Lesson 4: How to Write a Discussion Part of a Chemistry Lab Report

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This section is the most demanding part to write. It requires you to prudently think about the results you obtained during the experimental session(s), and relate them to the objectives of the experiment. Besides, it calls for the interpretation and generalization of the results. Therefore, students should relate the experimental results to the scientific knowledge under investigation and the objectives. In some chemistry lab reports especially short lab reports, discussion and conclusion are included in one section. However, in long chemistry lab reports, the discussion and conclusion have their distinctive sections.

Let us consider the results (Examples 1 and 2) of the lab report in the previous lesson (lesson 3) to help us write the discussion part of the chemistry lab report.

Discussion of example 1:

Sodium hydroxide-acetic acid titration curve has a lower initial pH and higher equivalence point as compared to aqueous ammonia-hydrochloric acid titration curve (Figure 1 and Figure 2). The pH of the NaOH-CH₃COOH titration curve increases from its initial pH as it approaches the equivalence point. On the other hand, NH3-HCl titration curve decreases from its initial pH as it approaches the equivalence point. NaOH-CH₃COOH and NH₃-HCl titration curves tend to follow the same pattern after the equivalence point (Figure 1 and Figure 2). The pH of the NaOH-CH₃COOH titration curve increases after equivalence point and then remains constant. Furthermore, the pH of the NH3-HCl titration curve decreases after equivalence point and then remains constant.

NaOH-CH₃COOH and NH₃-HCl titration curves do not start at zero points along x-axis and y-axis. Figure 2 along x-axis shows that more volume of HCl was used to attain equivalence point while Figure 1 along x-axis indicates that less volume of NaOH used to achieve equivalence point. The experimental pK_a value of acetic acid (4.3) is less than its accepted pK_a value (4.7). Though, the experimental K_a value of acetic acid (5.012 X 10^-5) is greater than its accepted K_a value (1.8 X 10^-5). The percentage error between pK_a values of acetic is small (10.23%) while its percentage error for K_a values is large (64.09%). Additionally, the experimental pK_b (9.57) value of aqueous is larger than its accepted pKb value (4.75). Nonetheless, its experimental K_b value (2.692 X 10^-10) is less than its accepted K_b value (1.76 X 10^-5). Moreover, the percentage error between the pK_b values of aqueous ammonia is moderate (50.37%), but its percentage error between Kb values is extremely high (more than 100%). Therefore, contaminated apparatus, acetic acid and sodium hydroxide, aqueous ammonia used might have led to experimental errors. These negatively affected the experimental results. However, the objectives of the experiment were achieved. NaOH-CH3COOH monitoring titration was more helpful than NH₃-HCl monitoring titration because there were limited experimental errors as in NH₃-HCl monitoring titration.

Discussion of example 2:

In Figure 1 and Table 1, there are peaks at about 3000cm^-1, 1350cm^-1 and 1714cm^-1. These imply that there were a C-H stretch, C-H bend, and C=O stretch. Therefore, these peaks best describe an aliphatic ketone. Hence, 2-pentanone was the possible identification of unknown organic solution #1 among the list of organic compounds present. Organic compounds such as ethyl benzoate, aniline, benzoic anhydride, benzaldehyde are aromatic organic compounds, thus they have different IR characteristics as compared to aliphatic organic compounds. For that reason, their IR properties ruled them out as possible candidates for unknown organic solution #1. Additionally, IR vibrational stretches and bends of an alkyl alcohol, acetamide, and propanoic acid are not the same as those of aliphatic ketones; for instance, if propanoic acid was present, then the C=O vibrational stretch peak could have appeared between 1760cm^-1 and 1690cm^-1. Moreover, O-H stretch, C-O, and O-H bend could have occurred at between 3000cm^-1 and 2500cm^-1, and 1320cm^-1 and 1210cm^-1, and 1440cm^-1 and 1395cm^-1. However, these are absent in Figure. Furthermore, if acetamide and ally alcohol we could have seen peaks between 3500cm^-1 and 3200cm^-1, and also 1550cm^-1 and 1475cm^-1. Though, these are absent in Figure 1, making 2-pentanone as the best organic compound represented by unknown organic solution #1.

In Figure 2 and Table 1, peaks appeared at 1714cm^-1, 1269 cm^-1, 3000cm^-1, 1980cm^-1, 1600cm^-1, and 1400cm^-1. The peaks indicate that there was the presence of C=O stretch, C-O stretch, C-H stretch, a weak overtone, and C-C stretch (in-ring). These peaks aptly define aromatic compounds. Therefore, an unknown organic solution #2 did not represent acetamide, alkyl alcohol, propanoic acid, and 2-pentanone. Besides, the presence of the peaks for C= O stretch and C-O stretch, which describe ester lock out the benzaldehyde, aniline, and benzoic anhydride as the representatives of the unknown organic solution # 2. The peak of a C=O stretch of the ester occurs between 1730cm^-1 and 1715 cm^-1 while its peak for C-O stretch appears between 1300cm^-1 and 1000cm^-1. The experimental peaks for C=O and C-O of unknown organic solution #2 (Table 1 and Figure 2) were within the range of the peaks in the literature of C=O stretch and C-O stretches of an ester (Smith, C. 2007, 43). So, this qualifies ethyl benzoate as the representative of unknown organic solution #2.

The experimental peak values for both unknown organic solutions #1 and #2 were almost the same as the peak values in the literature of 2-pentanone and ethyl benzoate. Hence, the experimental errors, which might have originated from contaminated unknown solutions and the sample cell, and improper handling of IR spectrometer, had insignificant effects on the experimental results. For that reason, the experimental results had high precision. Though, for one to achieve better results in the future, one should use industrial grade reagents. Moreover, sample cell should be properly cleaned with an appropriate solution and an IR spectrometer should be operated as enshrined in its operational manual.

The aims of the experiment were achieved. The unknown solutions # 1 and 2 were identified using the IR spectra generated by a Varian 640 IR. In addition, the experimental results were within the range of similar studies conducted before. As a consequence, this implies that the objectives of the experiment were justified. In literature, peaks of C=O stretch for aliphatic ketone appears at 1715cm^-1 (Smith, 2007). On the other hand, peaks of  C=O stretch for C=O and C-O stretch for unsaturated esters exist at 1730-1715cm^-1, and 1300-1000cm^-1 (Figure 2). In this experiment, the peaks of C=O stretch and C-O stretch were 1714cm^-1 and 1269cm^-1. Consequently, they are in agreement with the peaks of C=O and C-O for ketones and unsaturated esters in literature.

Final remarks:

In the examples above, the results of lesson 3 are carefully explained in relation to the scientific knowledge under investigation and the objectives of the experiment. In both examples, the conclusion part is not included.

Do you have any question regarding the discussion section of a chemistry lab report? If you do then do not hesitate to leave a comment. You can also check the previous lesson here .