Lab 1-Melting Points
In a melting point determination, the heating rate as the solid is melting should be _____ C per minute.
1.00 ° In a mp determination, to obtain an accurate mp, the heating rate must be very slow while the sample is melting. This allows the transfer of heat between the heating device, the sample, and the thermometer to attain equilibrium.
The melting point of a sample of compound X was found to be 121 - 125 ° C. Compound X was one of four possible compounds having melting points of 121, 123, 125, and 127 ° C. Which was it most likely to be? 125 121 123 127
127 Impurities usually cause a compound to melt over a temperature range rather than at a single temperature. The 4° melting point range observed for compound X indicates that the sample contains impurities. In addition to broadening the temperature over which a compound melts, impurities also lower the temperature at which the compound melts. The melting point of a pure compound will therefore be greater than the melting point range found for an impure compound.
A solid sample melted at 177.0-181.0°C. The compound was one of four possibilities, having the mp's shown below. Which was it most likely to be? 179.0 183.5 177.0 181.0
183.5
In this lab a melting range of _____C indicates that a compound is relatively pure.
2.00 ° A pure solid melts over a narrow range. In this lab, a compound is considered to be acceptable pure if its melting range is 2° C or less.
5.) What two pieces of information can a MP determination provide?
MP determination can be used to determine the identity of a compound and the relative purity of said compound.
4.) In a recrystallization (a technique that you will encounter later in the semester), a solid is dissolved in a solvent and later the solvent is removed. If a MP of the sample is taken while the sample is still moist with solvent, what effect would that have on the MP of the sample?
Since the solid is dissolved in a solvent and the sample is still moist with solvent, it has a soluble impurity. Soluble impurities would make the sample impure and therefore lower the melting point and widen the melting point range relative to a pure sample.
2.) For question 1, if the sample is one of four possible compounds the melting points of which are 133°, 135°, 137°, and 139°C, which is it most likely to be? Why?
Since this sample is an impure solid as explained in question 1 (melting point is over a wide range), it's most likely to melt at 139°C. Impure substances melt at melting point range lower than that of a pure sample, so it's most likely to be higher than the range recorded (133-137°C) in question 1, which would be 139°C.
Melting points can provide the following information about a sample. The heat of formation of the substance. The identity. The purity. The functional groups present in the compound. The crystal structure.
The identity. The purity. The melting point of a compound can be used to help identify the compound by comparing its melting point with the melting points of known compounds. A narrow melting point range indicates that the sample is pure. A wide range indicates that the sample is impure.
If a second melting point determination must be taken for a sample, it is necessary to use a fresh sample and not the one that has already been used. Why? The crystal structure may have changed. The sample may have decomposed slightly. A less pure sample may have been used the first time. The sample will be drier after it has been left for a longer time.
The sample may have decomposed slightly. It is occasionally necessary to determine the melting point of a sample more than once. For example, for an unknown substance, it is necessary to determine an approximate mp first by using a rapid rate of heating, and then to determine it more carefully a second time. Sometimes it is just necessary to confirm the first determination. Whenever a second determination is needed it is necessary to use a fresh sample because upon melting the sample may have decomposed slightly. Any decomposition product would act as an impurity and thus lower the mp and broaden its range.
1.) A solid sample has a MP of 133 - 137°C. What can one conclude about the sample?
This sample can be concluded to be impure because it has a wide range of MP (133 - 137°C), characterized by greater than 2 degrees Celsius
3.) Two test tubes contain compounds having the same MP. Using MPs, how could you determine whether the two test tubes contain the same or different compounds?
You can use mixed melting point technique, where you mix the two compounds to determine if they are the same or different compound. If they were different compounds, mixing them would create an impure solid which would lower and widen the melting point range for the known compound. If they were the same, the melting point range would be identical and narrow.