Unit 3: Topic 9 - Separation of Solutions and Mixtures Chromatography Flashcards
What are the different types of intermolecular forces that arise between components of a solution?
There are various intermolecular forces that can arise between components of a solution.
1) London dispersion
2) dipole-dipole
3) ion-dipole
4) dipole-induced dipole
5) ion-induced dipole
6) hydrogen bonding
Let’s say we have acetic acid (CH3COOH) in water. The intermolecular forces in this solution would be :
-London dispersion
-Hydrogen bonding (H-bonding is technically the strongest form of dipole-dipole force)
-Ion-dipole (less important interaction, but because acetic acid is an acid, we will have CH3COO- in solution, which is an ion )
How does chromatography separate different components of a solution?
Chromatography involves a stationary phase and a mobile phase. The mobile phase is the solvent that carries the solute, and the stationary phase is the material that the solute and mobile phase moves through.
When the mobile phase carries the solute in the solution through the stationary phase, the speed of solute movement depends on its polarity. Let’s say that our solution contains two solutes: solute A, which is very polar, and solute B, which is less polar. According to the principle of like attracts like, polar substances are attracted to other polar substances, so solute A will be more attracted to the polar stationary phase than solute B. If solute A is more attracted to the stationary phase, then it’s harder for the mobile phase to pull solute A along with it. The result is that solute A will move a smaller distance compared to solute B.
Solute A and B will be separated because of the difference in movement distance.
Another factor to consider is the polarity of the mobile phase. If both solute A and solute B are very polar, then the mobile phase should be quite polar as well. If both solute A and solute B are more nonpolar, then the mobile phase should be more nonpolar.
The more similar the polarity of the solute and mobile phase, the greater the movement distance.
What are the three main types of chromatography, and how do they differ from
each other?
There are three different types of chromatography: paper, thin-layer, and column. Their main differences are in their stationary phases.
Paper chromatography uses cellulose as the stationary phase. Thin-layer uses silica or alumina gel on a glass plate as the stationary phase. Column uses silica or alumina as well but is packed into a vertical tube. You would use column over thin-layer when you want to separate a much larger quantity of solution)
Cellulose, silica, and alumina are all polar stationary phases.
What is the Rf factor, and how is it relevant to chromatography?
The Rf value (retardation factor) is equal to (how far the solute moved) divided by (how far the mobile phase moved). The Rf value will always be between 0 and 1. A higher Rf value means that the mobile phase and solute are more similar in polarity. It also means that the solute and stationary phases are LESS similar in polarity.
How does distillation make use of the differential intermolecular attraction(s) between components of a solution to separate them?
The different strengths of intermolecular attractions between components of a solution give rise to different boiling points for each component of a solution.
The stronger the intermolecular attractions, the higher the boiling point, and vice versa.
Consider hydrogen peroxide (H202) and water (H20). H202 has a boiling point of 150 degrees Celsius, while water’s is about 100, mainly due to a greater number of hydrogen bonds.
If you boil a solution of 3% hydrogen peroxide (meaning it has 97 % water) at around 100 degrees Celsius, then the water would boil off
while the H202 remains because the temperature is under its boiling point. The result would be a solution with a higher concentration of H202.
What is an example of fractional distillation?
A real-world example of “fractional distillation” is the separation of crude oil into different distillates.
Crude oil is composed of hydrocarbon chains (molecules that only have hydrogens and carbons) of varying lengths. We know that the larger a molecule is, the stronger the London dispersion forces between them. Therefore, it makes sense that fuel oil, which has 50-70 carbons in its chain, boils at a higher temperature compared to gasoline/petrol, which only has 5-10 carbons in its chain.
In fractional distillation, the crude oil is heated up, which turns most of the sludge into a gas. As the gas rises, the temperature decreases, and molecules with higher boiling points will condense first. The lower the boiling point of the molecule (smaller molecules like C4H10), the higher it will rise before they condense into a liquid.
In this way, different components of crude oil are separated.
