PURIFICATION IS ESSENTIAL - THE MODERN CHEMISTRY LAB - The Handy Chemistry Answer Book (2014)

The Handy Chemistry Answer Book (2014)

THE MODERN CHEMISTRY LAB

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PURIFICATION IS ESSENTIAL

How do chemists purify compounds?

A few of the most commonly used purification methods are chromatography (see following question), recrystallization, or extraction methods. These methods typically exploit a difference in how a property (like polarity, for example) of one chemical species causes it to interact differently with the surrounding material than the others from which it is being separated.

What is chromatography?

Chromatography is a method for separating chemical compounds based on chemical properties as they travel over a distance. The sample to be separated may be in a solution or in the gas phase.

How is gas chromatography different than liquid chromatography?

Gas chromatography involves first vaporizing the sample, while liquid chromatography typically involves making a solution or suspension of the mixture to be separated. In gas chromatography all of the molecules in the vaporized sample will have the same average kinetic energy, but the different chemical species will each have different average velocities which are determined by their molecular weight. Heavier molecules will move with slower average velocities than lighter ones. This is how separation of different chemical components is achieved in a gas sample. In liquid samples, the mixture is dissolved in a solution that flows over a stationary phase, which interacts differentially with different chemical species in the sample. These different interactions with the stationary phase cause some compounds to move faster than others through the chromatography column, and this is the basis for separation via liquid chromatography.

What is the “mobile phase” in chromatography?

The mobile phase is the vaporized sample in gas chromatography or the solvent used to elute the sample over the stationary phase in liquid chromatography.

How is liquid-liquid extraction used in the chemistry lab?

Liquid-liquid extraction is a technique that exploits the differential solubilities of compounds between two liquid phases to extract the compound we want into a single phase. The liquids being used must be immiscible, meaning that they form two separate layers when placed in the same container. The goal is to have our compound of interest dissolved exclusively in a single liquid phase and also to have it be the only chemical compound dissolved in that phase. If this can be achieved, the liquid layer containing the desired compound is then separated and the solvent is removed, yielding the pure substance.

How does crystallization purify compounds?

Crystallization as a purification technique relies on the fact that it is usually much easier to form a crystal from a single chemical species than it is to form a crystal from a mixture of chemical compounds. Recrystallization is the process of dissolving a crude, impure product in a hot solvent (or mixture of solvents) and allowing it to crystallize out of the solution form as the solvent cools. Once a small crystal, even one so small we can’t see it, begins to form, it is relatively easy for other molecules of the same compound to add to the crystal. Other compounds will not be able to readily add to this crystal, which results in the formation of a crystal containing a single, pure chemical compound.

What is polymorphism?

Polymorphism is the capacity for a compound to exist in different crystalline arrangements. This can arise due to different packing of the individual units of the crystal or due to different conformations of the molecules making up the crystal.

Why do polymorphs matter?

Different crystalline polymorphs will have different material properties and physical properties. In medicine, for example, the body may absorb some polymorphs of a drug more readily, making them more potent or more effective. Polymorphism can also change the fundamental physical properties of a material, affecting things like its conductivity and thermal stability.

How are compounds separated by distillation?

Distillation is a purification technique that involves using the different boiling points of compounds to effect their separation. As a solution containing a mixture of chemicals is heated the compound with the lowest boiling point will evaporate first, and this can be collected on a cool surface after it has diffused away from the solution. In this way, a pure liquid component can be isolated from a mixture of liquids.

Does every compound have a unique boiling point?

No, and this is one limitation of distillation as a separation technique. If two chemicals coincidentally have very similar boiling points, then it will be difficult to separate them using distillation.

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A simple distillation device (or “still”) takes advantage of the fact that compounds usually have different boiling points and can be separated by applying heat to a solution. In this diagram, the parts include: 1) a heat source, 2) still pot, 3) still head, 4) thermometer/ boiling point temperature, 5) condenser, 6) cooling water in, 7) cooling water out, 8) distillate/receiving flask, 9) vacuum/gas inlet, 10) still receiver, 11) heat control, 12) stirrer speed control, 13) stirrer/heat plate, 14) heating (oil/sand) bath, 15) stirrer bar/anti-bumping granules, and 16) cooling bath.

Why would a chemist measure the melting point of a chemical sample?

The melting point of a compound can provide information on whether it is pure and about whether the correct compound has been made. This, of course, assumes that the melting point of the desired compound is already known. If a compound is being made for the first time, knowledge of the melting point can be useful to the next chemist who tries to synthesize it.

What affects melting and boiling points of chemical compounds?

Intermolecular (between molecules) forces govern the melting point of a chemical substance. These include Van der Waal’s interactions, dipole-dipole interactions, hydrogen bonding, and, in the case of ionic compounds or ionic solutions, ionic bonds or Coulombic interactions. The stronger the intermolecular forces between the molecules in a solid, the harder they will be to melt, so stronger intermolecular forces lead to higher melting points. The same is true with boiling points: stronger intermolecular forces make the molecules harder to separate, leading to higher boiling points.

For solids, the shape of a molecule can also affect its ability to pack into an ordered lattice. Having a shape that allows a well-ordered lattice to form will tend to stabilize the solid phase of a compound, leading to a higher melting point. The shape of a molecule can also affect the boiling point of a compound. In liquids that are able to form hydrogen bonds, the location of the hydrogen bond donor or acceptor can affect its spatial availability to serve as a donor or acceptor. In organic liquids, where Van der Waal’s interactions are important, molecules with larger surface areas will have stronger Van der Waal’s interactions, leading to a higher boiling point.