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16.3: Format and Grading Criteria for Prelabs and Lab Reports

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    The general format for most prelabs and lab reports is given below. Specific differences with examples will be discussed later. You are expected to include the following items in your lab notebook. All prelabs and lab reports are graded on a 100 point scale with the following distribution.



    INTRODUCTION – Depending on the type of experiment being performed, this section may contain the items shown below. The first two are relevant to all experiments. The last two are relevant to some experiments. If you cannot find some of the required information in textbooks and common sources such as MSDS or the Aldrich catalog, simply indicate so. If the information is not pertinent, write N/A for not applicable. If it is unknown, indicate so.

    a. Objectives of the experiment

    b. Table of physical constants

    c. Relevant chemical equations and reaction mechanisms (in the case of synthesis)

    d. Diagram or sketch of setup used (if it serves for illustration purposes or to enhance clarity)

    Objectives can be learning objectives and/or experimental objectives. For example, in a crystallization experiment the learning objective is to illustrate the technique of crystallization by having the student perform the steps involved. But the experiment objective is to purify a substance by crystallization and prove that it is reasonably pure by its melting point.

    A table of physical constants should include the following for every major chemical used in the experiment: chemical structure, molecular weight, boiling and/or melting point, density, and safety hazards such as flammability and toxicity. Under safety hazards, indicate the source of information with a footnote (Aldrich catalog, MSDS, etc.). If the substance poses special hazards, make a note under the table regarding safe handling and emergency procedures in case of contact. An MSDS must always be consulted before handling hazardous substances.

    Synthesis reports must include chemical equations. Include molecular mass and amounts used, in grams and in moles, for each reactant. This will enable you to find out which is the limiting reagent when the time comes to calculate the percent yield of the product. If the mechanism of the reaction is known (given in your book or in class), make sure to include it too.

    A picture can save you many words. If it serves an illustrative purpose, the introduction can also include a diagram or sketch of the setup used, such as a distillation apparatus. It doesn’t have to be a work of art and it doesn’t need to be done every time a distillation is performed. If you do distillations again, simply refer the reader to the first experiment where the setup was used, or to a source such as an article or book.

    Examples of different parts of a lab report are given at the end of this guide and in your textbook.

    EXPERIMENT OUTLINE or FLOW CHART OF OPERATIONS – An experiment outline is a list of the steps that go into performing an experiment, written in abbreviated form. The main purpose of this list is to save time by serving as a quick reference while performing the experiment. Most experimental descriptions in the textbook tend to be verbose and difficult to follow, usually covering several pages. By distilling these descriptions into a list of steps the student is able to perform the experiment faster, requiring less assistance from the instructor(s).

    A flow chart of operations (like those used in engineering) is similar to a list of steps, except that it relies on pictures, arrows, and brief descriptions rather than words. The choice of outline or flow chart depends strictly on the student’s personal preference, although some experiment may lend themselves better to one or the other.

    OBSERVATIONS OF PHYSICAL CHANGES – As you perform the experiment, some physical changes may or may not happen. For example, when you combine two substances, a change in color might occur, the solution might get hot, a precipitate may form, or bubbles might appear. The rule of thumb is to record any changes that can be detected with the senses. While most of them are visual (e.g. a change in color or formation of a prec.ipitate), some are tactile (a change in temperature), or even auditory (an explosion, which hopefully will not happen in this course!). On occasion a smell might be detected, and this qualifies as a physical observation.

    Keep in mind that in this section all you do is record such observations as objectively as possible, but you do not try to infer a meaning from them. This is done in the results and conclusions section, described ahead.

    TABLE OF RESULTS AND SUPPORTING MATERIALS – This is similar to the table of starting materials but it shows data that pertain to the products or results of the experiment. Supporting materials is any physical evidence such as spectral charts, chromatograms, or instrument readouts. Sometimes there are no supporting materials. For example when you take a melting point you simply read it from a temperature scale, but the instrument doesn’t produce any chart or physical record of it. On the other hand, when you run a chromatographic plate, you have the plate as evidence of the results.

    A report referring to a physical operation should contain the parameters indicative of the success of the operation. For example, in a purification procedure, give the parameters that indicate the degree of purity of the substance (e.g. melting point, IR spectrum). Also give the percent recovery of pure substance relative to impure substance, complete with relevant calculations.

    A report referring to a reaction or synthesis should contain any parameters that indicate you obtained the expected product. Examples are melting point or spectral data. Include the percent yield with relevant calculations (make sure that you know how to identify the limiting reagent when one of the reactants is in excess).

    RESULTS, DISCUSSION, AND CONCLUSIONS – This is where subjective interpretation comes in. By now you have gathered a certain amount of data, figures, charts, observations, etc. It is time to interpret what that information means. For example, is the melting point obtained close to the expected value? If not, what could possibly be the reason? What information can be gathered from a chromatogram or a spectral chart? Does it support the results you expect to obtain? If a precipitate forms or a change in color occurs, what can that possibly tell us? Speculations are OK as long as they can be backed up with logical arguments.

    Understandably, students find this the hardest part of the report to write. If you experience this type of difficulty, it might help you to go back and retrace all the parts of the lab report. Start with the introduction. Were the objectives and goals of the experiment met? If not, why not? This is where you start thinking about possible sources of error.

    Types of error can be systematic or random. Systematic error is a consistent error that can be detected and corrected. This occurs most commonly when using precision equipment that requires calibration, such as thermometers, burets, and pH meters. Miscalibration can give rise to systematic error. Random error results from limitations in our ability to make physical measurements. For example, when reading a scale, different people might report slightly different results due to their subjective interpretation of the values they read between marks. Random error can also result from variations in an instrument’s performance inherent in its physical makeup. For example, electrical noise might produce slightly different readouts at different times. Random error cannot be eliminated, but it can be reduced by improving the experiment, or by using better equipment.

    Next, you might want to go to section 3 and see if you can provide explanations for at least some of the physical changes observed here. For example, in a Grignard reaction, a change in color is observed as the reaction proceeds. This might be explained by invoking formation of an intermediate in the reaction mechanism. When a step calls for neutralization of excess acid with sodium bicarbonate, formation of bubbles is observed. This can be explained if you know that this reaction produces carbon dioxide gas. As you can see from these examples, many observations can be explained if you have a good understanding of the theory of the experiment.

    The last section to draw from is section 4, where you have presented all the raw data that you gathered during the experiment. The term raw data refers to information that has not yet been organized or interpreted. You can start by answering the question, were the outcomes of the experiment as expected? If yes, what evidence supports this conclusion? For example, was the melting point within the range reported in the literature? If not, what does the evidence indicate that might have happened instead? Maybe the spectrum of what you think is a product really corresponds to the spectrum of the starting materials, indicating that there was no reaction. Maybe it is a mixture of products and starting materials, indicating partial reaction. Maybe it is the spectrum of a byproduct, indicating that the conditions of the experiment favored formation of a product other than the expected one.

    If the experiment worked as expected, there is little need for superfluous elaboration in this section. Remember that the quality of a report does not correlate with the amount of writing done. It correlates with the substance and the clarity of the material presented.

    This page titled 16.3: Format and Grading Criteria for Prelabs and Lab Reports is shared under a not declared license and was authored, remixed, and/or curated by Sergio Cortes.

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