Analysis of a Mixture: Applying Green Chemistry
(Modified from the 2013 AP Chemistry Lab Manual by Phil Palko)
Scenario:
You are assuming the role of a chemist who will review a chemical process to analyze a mixture. You will be given a sample manuscript (lab report) that has been submitted to a journal on green chemistry. You will first investigate the analysis process and then assess the quality of the submitted manuscript and write a review to submit to the editor of the journal.
Background:
Analytical chemists often are asked to determine the composition of a mixture. How this is accomplished depends largely on the properties of the mixture itself and the properties of the components of that mixture. In this experiment, you will rely on the fact that the two substances in the given mixture (NaHCO3 and NaCO3) differ in how they behave chemically when heated.
It is also an important consideration in chemistry to evaluate the efficiency of a particular chemical process. Percent yield, as calculated using equation (1) below, can be used to give one an idea of how much desired product resulted from a process (usually a chemical reaction) in comparison to some theoretical or expected amount. Ideally a process is able to convert 100% of the starting material to product but numerous factors can reduce efficiency. As part of research, a chemist maybe tasked with improving the percent yield of a particular reaction which would then reduce the costs associated with it.
Equation (1): Percent Yield = (Actual Yield/Theoretical Yield ) x 100
Another way to examine the efficiency of a process is to determine the atom economy. This takes into account by-products of a process that could be considered wastes. A 100 % atom economy then indicates that the amount of desired product is maximized and there is no waste or by-product. This calculation is shown in equation (2) below.
Equation (2): Percent Atom Economy = (Mass desired product/Mass of all products) x 100
Various chemical processes have different atom economies because the desired product can range from being only one substance to being only one of several substances that result from a reaction. If several processes are being compared, then the one with the atom economy closest to 100% would be considered as one that most cleanly produces the desired product.
Procedure:
(Your group must devise a written procedure and have it approved before doing anything in the lab!) Be sure to record all data and show all calculations in your lab notebook!
Part I: Investigating the properties of Na2CO3 and NaHCO3
Obtain a sample of Na2CO3 and a sample of NaHCO3 and two crucibles and devise a simple procedure to compare the chemical behavior of the two substances when they are heated using your samples and the crucibles. You must collect data to verify that the two substances do indeed behave differently. Write any chemical reaction equations that are appropriate to describe any chemical changes that you observe.
Part II: Analysis of a mixture of Na2CO3 and NaHCO3
Based on your initial investigation, develop a procedure to determine the mass percent of each component in an unknown mixture of Na2CO3 and NaHCO3. You must include how you will use the collected data to calculate the relative amounts of the two components.
Part III: Manuscript Review
Now that you are familiar with the chemistry of analyzing this particular mixture, your task is to review a lab report pertaining to the same problem and report on its strengths and weaknesses. The journal to which the lab reports have been submitted is a green chemistry journal. Determine the atom economy of the reported reaction and assess the “greenness” of the process in at least one other way according to the principles of green chemistry.
Additionally, you will review the lab report in terms of effectiveness in reporting information. You will be provided with a review template in order to accomplish this task.
Safety:
Goggles and Aprons are required!
Do not heat a completely covered crucible! Be sure the lid is in the position shown above.
Remember you are heating objects to high temperatures! Do not touch anything you think maybe hot.
Equipment for the Lab:
Balance
Crucible + Lid
Crucible tongs
Bunsen burner
Ring stand and iron ring
Ceramic triangle to hold the crucible
Beakers
Lab Report Review
Lab Report Reviewed: 1 2 3 (circle)
1. What was the quality of the science in the lab report?
a. Was the investigation conducted well?
b. Were sufficient trials run to instill confidence in the results reported?
c. Is there evidence that the data were collected carefully?
d. Are the explanations of chemistry in the lab report correct?
e. Are the calculations done correctly?
f. Does the report account for any problems in the investigation?
2. What is the quality of the reporting the scientist did in the lab report?
a. Does the lab report communicate the investigation effectively?
b. Is the procedure detailed enough that it can be repeated to verify the results?
c. Were the materials and equipment specified sufficiently?
d. Are the calculations presented clearly?
e. Are any of the sections of the lab report weak, and how can they be strengthened? (For example: Is the procedure clear? Are the data reported effectively?)
3. How green is the chemical process that is reported?
a. What is the atom economy of the chemical reaction in the lab report? Identify the desired product (It is not the water or carbon dioxide!) and show your calculation.
b. Provide an argument based on a different principle of green chemistry for or against the greenness of the chemical process reported in the lab report.
Pre-Lab Exercises
1. Go to http://www.epa.gov/sciencematters/june2011/principles.htm and list the 12 Principles of Green Chemistry:
2. Select one of the principles of green chemistry. Explain in your own words how this principle shifts chemistry toward more environmentally conscious practices.
3. The following two reactions are possible methods for refining copper in the final step of a smelting process which is used to obtain pure copper from copper ore. Calculate the atom economy for each reaction. Decide based on this calculation, which method is greener.
a) 2 CuO (s) + C(s) à 2 Cu (s) + CO2 (g)
b) CuO (s) + CO (g) à Cu (s) + CO2 (g)
4. What does calculating the atom economy tell in terms of “greenness” of a process?
5. What is “peer review”? Why is this process important?
6. Go to: http://group.chem.iastate.edu/Greenbowe/sections/projectfolder/percenttutorial.htm.
Do all parts of the activity you find there and write up a short lab report to show your findings. Include the results (measured data and calculated results) of the virtual experiment and any calculations necessary in this report. Print the page from the simulation after you heat the hydrate and submit it with your report. This is the link to the simulation: http://group.chem.iastate.edu/Greenbowe/sections/projectfolder/flashfiles/stoichiometry/empirical.html
7. How are the two ways of measuring the efficiency of a reaction the same? How do they differ?