ChemEd X recently made a Call for Contributions soliciting input regarding the big ideas being put forth by organizations like AP. The first thing that came to mind was a lab I modified that is centered around making connections between topics. Admittedly, this lab is not a "big idea" per se. Rather, it's the big idea that students should be able to make connections between topics we study to solve problems. So in this blog post, I would like to share a lab activity that relies on these connections - between stoichiometry, esterification, equilibrium, kinetics, titrations and uncertainty of calculations. I will also share the resources I have created to support my students through the process of working through these calculations.
In terms of attribution, I will admit I created this lab modified from a number of sources (see list below). I have modified it so much that it is difficult to know which source contributed which idea I borrowed. I offer my humble thanks to all of the sources used.
I would suggest reading the lab handout at this point (linked below). The reaction used is an esterification reaction between ethanol and ethanoic acid. As mentioned earlier, I think students benefit from discussion of the connections between topics. Depending on sequencing, this can be review of an esterification reaction already learned. It is also review of equilibrium calculations and how titration can be used to find equilibrium concentrations.
As for the pragmatics, the lab takes three days of class time.
Day 1 is simply making the equilibrium mixture - and this typically takes 15-25 minutes depending on the comfort level of my students with general lab practices. The most efficient method of delivering the liquids I have found is setting up burettes around my lab with the required liquids. I use multiple burettes of each liquid to prevent log jams. The students simply move from station to station collecting the required liquids - recording burette readings as they go. You can certainly modify this to use volumetric and/or graduated pipettes for liquid transfer.
Day 2 is the titration. I try to have at least two students with each mixture (I have small classes) so that their results can be compared. Each student completes his/her own titration. With a bigger class, this could be modified so that each group conducted one or more titrations - trying to give each student time on the burette for the experience. The students also titrate the 3 M hydrochloric acid for their data processing. These two titrations typically take about an hour - plus or minus 15 minutes depending on the comfort level of the students with the process of titration - and whether you make the NaOH solution for them.
Day 3 involves going through the calculations. I typically start this at the end of Day 2 and ask students to put in their best effort before the following class, using the guideposts below:
Guideposts
For each mixture:
1. Calculate the amount (in mol) of ethyl ethanoate, ethanoic acid, ethanol and water present in the original mixture (i.e. before any reaction has taken place). When calculating this for the water remember to take into account the water present in the dilute HCl.
2. Using the titration information, calculate the amount (in mol) of ethanoic acid present in the equilibrium mixture and hence the amount (in mol) of ethanol, ethyl ethanoate and water in the equilibrium mixture. (Use ICE here after the titration calculation.)
3. Calculate the equilibrium concentrations of the water, ethyl ethanoate, ethanoic acid and ethanol in the mixture and hence the equilibrium constant.
4. Compare to the true value of 4.0 at 298 K.
The guideposts are not very detailed - and yet this series of calculations requires several steps that are linked together to come up with a final value for Kc. Based on the difficulty - and my desire to guide students without doing all of the work for them - I created a detailed guide to the calculations that literally "talks" them through each step (see the documents included in the Supporting Information at the bottom of the post). My students commented that they could hear my voice as they read the directions. It is a casual set of directions that take the students through every step.
And to verify the student calculations, I created a spreadsheet (see the documents included in the Supporting Information at the bottom of the post) that allows me to quickly verify any set of calculations by using some pre-measured data (such as the mass and volume data of the substances) and the student data to calculate Kc. I have created many similar spreadsheets for labs that are calculation-heavy so I can efficiently check student work. Take a look and let me know what you think.
I am curious to know if you have any favorite "big idea" labs, or labs that involve making connections between a number of topics. I am always looking for new ways to help students establish these connections - and I hope this lab can contribute to your classroom as well.
Sources of Inspiration
Dr. Geoffrey Neuss and his InThinking site, http://www.thinkib.net/chemistry
Teacher Resource Bank, http://filestore.aqa.org.uk/subjects/AQA-2420-W-TRB-PSA14.PDF
San Mateo County Community College District Lab Manual, http://accounts.smccd.edu/batesa/chem220/lab/labmanual-sum12/4-Keq_Ester-SUM12.pdf
Harvard Westlake Lab Handout, http://www.hwscience.com/chemistry/ap/aplab/eqetac.pdf
Esterification
Organic Nomenclature
Equilibrium
Titration
Stoichiometry
Equilibrium Constant
Three class periods
Day 1: setup of equilibrium mixture; roughly 30 minutes
Day 2: titration of equilibrium mixture (approximately 1 week after Day 1); roughly 60 minutes
Day 3: calculations; variable time required - typically 30-90 minutes depending on the student group
3 M HCl
Distilled Water
Ethyl Ethanoate (aka ethyl acetate)
Glacial Ethanoic Acid
Ethanol
Esterification is a common type of organic reaction studied in a variety of courses. The reaction here is quite slow. Thus, equilibrium concentrations of the reactants and products can be determined by titration – as this process won’t shift the equilibrium amounts by a great deal given the slow reaction rate.
Day 1: Make up two flasks of ONE the following mixtures using the solutions provided in the burettes. Run the liquids into the reagent bottles to give a total volume of 10 cm3 then stopper immediately to prevent evaporation. Shake well then allow the bottle to stand at room temperature for one week to allow the mixture to reach equilibrium. Note: You need to LABEL your bottle and store it in the fume hood.
Note: You need to RECORD the starting and ending volumes on the burette so you know exactly how much of each solution you added.
Weigh separately 5.00 cm3 of each of the liquids used to make up the mixtures (including the HCl) and record the mass. This will be used to calculate the density for the calculations.
|
Mixture |
HCl cm3 |
Water cm3 |
Ethyl Ethanoate cm3 |
Ethanoic Acid cm3 |
Ethanol cm3 |
|
1 |
5 |
0 |
5 |
0 |
0 |
|
2 |
5 |
1 |
4 |
0 |
0 |
|
3 |
5 |
2 |
3 |
0 |
0 |
|
4 |
5 |
3 |
2 |
0 |
0 |
|
5 |
5 |
0 |
4 |
0 |
1 |
|
6 |
5 |
0 |
4 |
1 |
0 |
|
7 |
5 |
0 |
0 |
1 |
4 |
|
8 |
5 |
0 |
0 |
2 |
3 |
Day 2: After one week, add approximately 20 mL distilled water to your first reaction mixture. Then quickly titrate with 1.0 mol dm-3 sodium hydroxide using phenolphthalein as the indicator. Repeat with your second trial. (Note: Don’t add water until you are ready to start the titration.)
In order to find the exact concentration of the HCl catalyst, also titrate 10.00 cm3 of the 3 mol dm-3 HCl with the 1.0 mol dm-3 sodium hydroxide solution.
Day 3: Use the information from the titrations to calculate the equilibrium constant, Kc, for this reaction.
I found it best to place the liquids in burettes around the room to give students easy access for creating their flasks for the equilibrium mixture.
For Day 2, I require students to make their own NaOH solution. However, to speed up the process you could certainly make the solution for them. I used 1.00 M NaOH.
I am the author of the activity and supporting resources. The original lab handout was inspired by many sources (see Credits).
Safety
General Safety
General Safety
For Laboratory Work: Please refer to the ACS Guidelines for Chemical Laboratory Safety in Secondary Schools (2016).
For Demonstrations: Please refer to the ACS Division of Chemical Education Safety Guidelines for Chemical Demonstrations.
Other Safety resources
RAMP: Recognize hazards; Assess the risks of hazards; Minimize the risks of hazards; Prepare for emergencies
NGSS
Students who demonstrate understanding can refine the design of a chemical system by specifying a change in conditions that would produce increased amounts of products at equilibrium.
*More information about all DCI for HS-PS1 can be found at https://www.nextgenscience.org/dci-arrangement/hs-ps1-matter-and-its-interactions and further resources at https://www.nextgenscience.org.
Students who demonstrate understanding can refine the design of a chemical system by specifying a change in conditions that would produce increased amounts of products at equilibrium.
Assessment is limited to specifying the change in only one variable at a time. Assessment does not include calculating equilibrium constants and concentrations.
Emphasis is on the application of Le Chatelier’s Principle and on refining designs of chemical reaction systems, including descriptions of the connection between changes made at the macroscopic level and what happens at the molecular level. Examples of designs could include different ways to increase product formation including adding reactants or removing products.
All comments must abide by the ChemEd X Comment Policy, are subject to review, and may be edited. Please allow one business day for your comment to be posted, if it is accepted.
Comments 3
solubility of ethyl ethanoate
Hi,
very interesting topics, nice experiment to determine the equilibrium constant.
A practical question that I had is the following: ethyl ethanoate is not that soluble in water. Basically, one gets two layers (let's take for instance the first mixture with 5 mL of ethylethanoate added to 5 mL HCl 3M).
May this have an impact for the final detrmination of the constant ? I would argue that the answer is yes, since mixing is not complete and reaction may not actually get to equilibrium, but I would appreciate to listen to your opinion.
Thanks in advance,
Paolo
Thank you, and a thought
Thank you for the comment and the question.
This is admittedly something I hadn't put thought into, as I got the idea for the lab from a reputable source and figured the reaction must work. Your question, of course, got me thinking.
I found an article from the chemistry archives which puts the solubility at about 8 grams/100 grams H2O. Thus, it's not completely insoluble. So I would propose that as the ethyl ethanoate that is dissolved completes the reaction, more will dissolve (thinking about how removing the product of dissolving will shift the equilibrium) - and thus can react to allow the overall system to reach equilibrium.
Additionally, the mixture was left for about a week - so I would imagine that some reaction could occur even at the water-ethyl ethanoate layer over that time. One step I can think to modify the procedure, based on your question, is to have the students shake up the mixture each day of class over the 7 days of waiting.
Does that sound reasonable to you? I'd be interested in your thoughts.
Thanks.
Lowell
reply to solubility of ethyl ethanoate
My results agree well to what you were arguing. Indeed, reaction occurs and after few days, separation between the two layers disappear.
I have kept the samples at a temperature of 30oC. Acetic acid titration was then performed after not less than 72 hours. Measured NaOH volumes are in agreement to what I was expecting, if we admit the equilibrium constant to be equal to ~4.
I can then definitely conclude that this is a very nice experiment, allowing the study of equilibrium and in particular showing that reactions can evolve towards one side or the opposite one, depending on the initial mixture composition.
Thanks again, regards
Paolo