Separation and Purification Flashcards
Separation and purification techniques are constant favorites of the MCAT test-makers. From extraction to distillation to chromatography, use these cards to master these concepts as they are tested on the exam.
What does separation of a chemical mixture accomplish?
It separates a mixture into at least two distinct products, enriching one in a specific chemical component while depleting it in others.
In some cases, but not always, a separation may fully divide the mixture into its pure constituents. Separations are classified based on the technique and apparatus as well as the specific chemistry of the mixture.
What criteria must be satisfied for a successful separation process to be possible?
It require differences in chemical properties such as size, shape, mass, intermolecular forces, or chemical affinity between the constituents of a mixture.
If no single difference can be used to accomplish a desired separation, multiple processes can be performed in combination.
Define:
extraction
It refers to the process of moving a solute from one phase to another for easier separation.
There are many forms of extraction, but the two most commonly tested on the MCAT are solid-liquid and liquid-liquid (or aqueous-organic).
Describe the general process for aqueous-organic extraction.
It is a special type of liquid-liquid extraction. This is used to separate dissolved compounds based on relative solubility.
A dissolved solution is exposed to an immiscible solvent, resulting in an aqueous layer and an organic layer. One constituent dissolves in the new layer, leaving the remainder in the original solution. Since the two layers are immiscible, they can then be separated, and the dissolved constituent is separately reconstituted through a work-up step.
Describe the general process for solid-liquid extraction.
It is used to separate solid mixtures based on relative solubility in a liquid solvent.
A solid mixture is exposed to a solvent which dissolves at least one constituent and leaves the remainder in the solid phase. The entire mixture can then be filtered, separating out the solid; the dissolved mixture can then be separately reconstituted.
What procedure could be used to separate a mixture of white sand and table salt?
Solid-liquid extraction could be used simply by adding water to the mixture.
The table salt will dissolve, or move into the aqueous phase, while the sand will not. As a solid, the sand can easily be filtered out. The salt can then be recovered through evaporation.
What characterizes organic compounds that are soluble in water?
Any compound with similar chemical properties to water will dissolve in it readily (“like dissolves like”). Generally, polar compounds fulfill this requirement.
Specifically, molecules that can hydrogen bond (including ethanol, ammonia, and hydrogen fluoride) are especially water-soluble. Additionally, charged ions (such as Na+ and Cl-), and molecules with a large dipole moment (such as acetone and dimethylsulfoxide) dissolve in water.
What characterizes organic compounds that are soluble in nonpolar solvents?
Nonpolar compounds tend to dissolve well in nonpolar solvents. These compounds, which do not dissolve in water, are considered hydrophobic or lipophilic.
Specifically, hydrocarbons (such as cyclohexane and benzene), ethers, and very large aldehydes and ketones fit into this category.
What is the role of a separatory funnel in a chemical extraction?
It helps precisely separate the aqueous and organic layers.
It is shaped like an inverted water droplet, with a valve called a stopcock blocking the bottom of the funnel.
When two immiscible liquids are present in the funnel, they settle into two distinct layers. The denser layer, which is located on the bottom, can be poured off by opening the stopcock. The narrow shape of the funnel allows for maximal separation of the layers.
Describe an aqueous-organic extraction procedure that could separate ethanol and benzene.
- Add both water and a suitable organic solvent, such as carbon tetrachloride, to a separatory funnel. The two solvents will separate into immiscible layers.
- Add the mixture of ethanol and benzene.
- The ethanol will dissolve in the water, while the benzene will move to the organic layer. These layers can then be physically separated.
- The ethanol and benzene can be recovered later in a work-up step.
When water is mixed with hexane in a separatory funnel, what constitutes the organic layer?
The organic layer will contain hexane. The other layer, which contains water, is known as the aqueous layer.
On the MCAT, extractions generally require both a nonpolar solvent and a polar one. These will form immiscible layers, with the more dense layer at the bottom of the funnel.
How can an acidic compound be extracted from an organic mixture?
The acidic species can be deprotonated by the addition of a base. This causes the molecule to become an anion, moving it to the aqueous layer.
Remember, charged molecules are especially water-soluble.
This is a type of aqueous-organic extraction.
Benzene and phenylamine (C6H5NH2) are dissolved in diethyl ether. For these constituents to be separated by extraction, what type of compound must be added?
Aqueous acid. This would protonate the basic -NH2 group on phenylamine, moving that molecule to the aqueous layer.
Specifically, protonation results in C6H5NH3+, an ionic species. Such charged molecules will preferentially dissolve in water over ether, a comparatively nonpolar solvent. Benzene will not be protonated, so it will remain in the organic layer.
How can a basic compound be extracted from an organic mixture?
The basic species can be protonated by the addition of an acid. This causes the molecule to become a cation, moving it to the aqueous layer.
Remember, charged molecules are especially water-soluble.
This is a type of aqueous-organic extraction.
Benzene and benzoic acid (C6H5COOH) are dissolved in hexane. For these constituents to be separated by extraction, what type of compound must be added?
Aqueous base should be added to the mixture. This would deprotonate the -COOH group on benzoic acid, moving that molecule to the aqueous layer.
Specifically, deprotonation results in C6H5COO-, an ionic species. Such charged molecules will preferentially dissolve in water over hexane, a comparatively nonpolar solvent. Benzene will not be deprotonated, so it will remain in the organic layer.
What properties of a compound determine its boiling point?
- The size and branching of the molecule
- The strength of the intermolecular attractive forces between the molecules
Typically, size trumps intermolecular attractive forces; very large nonpolar compounds have higher boiling points than small polar ones.
Which has a higher boiling point, water or ethanol?
Water
Hydrogen bonding, a relatively strong type of intermolecular interaction, tends to increase boiling point. Both of these compounds can hydrogen bond, but water contains two hydroxyl groups compared to the single -OH in ethanol. Water is thus capable of twice as much hydrogen bonding, giving it a higher boiling point.
Which has a higher boiling point, water or acetone?
Water
The -OH groups in water are capable of hydrogen bonding, while the C=O group in acetone only results in dipole-dipole interactions between the molecules. Hydrogen bonding is a stronger type of intermolecular interaction than dipole-dipole forces, and the stronger the attraction between molecules, the higher the boiling point.
Which has a higher boiling point, hexane or propane?
Hexane
Higher molecular weight results in a higher boiling point. As a 6-carbon compound, hexane is heavier than propane, a 3-carbon compound.
Which has a higher boiling point, n-butanol or tert-butanol?
n-butanol
As isomers, these two molecules are identical in their molecular weights and ability to hydrogen bond. They only differ in their amount of branching; tert-butanol is a highly branched compound, while n-butanol has a straight-chain structure. More branching reduces the surface area available for intermolecular interactions, lowering the boiling point.
What physical property is most closely related to distillation?
Boiling point relates to distillation, which is a separation technique used for mixtures of liquids.
The mixture is heated until the more volatile component boils away.
The boiled vapor is condensed back to its liquid phase in a separate container, resulting in two product solutions that are relatively more pure than before.
What conditions must be satisfied for two liquids to be separated by simple distillation?
The liquids must differ significantly in their boiling points. Though the MCAT will not test this exact number, simple distillation requires a boiling point difference of at least 25°C.
If the boiling points are too close, boiling will produce vapor with a significant amount of both compounds, which is not an effective separation.
What separation process should be used if the boiling points of two miscible liquids differ by less than 25°C?
The two liquids can be separated using fractional distillation. This technique is used instead of simple distillation when the components’ boiling points are fairly close.
In fractional distillation, a separation column is added to the distillation apparatus. The column effectively lengthens the distance over which the vapor travels, allowing only the more volatile component to separate off as gas while the less volatile component recondenses.
Can distillation be used to separate two liquids if their boiling points differ by 15°C?
Yes. The two liquids can be separated if fractional distillation is used.
Only simple distillation requires the liquids to have a relatively large boiling point difference. Fractional distillation is used to separate compounds that are more similar.