Exploring Boiling Points
Every substance has a unique set of properties that allow us to differentiate between them. These properties are classified as physical properties and chemical properties. Physical properties are those that can be determined or measured without changing the composition or identity of the substance. These properties include color, odor, taste, density, melting point, boiling point, conductivity, and hardness. Chemical properties tell us how the substance interacts with other substances and may include reaction with oxygen (oxidation), chlorine, metals, etc. Determination of chemical properties results in the change of the identity of the substance. Some properties, such as solubility, melting point, boiling point, and density are independent of the amount of substance being examined. These properties are known as intensive properties and are used to identify a substance. Extensive properties, such as mass and volume depend on the amount of substance present and are not useful in the identification of a substance. The physical properties of a pure substance can be used to identify the substance and distinguish it from other pure substances. Boiling temperature is one such physical property. Boiling is characterized by the formation of vapor bubbles within the liquid phase as a substance changes from a liquid to a gas. But in order for this to occur, we must apply heat to the liquid at a constant pressure and observe the temperature increase. The point at which the temperature no longer increases even when heat is being added, and when bubbles begin to form and the liquid is being converted to a vapor, is known as the boiling point of the liquid. This can be formally described as the temperature at which a substance (solid or liquid) boils when the pressure is 760mmHg or 1 atm. At the boiling point, the temperature of the liquid is the same as the escaping vapor (or gas). Although the boiling point does vary slightly with the prevailing atmospheric pressure, we will use the normal boiling points at one atmosphere Different substances have different boiling points, and this is due to different trends. In this experiment, we would be observing the boiling points of a group of similar compounds by looking at the straight-chain alkane hydrocarbons. Different trends such as Molecular weight, symmetry, density, solubility and Intermolecular forces can be observed. We will begin by analyzing group of similar compounds – hydrocarbons to see how their boiling point vary with molecular weight. Normal Boiling Points of the Straight-Chain Alkane Hydrocarbons Compound
To observe the boiling points of a group of similar compounds by looking at the straight chain alkane hydrocarbons. Firstly, we will create a graph of the data for the straight chain alkane hydrocarbons, then, look up their structural formulas, molar masses and boiling points of five other compounds. Methods:
Microsoft excel to plot graph.
Molar Mass (g/mol)
Normal Boiling Point (K)
A (11) 2,2,3-trimethylbutane
354.1 ± 0.2
B (18) 2-pentanone
375. ± 1
C (21) 3-methyloctane
417. ± 1
D (30) methyl alcohol
337.8 ± 0.3
E (32) propene
225.6 ± 0.6
Excel is used to make a plot of the normal boiling points of the straight chain alkane hydrocarbons using the data in the introduction. •
Any point on the graph is right clicked, trend line added and polynomial option with an order of 2 is selected. •
References: • S.E. Stein, NIST Mass Spec Data Center, “2 Pentanone” NIST Standard Reference Database 69: NIST Chemistry WebBook 291264
• Wizard, Mr., “Don’t try this at home” – Experiments for General Chemistry, 1st Ed., Explosive Info Co., Ground Zero, 1978, Experiment 2, pp. 10-15.
• Petrucci, Ralph; General Chemistry, 5/e; Macmillan, N.Y., N.Y.; 1989.
• Jim Clarke, www.chemguide.co.uk
The purpose of this experiment was to find out if there is any trend we can observe to help us predict what the boiling point of a compound will be. And if not, how can we explain the discrepancies based on the chemical nature of the substances?
I worked with a group of organic compounds and compared the boiling points of a group of straight chained alkanes to 2 branched alkanes, a ketone, an alcohol and an alkene. I plotted a graph of boiling points of straight chain alkanes against its molar mass. Here, it was noticed that as the molar mass increased, the boiling point increased as well.
However, when I plotted the graph of the 2 branched alkanes, the ketone, alcohol and alkene, I noticed that they didn’t follow the same trend as the straight chain alkanes. The branched alkanes had a lower boiling point compare to the straight chain alkanes due to a decrease in surface area. The ketone and alcohol had a higher boiling point compared to that of the straight chain alkanes due to the presence of hydrogen bonds, which are strong intermolecular forces that require a lot of energy to break. The alkene however, followed the same trend as the straight chain alkane.
So, no. There is no trend we can observe to help us predict the boiling point of any substance. Unless we observe how the boiling points of elements in the same functional group, period or group vary, then it may be similar. However, it does not apply in all cases.
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