Melting Point and Boiling Point of Organic Compounds

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Social Alchemy
2nd Quarter / SY 2012 – 2013
Eng’r. Josephine A. Ng

Concepcion, Jermin B.
Group# 2

Experiment No. 1

The melting point (MP) and the boiling point (BP) are probably the most widely used physical constant in the field of science. Determining the boiling point and the melting point of a compound helps you to characterize an unknown solid in a quick, easy and cheap way. The temperature at which a compound turns from a solid state to a liquid state is known to be the melting point. On the contrary, the boiling point of a compound pertains to the temperature where the compound changes from liquid state to its gas state. Many factors affect the boiling point and melting point of an organic compound, and one of which is the molecular structure. Purity and branching also affects the boiling and melting point of a compound. There are three trends that affect the boiling and melting points and these are the following: The relative strength of the four intermolecular attractions is factor significally affects the boiling point and the melting point of a compound. The overarching principle involved is simple: the strong the non-covalent interactions between molecules, the more energy are required, in the form of heat, to break them apart. The higher melting and boiling points signify the stronger non-covalent intermolecular forces. For molecules with a given functional group, boiling point increases as molecular weight increases. The key force acting here are Van der Waals dispersion forces, which boiling points and melting points are proportional to surface area. So as you increase the length of the chain, you also increase the surface area. And the increase also increases the ability of each individual molecule to attract each other. Lastly, the symmetry or lack thereof. Symmetry is another byproduct of the Van der Waals dispersion forces- in this case; the more rod-like molecules are, the better they are able to line-up and bond. However, we can’t base on mass evidences alone, for we are aware that certain compounds can exhibit same formula mass through the effects of isomerism. Keywords: melting point (MP), boiling point (BP), molecular structure, branching, intermolecular attractions, isomerism, purity

The melting point of a compound is the temperature at which it changes from a solid to a liquid. It is also determined as the temperature where the thermal energy is great enough to overcome the internal forces that holds the molecule is reached. It is one of a number of physical properties of a substance that is useful for characterizing (describing) and identifying the substance. Determining the melting point of a compound is one way to test if the substance is pure as well. A pure substance generally has a melting range (the difference between the temperature where the sample starts to melt and the temperature where melting is complete) of one or two degrees. Pure crystalline substances have a clear, sharply defined melting point. Impurities tend to depress and broaden the melting range so the purified sample should have a higher and smaller melting range than the original, impure sample. To melt a compound, energy must be supplied to break up the intermolecular forces that bind organic molecules in the solid state so that they can go into the freer liquid state. The amount of energy needed to break these attractions in turn depends on the strengths of the intermolecular holding the molecules together.

When the vapour pressure of a liquid is equal to the atmospheric (or applied) pressure then boiling occurs. Boiling is involved in the breaking away of the oppositely charged ions contained in the solution. The temperature, at which this occurs, for a given pressure, is...
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