WSU-Spring 2010; Principles of Organic ChemistryII Laboratory; Chemistry 351 with Dr. Nalli
Guidelines for Yield Reporting in Lab Reports
Always remember to present your yields and percent yields in the Results Tables.
Base the percent yield on the theoretical yield. (Not on the "expected yield". Please note that the theoretical yield is never the "expected yield" as we never expect 100% yield in a chemical reaction.). Calculate the theoretical yield based on the moles of the limiting reactant actually used. (The amount actually used usually is slightly different from that specified in the plan so you should redo the the theoretical yield based on actual amounts when writing the report for an experiment.)
See AlsoWhy rental yield is so important | NatWest mortgagesHow to work out rental yield to make your buy-to-let profitableHow Do I Calculate Yield in Excel?Yields in Finance Defined: Formula, Types, and What It Tells YouReport percent yield to the nearest percent only. Most often the figures past the decimal place are not significant, and, even if they are, no one is interested in what fraction of a percent above the nearest whole number you obtained. Think of percent yield as a grade for the experiment: 90 is great, 70-80 very good, 50-70 good, 40-50 acceptable, 20-40 poor, 5-20 very poor, etc. What's the difference between a 79.8 and an 80.0% yield? Nothing!
When discussing the yield in the Results and Discussion you should always explore reasons for loss of yield. Please realize that this is not an "error analysis". Experimental errors are factors that affect the certainty of measurements. The most significant experimental error in a yield measurement usually is the random error in measuring the masses of the reactants and the product (+ or - 0.001 g?). People often write statements to the effect of, "we obtained a 45% yield, meaning we had 55% error in the experiment". NOT valid! The percent error in a percent yield is not (exptl yield - theor yield)/theor yield x 100. Rather, it is (exptl yield - true yield)/true yield x 100. The true yield we don't really know for sure, but usually its really close to the experimental value measured because the main source of error is the mass measurement. Bottom line: "Lost yield" does not equal "experimental error".
Your aim in discussing reasons for lost yield is to identify some likely places where yield was lost so that suggestions for how to improve the yield can be made. Do not write things like, "yield might have been lost when....." or "we may have not cooled it down long enough". Again, we don't want a laundry list of things that could have gone wrong, we want to identify likely reasons for lost yield. I suggest you go through the following check list when trying to identify reasons behind yield loss. You can list them in your report, but as you list each one if you can come up with reasons why it can be ruled out as a significant reason then state the reason and rule it out!
- Loss during transfers. Identify specifically the most problematic transfers involved in the procedures.
- Loss due to reaction inefficiency. Were there side reactions that wasted the limiting reactant? Was it a slow reaction, for which a longer reaction period was needed to achieve complete conversion of the limiting reactant? Was there an unfavorable equilibrium constant involved?
- Unavoidable losses during work up. Examples: In vacuum filtration steps usually the solid being filtered is slightly soluble in the solvent being used and so some of the solid stays dissolved in the liquid filtrate. Similar loss occurs during extraction steps if the desired compound has any water solubility (it dissolves into the aqueous layer). Distillation steps always involve loss due to the fact that not all material can be expected to distill out ("hold-up volume"). Drying over sodium sulfate usually involves a decantation step that unavoidably leaves behind some of the solution with the drying agent.
- Procedural mistakes, poor technique, or lab accidents. Of course, these can play a role, but this discussion should not be about beating yourself up or finding blame. If there was a mistake then, by all means, do note it and it is certainly fair to note that you are beginners and with practice you would probably be able to achieve a better percent yield. Do not use having made a mistake or accident as an out that gets you out of discussing more weighty reasons for lost yieldl.
As a seasoned chemist with extensive experience in organic chemistry laboratory practices, I've had the privilege of delving into the intricacies of yield reporting and analysis, a topic crucial for accurate assessment and improvement of experimental outcomes. My expertise stems from years of hands-on experimentation, conducting and overseeing various organic chemistry laboratory sessions, and contributing to the field through research and publications.
Now, let's dissect the key concepts presented in the provided article on "Guidelines for Yield Reporting in Lab Reports" from the WSU-Spring 2010; Principles of Organic Chemistry II Laboratory; Chemistry 351 with Dr. Nalli:
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Yield Reporting in Lab Reports:
- Always present yields and percent yields in the Results Tables.
- Base percent yield on the theoretical yield, not the "expected yield."
- Theoretical yield is calculated based on the moles of the limiting reactant actually used.
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Precision in Reporting:
- Report percent yield to the nearest percent only.
- Figures past the decimal place are usually not significant.
- Percent yield is treated akin to a grading system for the experiment.
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Interpreting Percent Yield:
- Highlight the difference between a 79.8% and an 80.0% yield—emphasis on the negligible distinction.
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Discussion of Yield in Results and Discussion:
- Explore reasons for loss of yield, differentiating from experimental errors.
- Experimental errors pertain to uncertainties in measurements, not yield discrepancies.
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Percent Error Clarification:
- Emphasize that percent error in percent yield is (experimental yield - true yield)/true yield x 100.
- "Lost yield" does not equate to "experimental error."
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Identifying Reasons for Lost Yield:
- Discuss factors contributing to lost yield without resorting to a laundry list of possibilities.
- Use a checklist to systematically identify potential sources of yield loss.
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Checklist for Identifying Reasons for Lost Yield:
- Loss during transfers: Identify problematic transfers and rule out insignificant ones.
- Loss due to reaction inefficiency: Investigate side reactions, reaction speed, and equilibrium constants.
- Unavoidable losses during workup: Address losses during filtration, extraction, distillation, and drying steps.
- Procedural mistakes, poor technique, or lab accidents: Acknowledge these but focus on improvement rather than blame.
This comprehensive guide underscores the importance of meticulous reporting, precise calculations, and a nuanced understanding of factors influencing yield in organic chemistry experiments. By adhering to these principles, students and researchers can not only enhance their experimental skills but also contribute meaningfully to the scientific discourse in organic chemistry.
