What would be the effect on the observed melting point if sample were: a) too sm

Question

What would be the effect on the observed melting point if sample were: a) too small- b) too large - c) poorly packed - d) heated too rapidly- Some compounds sublime in the capillary and some decompose before melting. How do you determine melting point of these compounds? A student was given a white solid for an unknown. Its melting point range was 119-121 degree C. The student has previously worked with benzoic acid, and had observed that it was a white crystalline solid with a melting point of 122 degree C. (a) Can the student conclude that the unknown is benzoic acid on the basis of her work to this? Why or why not? (b) What additional experimental work should be done to verify this compound? You and your lab partner take melting points of the same sample. You observe a melting point of 101-107 degree C, while your partner observes a value of 110-112 degree C. Explain how you can get two different values with exactly the same sample. An unidentified compound is observed to melt sharply at 111 degree C with the vigorous evolution of a gas. The sample then solidifies and does not melt until the temperature reaches 155 degree C, at which time it again melts sharply. Briefly explain these observations.

Explanation / Answer

1. a) If the sample is too small- then the melting point observed will be less than the actual melting point because it will melt very quickly.

b) If the sample is too large - Then it will take more time to melt and the reading would be wrong.

c) If the sample is poorly packed, then the heat transfer would not be efficient and the sample will not get heated up evenly.

d) If the sample is heated too rapidly, then the reading at which melting starts and the point at which compound completely melts will not be observed properly.

2. Sublimation is directly conversion of solid into gaseous form. To avoid sublimation, seal the tube from top after the compound is filled in it so that compound does not escape. The compounds which decompose before their melting point are measured at greater ramp rates.

3. a) The student can conclude that the compound is benzoic acid but there are impurities in it. Because benzoic acid was a crystalline white solid and this unknown compound is white solid and not crystalline. This means that the compound is not in pure form now. The impurities in the compound lower or elevate the melting points.

b) Other techniques can be used to find if the compound is benzoic acid or not like NMR, IR spectroscopy, CHN analysis. She can also measure density or other physical properties of the compound.

4. The reasons for different values can be:

(i) The calibration of the instrument is done in different ways. They did not calibe=rate it properly, so the values are different.

(ii) The heating ramp selected is not correct. It is possible that the heating was done sharply, then the melting point observed would be wrong.

(iii) The method of noticing the melting point could be wrong. Melting point is the range in which the initial temperature is the point when you see 1st drop in capillary. It is possible, that it is not observed properly.

5. If there is vigorous evolution of gases, it means that the compound has decomposed and produced some gas and the other compound which is formed by decomposition is a solid. That is why, the compound becomes solid after evolution of gas. The compound formed has melting point of 155 oC, so it does not melt till 155oC and after that it sharply melts.

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