Experiment No. 1: Melting Point and Boiling Points of Organic Compounds

Topics: Van der Waals force, Carboxylic acid, Hydrogen bond Pages: 13 (2159 words) Published: June 12, 2013


The properties of organic compounds depend on their chemical structures. Intermolecular forces of attraction affect physical properties such as melting and boiling point. Through the Thomas Hoover apparatus, the melting point of 8 test compounds was determined. Salicylic acid exhibited the highest melting point while naphthalene, the lowest. The stronger the intermolecular forces of attraction, the higher the melting point. Boiling point determination was done on 7 different test compounds using micro method. Results show that propanoic acid and n-hexane had the highest and lowest boiling point respectively. The former had the strongest intermolecular force of attraction among the test compounds. Thus, there exists a direct proportionality between intermolecular forces of attraction in a compound and its melting or boiling point. In addition, pure organic solids exhibit a sharp melting point range.

I. Introduction

Molecular structures provide an
understanding of the physical and
chemical properties exhibited by
various organic compounds. They
account for the forces of attraction
that bind molecules of a compound
together. Intermolecular forces
include dispersion forces, dipoledipole
interactions and hydrogen
bonding. Dispersion forces, the
weakest among the three, are
brought about by an instantaneous
or induced polarization of
molecules. They are also known as
van der Waals forces or London
forces of attraction. Dipole-dipole
interactions exist among polar
molecules and are stronger than
dispersion forces. A special kind of
dipole-dipole force is hydrogen
bonding which is the strongest
intermolecular force of attraction.
This is present among molecules
with hydrogen bonded to an
electronegative atom (F, O or N).
Physical properties such as boiling
point and melting point are largely
influenced by these intermolecular
forces. The melting point of a solid
compound refers to the
temperature in which its solid and
liquid phases are in dynamic
equilibrium with each other at a
given pressure. On the other hand,
the boiling point of a liquid
compound is the temperature at
which its vapor pressure is equal to
the atmospheric pressure. These
properties are often used in
characterizing or determining a

II. Methodology

I. Melting Point

A. Structural Effect
a. Intermolecular Forces of Attraction

Test Compounds: Finely ground
naphthalene, benzoic acid, salicylic
acid, benzoin and urea.
1. Get capillary tubes from the
laboratory assistant and seal one
end of each tube using heat. The
instructor will demonstrate how
this is done. These tubes will serve
as the sample holders.
2. Get a small amount of the solid
sample and press it into the open
end of a capillary tube until enough
samples are inside the tube. Tap
the closed end of the capillary tube
on the table in order to move the

sample to the bottom. If the
sample remains on the upper
portion of the tube, drop the
capillary tube inside a one meter
glass tubing in which one end is
placed on the floor. The sample
should be at least 5-6 mm in
height and must be tightly packed.
3. Place the capillary tube
containing the sample inside the
sample holder of the Thomas
Hoover Apparatus for melting point
4. Switch on the apparatus. Adjust
the heat and turn on the stirrer.
Record the temperatures at which
the compound starts to liquefy and
at which it has completely liquefied
as the melting-point range.
5. Repeat the above procedures
until all the samples were tested.
Note: Allow the silicone fluid to
cool down first before performing
another test.
b. Geometric Isomers
Test Compounds: Finely ground
maleic acid and fumaric acid
Procedure: Follow the same
procedure as of Part I.A.a.
B. Effect of Purity on Melting Point
Test Compounds: Finely ground
urea and...

References: Lim-Sylianco, C. (1994). Principles
of Organic Chemistry
Morrison, R. & Boyd, R. (1987).
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