Distillation

Distillation Definition

First, let's define distillation. Distillation separates liquid mixtures based on the differences in the volatility of components.

Let's look at a simple distillation setup in the laboratory. For now, don't worry about what's going on, and just familiarize yourself with how they look! For the most part, they look like this:

They usually have a heating device, a flask containing the original mixture, a condenser to cool and condense the vapors, and another container to collect the condensate (or distillate)

Types of Distillation

For your AP chemistry exam, you will need to know two types of distillation: simple and fractional distillation.

Simple Distillation

Simple distillation is used to separate volatile liquids or separate liquids from nonvolatile solids or oils. This technique can separate liquids that have different boiling points. The boiling point is the temperature at which the vapor pressure of a liquid equals atmospheric pressure. Simple distillation is most effective when volatile components of a mixture have boiling points that differ by at least 40-50 °C.

This technique involves a single vaporization/condensation cycle. But, how does simple distillation work? Let's look at an example to explain this better.

Suppose we have a mixture of two different liquid components. Liquid, A, has a boiling point of 50 °C, whereas liquid, B, has a boiling point of 90 °C. When the mixture is heated, liquid, A (the more volatile liquid), will vaporize faster because it takes liquid, A, less energy to break its intermolecular forces. At this temperature, the vapor will consist of a large percentage of the compound, A, leaving the rest of the liquid enriched in the compound, B.

The liquid-vapor, A, will pass through the condenser, which will turn it back into a liquid. Then, liquid, A, will be collected in another flask! After we collect liquid, A, we can keep heating the mixture (which is now composed of a large percentage of liquid, B), until it reaches its boiling point, and we then can collect it in another flask the same way we did with liquid, A!

The problem with single distillation is that sometimes it does not separate, A, and, B, completely. So, multiple simple distillations might be needed. Luckily, fractional distillation allows us to repeat the distillation process multiple times at once!

Fractional Distillation

Fractional distillation is used to separate liquid mixtures where the compounds have similar boiling points or a boiling point difference of less than 40-50 °C. The laboratory setup for fractional distillation looks similar to the one for simple distillation, but it contains additional equipment: a fractionating column.

This column is packed with either glass beads or steel wool, and this packing material provides a larger surface area to create a temperature differential within the column (hotter at the bottom and cooler at the top). So, by having a larger surface area, multiple "mini-distillations" can occur! In other words, the vapor condenses on multiple surfaces in the fractionating column, and the resulting liquid vaporizes again.

Distillation in chemistry

Let's go back to our example involving a mixture containing compounds, A, and, B. When dealing with distillation, we would expect that at the lower boiling point of compound, A, only compound A would turn into vapor and come out first and be collected. Then, as the temperature rises to the boiling point of, B, only compound B would turn into vapor and be collected separately!

However, in reality, this usually does not happen. Why? Because some of the higher-boiling compound (in this case, compound, B) typically begins to vaporize below their boiling point temperature. But, if you heat it slow enough, you might approach a more ideal result. Just don't do it too slow as you might end up evaporating your liquids!

Distillation depends on the strength of intermolecular interactions between and among the components, and also on how these forces affect the vapor pressures of the components in the mixture. Stronger intermolecular forces result in higher boiling points. The relative strength of intermolecular forces are:

Ion-dipole > Hydrogen-bonding > Dipole-dipole > London dispersion forces

Distillation Process

Let's look at the fractional distillation of a 10 mL 1:1 mixture containing ethyl acetate (C₄H₈0₂) and butyl acetate (C₆H₁₂O₂). The molecular weight of butyl acetate is 116.16 g/mol, while the molecular weight of ethyl acetate is 88.11 g/mol. Note: The boiling point of ethyl acetate is 77.1 °C. The boiling point of butyl acetate is 120.6 °C.

Fig. 5: Structures of Butyl and Ethyl Acetate - Isadora Santos, StudySmarter Original.

1. In a 25mL round bottom flask, add 10 ml of the 1:1 ethyl acetate and butyl acetate mixture. Add a boiling chip inside the flask to smooth the boiling.

2. Separate a clean, 10 mL graduate cylinder to collect the distillate.

3. Start heating up the mixture and adjust the rate of distillation to about one drop per second.

4. When you see the first drop of the distillate, record its temperature. Keep recording the temperature from every 0.5 mL of distillate collected in the graduate cylinder.

5. Collect three fractions of a 1mL volume at the following temperatures: 76-80 °C, 88-92 °C, and >110 °C. These fractions will be used for Gas Chromatography analysis.

Distillation Examples in Chemistry

Distillation is widely used in industry. For example, distillation can be used to purify water, especially in those areas where freshwater is inaccessible. So, they use distillation to purify seawater and make it safe for drinking. Water distillation is also used to remove minerals and other impurities.

Another application of distillation is the recycling of oils. By using distillation, oil can be purified by removing water and other contaminants.