break down the reaction between the water and alka seltzer by each part:
- How did the molecules collide? What was your evidence?
- Around the edge of the tablet, because that's where the bubbles were.
- Did the molecules have enough energy? What was your evidence?
- Yes, the reaction started immediately and didn't change temperature much.
- Were there enough properly lined up collisions? What was your evidence?
- Yes, the reaction went to completion.
Then I ask them to pick one factor from what they talked about and decide how changing it might make the reaction faster or slower. I then ask them if their change deals with the collisions, the kinetic energy, or the alignment of the collisions. Once they have this worked out, I remind them to use my procedure, and change just what they want to focus on to test their new hypothesis.
This example did just that, altering the procedure to allow for heating the water, but keeping the rest completely the same.
The timing of this section is very fluid depending on the aptitude of the groups. Some students may need the whole period to design the procedure and then come after school to perform the lab, whereas others will finish and have more than enough time to begin the posters.
The timing of this portion of the lesson is very fluid as well. Students can achieve the actual tests in under 10 minutes, but need additional time for the water to heat or cool, or to analyze their data.
While students are working, I am checking that they are recording their data correctly, following their lab procedures, and working safely and cleaning up correctly. I have a class roster that I am checking off for scoring, as this is part of our grading rubric, but will not be represented on the poster.
When students finish, I have them begin to mock up their poster on a piece of computer paper. In future years, I will have the posters available on this day for the faster working students. Students find the averages of their own investigation times and begin to draw conclusions from the comparison of baseline and test data regarding the validity of their hypotheses.
The students who changed temperature or surface area see the cleanest results, aligned correctly with their hypothesis. The students who changed the amount of water may struggle with why it didn't help. I prompt them to think about where the reaction happens: "On the edge of the tablet" and whether adding more water to the beaker makes more collisions at the edge of the tablet. Below is a sample conclusion from one of these groups:
To illustrate the reaction locations, the students traced the edge of the tablet to show where the water could collide with the tablet, and also explained it in the text.
The two oddball groups, testing Coca-Cola and seltzer water are at a loss as to why their reactions are slower than the baseline test. I ask them to think about it overnight and promise to help them think it through tomorrow while they make their posters.