Mapua Institute of Technology
School of Chemical Engineering and Chemistry
Muralla St., Intramuros, Manila
Melting Point and Boiling Point of Organic Compounds
Group No. 5
Manacup, Cris Vincent L.
Oblena, Adrian D.
Ong, Joshua Jyro B.*
In compounds, two of the physical properties affected by the varying structures are melting point and boiling point. Through the use of the Thomas-Hoover Melting Point Apparatus or the micro method, the melting and boiling point of organic compounds are now determined. The aim of this experiment is to know the factors affecting the trend of melting point and boiling point. These factors are all based on the structural theory of compounds. Including are the intermolecular forces of attraction between molecules, geometric isomerism, purity for melting point determination; and intermolecular forces of attraction between molecules and branching for the boiling point determination. From the structure, intermolecular forces can be observed (hydrogen bonding, dipole-dipole interaction, or London dispersion forces)*. Along with these forces, geometric isomerism affects also. In terms of purity, a pure compound has higher melting point than compound with impurities. Branching, also a factor, makes a compound smaller in terms of surface area that lowers boiling point.
*arranged from strongest to weakest
Melting point of a compound is the temperature at which liquid and solid coexist in equilibrium where a substance in solid form for changes to a liquid. Boiling point, on the other hand, is the temperature at which the vapor pressure of a liquid is equal to the external pressure where a liquid form of the substance changes to a gas.
These properties vary from structure to structure and can be explained by the structural theory. This involves the intermolecular forces of attractions for each molecule, the order in which atoms are put together in molecules and the electrons that hold them together. This is the reason behind the melting and boiling point of each compound. At this point in time, there are inventions of apparatus for the melting point and boiling point determination that are easy to use, fast and digital. These sets are available to the market for a more accurate reading in these physical properties.
Melting Point Determination
Capillary tubes were sealed first on one end by heating it in a Bunsen burner.
Using reagent-contained capillary tubes about 5-6 mm in height, it was subjected into the Thomas-Hoover melting point apparatus (Fig 1). Melting point range determined by two measurements: the temperature at which the reagent starts to liquefy and the temperature at which the reagent was completely liquefied.
Thomas-Hoover Melting Point Apparatus
REAGENTS: Benzoic acid, Benzoin, Fumaric acid, Maleic acid, Naphthalene, Pure and Impure Urea, Salicylic acid, Silicone fluid
Boiling Point Determination
For the boiling point, micro method was used for determining it. Micro method is shown in Fig 2. The capillary tube is placed in a micro test tube containing 2-3 drops of the reagent. Firmly attached is the thermometer beside the test tube with a rubber band. Boiling point is determined by two measurements: the temperature at which bubbles come out of the capillary tube and the temperature at which the liquid comes out of the capillary tube.
REAGENTS: 2-Butanone, Glycerol, n-Butanol, n-Hexane, n-Pentane, Propanoic Acid, sec-Butanol, tert-Butanol
RESULTS AND ANALYSIS
I. Melting Point
A. Structural Effect
a. Intermolecular Forces of Attraction
| MELTING POINT
| 123 deg C
| 131 deg C
| 81.5 deg C
| 156.5 deg C
| 131 deg C
Based on the data obtained from the experiment, salicylic acid has the highest melting point; then, benzoin and urea; next is benzoic acid; and...
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