You have probably heard of crude oil before. Crude oil is a complex mixture of hydrocarbons that gets processed into fuels like diesel, petrol, and kerosene! Crude oil was actually made over millions of years from the ancient remains of marine organisms that were covered by mud and heated under pressure without air. This caused the remains to turn into oil, and the mud to change to sedimentary rock!
Crude oil is separated into its many compounds according to boiling points, and this is through a process called distillation.
Feeling curious about how distillation works? Keep reading to find out!
- This article is about distillation.
- We will talk about what distillation means.
- We will dive into the process of distillation and talk about two types of distillation: simple and fractional distillation.
- Lastly, we will look at some examples of distillation.
Distillation Definition
First, let's define distillation. Distillation separates liquid mixtures based on the differences in the volatility of components.
Distillation is a process where a liquid mixture is separated into its components by continuous vaporization and then condensation.
Volatility is the tendency of components of a mixture (usually a liquid mixture) to evaporate at different temperatures.
Vaporization is the process of a substance changing from the liquid or solid phase into the gas phase.
Condensation is the process of a substance changing from the gas phase to the liquid or solid phase. A condensate is the product of the condensation process.
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!
A mixture that cannot be completely separated by simple distillation is known as an azeotropic mixture.
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
If you need a refresher on the different types of intermolecular interactions, check out "Intermolecular 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, Vaia Original.
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.
Separate a clean, 10 mL graduate cylinder to collect the distillate.
Start heating up the mixture and adjust the rate of distillation to about one drop per second.
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.
- 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.
To learn more about GC, check out "Gas Chromatography"!
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.
Distillation - Key takeaways
- Distillation is a process where a liquid mixture is separated into its components by continuous vaporization and then condensation.
- Simple distillation is used to separate volatile liquids or volatile liquids from nonvolatile solids or oils. Simple distillation is most effective when their boiling points differ by at least 40-50 °C.
- 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.
- 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.
References
- Organic Chemistry 211 Laboratory Experiment 5b: Fractional Distillation. (n.d.). Retrieved June 10, 2022, from https://www.cerritos.edu/chemistry/chem_211/Documents/Lab/5b_fractional%20distillation.pdf
- Experiment 3: Separation of Liquid Mixtures by Simple and Fractional Distillation and Gas Chromatography. (n.d.). Http://Sayhellotochemistry.weebly.com/Uploads/5/4/8/4/54842741/Simple_and_fractional_distillation.pdf; Chem-105, Glendale Community College.
- N Saunders, Kat Day, Iain Brand, Claybourne, A., Scott, G., & Smithsonian Books (Publisher. (2020). Supersimple chemistry : the ultimate bite-size study guide. Dk Publishing.
- Malone, L. J. (2009). Basic Concepts of Chemistry, Student Study Guide, 8th Edition. John Wiley & Sons.
- Theodore Lawrence Brown, Eugene, H., Bursten, B. E., Murphy, C. J., Woodward, P. M., Stoltzfus, M. W., & Lufaso, M. W. (2018). Chemistry : the central science (14th ed.). Pearson.
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