student perform 2 melting point determinations on a crystalline product. In one determination, the tube contained a sample 1 - 2 mm in height, and the melting point was 141 - 142 ¢ªC. In the second determination, the sample hight was 4 - 5 mm and the melting range was 141 - 145 ¢ªC. Explain what may have caused the broader range in the second determination. (The actual melting point is reported to be 143 ¢ªC.)
This is my reasoning but I don't know if it is correct. The broader melting range is observed in the second one because the sample of the height was greater.
You are correct; however, your prof won't give you any credit for that (at least I wouldn't). Example: Why is grass green? Answer: because the sun is shining. (I wouldn't get any credit for that either. You see what I mean. (The sun shining lets there be enough light that I can see the green color BUT it doesn't answer why grass is green in the first place.) So what is the real reason why the melting point range was increased. Because the oil bath in which the melting point tube was immersed had temperature fluctuations from the bottom of the tube to the top of the tube. If you had two thermometers, one located at the lower end of the tube and the other at the top of the tube you would have been able to see the difference in the temperature of the two places. Check my thinking. (I'm assuming an oil bath was used.)
Your reasoning is partially correct. The broader melting range observed in the second determination can indeed be attributed to the larger sample height. However, there are a few more factors that can contribute to this broader range.
1. Heat transfer: In the second determination, with a larger sample height, the heat transfer process may not be as efficient. The heating rate may not be consistent throughout the entire sample, resulting in different portions of the sample melting at slightly different temperatures. This can lead to a broader melting range.
2. Impurities: The presence of impurities in the sample can also contribute to a broader melting range. Impurities can alter the arrangement of the molecules within the crystal lattice, making the melting process less uniform. Different impurities may melt at slightly different temperatures, leading to a broader range.
3. Variation in crystal structure: Crystalline substances can exist in different crystal structures, known as polymorphs. If the sample contains different polymorphs of the same compound, they may melt at different temperatures, resulting in a broader melting range.
It is worth noting that the actual melting point of the compound, reported to be 143 ¢ªC, falls within the range of both determinations. This suggests that both samples are indeed the same compound, despite the difference in melting range.