Writing Conclusions History We used to have students write entire formal lab reports. Date of experiment Title of experiment Reference Purpose Chemical equation (if applicable) Reagents Procedure Data Calculations Results and Conclusions Your signature and date History We quickly discovered that the problem areas were Purpose Results and Conclusions In this course, the focus will be on Results and Conclusions A restatement of the purpose is included in the conclusion. Remember This When you write your conclusions, write them as if they are at the end of a formal lab report. So, you have already discussed reagents, procedure, and calculations. In the conclusions, you DO reiterate the purpose. In the conclusions, you only repeat important data and show how the data impacts your success at achieving the purpose. The Conclusion Starts with a Statement of Purpose Literally. “The purpose of this experiment was to separate an unknown acid (#8B) from a mixture using acid base extraction, purify it by recrystallization from methanol, then measure its melting point and use it to confirm the identity of the acid from a list of possible unknowns.” If you feel this is too simplistic a writing style, you are welcome to make it more sophisticated…but not at the expense of covering the facts! The Purpose is Different with Virtually Every Experiment As such, this means thought must be given as to why you are doing this particular lab. A clear statement of purpose makes writing the rest of the conclusion much easier as you then show whether you achieved the purpose and show supporting data or whether you failed to achieve the purpose…again showing supporting data. The statement may be short and simple, or it may require several sentences. Often, your conclusion should include a physical description of the substance you are working with solid or liquid, color (clear is not a color), odor if distinctive (do NOT intentionally smell organic chemicals!) relevant data bp, mp, solubility, RI, IR, NMR, MS are possibilities These will depend on the purpose. If you are identifying an unknown from a list of possible compounds Use one piece of physical data to narrow the possible compounds to a “short list,” being careful to state explicitly your criterion. Often this is mp or bp, but may be solubility, refractive index, IR absorptions, etc. Explain How You Created the “Short List” Explicitly state the criterion you used, being realistic. “I measured the mp of the unknown as 120-122°C. I chose all compounds within 5% of that value (compounds with mp’s in the range 114 - 128°C).” If there are 10 compounds which meet this criterion, so be it and report each one with its mp. Explain How You Created the “Short List” Understand that all compounds in the short list are equal! 114°C is not better than 120°C. Try not to have too high an opinion of your infallability. Identifying your unknown from a second data point For instance, refractive index. State the new criterion, then apply it only to the compounds in the short list, listing RI values for every compound in the short list, and explaining how the compounds fit / don’t fit the criterion. At this point, you may have identified your unknown. Use remaining data points to confirm your choice If you have, say, an IR spectrum, of the compound, use it to confirm your identification. If it cannot be used to confirm, it is likely you picked the wrong compound. Dealing with IR Spectra First, identify the major peaks (not all of which will be large) the alcohol or acid O-H str the sp2 and sp3 C-H str the C=O str and state aldehyde, ketone, etc and whether it is conjugated the C=C for isolated double bonds and for aromatic compounds all major peaks above 1500 cm-1 Dealing with IR Spectra Based on your peak identification - which is of bonds! - state what functional groups must be present. It may also be necessary to state what functional groups are NOT present (in particular, the absence of C=O eliminates a lot of compounds). Dealing with IR Spectra Your other data MUST be consistent with your IR data! If your bp data says your compound can only be an aromatic amide and your IR spectrum has an acid O-H str, which is right? What do you do? Be prepared to consider the possibility that you may have performed a technique incorrectly.