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Experiment No. 5: Boiling Point and Melting Point Determinat

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Jane Merca

on 7 February 2014

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Transcript of Experiment No. 5: Boiling Point and Melting Point Determinat

Experiment No. 5: Boiling Point and Melting Point Determination
Intermolecular Forces of Attraction
One end sealed
1mm in diameter; 6cm in length
Powdered 0.5g benzoic acid
Melting Point Determination of Benzoic Acid & Benzoic Acid-Urea Mixture
Boiling Point Determination of Distillate & Hexane
Three capillaries;
One end sealed
5-mL test tube (with 10 drops of sample) attached to the thermometer
Capillary tube immersed in the liquid sample with the open end
Heat the set-up
Record the temperature at which the sample
starts to liquefy
and the temperature at which the
solid has completely melted
Record temperature as the
instant bubbles cease to come out of the capillary
and just
before the liquid sample enters the capillary tube
Answers to
Guide Questions
1. Give an explanation of the observed melting points.
The high melting point of benzoic acid is due to its high vapor pressure (solid to liquid). The melting point of the “pure” benzoic acid is relatively high but the opposite happened in the benzoic acid – urea mixture.

According to Raoult’s Law, the vapor pressure (as well as the melting point) will be lowered when an impurity (another substance) is mixed with a pure compound. The more impurity added, the lower the melting point.

2. Give an explanation to the observed boiling points of the two distillates.
An azeotropic mixture is formed, two or more liquids in a ratio such that it cannot be simply separated by simple distillation.
3. What effect would poor circulation of the melting point bath have on the observed melting point?
For the melting bath to reach thermal equilibrium (the temperature in the bath is the same temperature of the sample), the melting bath should be heated slowly and provided by a good circulation. On the contrary, providing poor circulation or uneven heating will affect the melting bath liquid and cause the melting point to increase.
4. What effect would incomplete drying of a sample have on the melting point?
In incomplete drying of a sample, an impurity is present (water). Thus, the melting point of the sample will decrease.
5. Three test tubes, labeled A, B, and C, contain substances with approximately the same melting points. How could you prove the test tubes contain three different compounds?
If substances A, B, and C are identical and have approximately the same melting points, and if a mixture of them (A+B, A+C, and B+C) is made, then the resulting temperature should still be the same. But if the melting point decreased, then one of them is an impurity and therefore contributed to the depression of the observed melting point of the mixture.
6. Which would be expected to have higher boiling point – t-butyl alcohol or n-butyl alcohol? Explain.
N-butyl alcohol is expected to have higher boiling point than t-butyl alcohol. The more linear structure of n-butyl alcohol causes it to have larger dipole moment and stronger London dispersion forces than will be present in t-butyl alcohol.
7. Calculate the vapor pressure of a solution containing 30 mol% hexane and 70 mol% octane at 90 ⁰C assuming Raoult’s law is obeyed. (Given: vapor pressure of pure compounds at 90 ⁰C: hexane = 1330 torr; octane = 253 torr).
These are forces of attraction present / exhibited between molecules.
By identifying the IMF’s present among substances, one can easily determine other physical and chemical characteristics of a substance.
Melting Point
- the temperature at which a solid substance changes its phase into liquid

Boiling Point
- the temperature at which a liquid substance transforms into its gaseous phase

Vapor Pressure
- equilibrium pressure of the vapor with its condensed phases in a closed container
Factors the affect boiling and melting points:
→ Molecular size
→ Molecular symmetry
→ Branching
→ Polarity
→ Intamolecular H-bonding
Melting Point
Benzoic Acid
IMF present: H-bonding, dipole-dipole, LDF

Molecular Weight: 122.12 g

Molecular Structure (Branching): ring

Theoretical Boiling pt.:

IMF present:
dipole-dipole, LDF

Molecular Weight:
60.06 g

Molecular Structure (Branching):

Theoretical Boiling pt.:
Boiling Point
IMF present:

Molecular Weight:
86.18 g

Molecular Structure (Branching):
straight chain

Vapor Pressure:
17.60 kPa at 20°C

Distillate 1 (from Simple Distillation)
IMF present:
H-bonding, dipole-dipole, LDF

Molecular Weight:
46.07 g

Molecular Structure (Branching):

Vapor Pressure:
if pure (5.87 kPa at 20°C)

IMF present:
H-bonding, dipole-dipole, LDF

Molecular Weight:
46.07 g

Molecular Structure (Branching):

Vapor Pressure:
if pure (5.87 kPa at 20°C)

Distillate 2 (from Fractional Distillation)
Polarity: Direct relationship
MW: Direct relationship
Branching: Direct relationship
Molecular Symmetry: Direct relationship
Intramolecular H-bonding: Direct relationship
Polarity: Direct relationship
MW: Direct relationship
Branching: Inverse relationship
Molecular Symmetry: Inverse relationship
Intramolecular H-bonding: Direct relationship
Structural features that affect boiling point:
Structural features that affect melting point:
Mixed Melting Points
Two sample of the same compound, having the same melting point separately, should have the same melting point when they are mixed together. Also, in general, sample of two different compounds having the similar melting points separately, will act towards each other as impurities when they are mixed together; hence such mixtures will have broadened and depressed melting point ranges.
learn the methods used to get the melting and boiling points
determine the effects of impurities in the melting and boiling points of the substance
recognize how a structure affects a compound's physical and chemical properties
study the factors the affect boiling and melting points
Conclusion and Recommendation
• one must be familiar with other physical characteristics that can influence or affect the boiling and melting points of a substance

• melting and boiling point of a substance distinguishes itself from other substances
It is recommended to be careful and keen in performing the experiment. Changes in the substances, especially during heating, are rapid. This minute changes must not be neglected to yield accurate results.
Brown, T., et al. (2009). Chemistry: the central science.
Singapore: Pearson Education South Asia Pte Ltd.
Bruice, P. (2003). Organic chemistry. USA:
Carey, F. (2000). Organic chemistry. USA: McGraw-Hill
Higher Education.
Physical Properties of Organic Compounds. Retrieved
January 18, 2014 from http://www.personal.psu
Understanding Chemistry. Retrieved Januray 22, 2014
from http://www.chemguide.co.uk/orgpropsmenu

Chem31.1 WEJ1
Group 5
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