experiment on simple distilliation
Answers
Answer:
The distillation process is a very important technique used to separate compounds based on their
boiling points. A substance will boil only when the vapor pressure of the liquid is equal to the external
pressure being applied by the surroundings. Distillations can be used to efficiently purify volatile (i.e.
low boiling) compounds. The general concept of distillation involves the boiling of a mixture, resulting in
the lower boiling compounds boiling off first. This compound is then collected when the vapors are
cooled on a condenser.
Several different techniques exist for specific applications. The most commonly used method is the
simple distillation apparatus (Figure 1). This results in the vapors being collected and affords one
solution in moderate purity. It is difficult to separate compounds by this method unless they have a
large difference in boiling points (>25 °C), or if ones is trying to removing a liquid from a solid. A similar
apparatus is used during a fractional distillation (Figure 2). The primary difference is that a fractionating
column provides significantly more surface area, and essentially results in repetitive simple distillations
being formed throughout the column. The consecutive evaporations and condensations allow for the
separation and purification of compounds with similar boiling points.
Boiling points are directly proportional to pressure; therefore as the pressure is decreased the boiling
point will also decrease. Compounds with very high boiling points can be distilled much easier if a
vacuum is applied. Vacuum distillation is a common technique which allows for the distillation of high
boiling compounds under mild conditions. Both the simple and fractional distillation apparatus can be
performed under vacuum by attaching the vacuum line to the fume hood vented adapter near the
collection flask.
The Hickman distillation apparatus is a micro-scale purification apparatus that is used to distill small
amounts of materials (Figure 3). This is similar to a simple distillation and only one fraction is usually
collected. The purity is again related to the difference in boiling points, and fractionating columns can be
employed to increase separation and purity. We will be using this apparatus today to separate a mixture
of o-xylene and cylcohexane.
Experiment #2: Distillation CHEM 213 – Fall 2009
Distillation is an inexpensive and relatively simple technique used to purify liquids. Chemists often use this method
to separate homogeneous solutions of two or more liquids. In industry, distillation is used to separate the
economically important components of fossil fuels including natural gas, gasoline, kerosene, heating oil, and
lubricants. In the food industry, distillation is used to concentrate the alcohol in wines and other beverages obtained
from the natural fermentation of fruits and vegetables. Both of these economically important processes separate
liquids, which do not interact with one another, by differences in their boiling points. In practice, liquids can be
separated by simple or fractional distillation as discussed in Mohrig.
In this experiment, you will be using two distillation methods to separate an alcohol from an organic solvent. Using
either the blue or white glassware kits, you should separate the 60-mL unknown sample with both simple (figure
11.6) and fractional macroscale distillation (figure 11.15.) You and a partner will be randomly assigned one
unknown alcohol/solvent mixture to purify by both methods. On the following two pages the apparatus for each
distillation is represented. After comparing the quality of the two methods by constructing a distillation curve and
analyzing the purified liquids by gas chromatography (GC), you will identify the pure alcohol from boiling point
data, refractive index (RI) and infrared spectra (IR).
Objectives:
1. Separate a liquid alcohol/solvent mixture using both macroscale simple and fractional distillation.
2. Determine the boiling point of the purified alcohol by constructing a distillation curve (figure 11.14).
3. Determine the success of the separations (purity) and compare the methods by gas chromatography (GC).
4. Identify the pure alcohol using the boiling point, refractive index (RI) and infrared spectroscopy (IR).
Pre-lab* Questions:
1. In this lab you will separate a mixture of organic liquids by two methods; simple and fractional distillation.
In theory, which method should give the better separation? Briefly explain why.
2. Suppose you measured a refractive index of your purified unknown liquid to be 1.3973 at 17.6 °C. Using
the attached table, what would you concluded your unknown to be?
3. After the distillation was completed, you analyzed your alcohol product by infrared spectroscopy (IR).
You observe a strong peak at 1205 cm-1. Given on the three choices below, which of the following
compounds would be your unknown?
OH
OH
OH
* Distillation is a purification technique; therefore, a balanced equation and data table are not appropriate.
Post-Lab Questions:
1. What were the amounts of the two components in your original mixture? What is the % recovery of the
alcohol?
2. Based on your results, which method of distillation provided you with a better separation? Briefly explain
how you came to this conclusion from your data.
3. How can you determine the relative amounts of components in a mixture by looking at a gas
chromatogram?
4. Provide two reasons why the boiling points of the two fractions you separated from your fractional
distillation may be lower than the literature values.