OC Lab: Distillation and Gas Chromatography

If you used a more polar column, how would it effect retention time?

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What is the mobile phase used in gas chromatography? What is the stationary phase? Give an example of a stationary phase.

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What is adsorption?

Adsorption is the adhesion of atoms, ions, or molecules from a gas, liquid, or dissolved solid to a surface.

How would an increase in the column temperature effect retention times?

An increase in temperature would speed up elution rates and worsen separation. The retention rate would slow down if there was an increase in the column temperature. (Double Check).

Why should one never heat a system that is totally closed from the outside atmosphere?

Because an explosion is very likely to occur as pressure builds up. Never heat a system to dryness because of possible explosion.

What is chromatography?

Chromatography is the separation of components of a mixture by differential adsorption between a stationary phase and a mobile phase. The word "chromatography" means to "write" in "color".

There are many types of chromatographic methods used by organic chemists. When is gas-liquid chromatography used?

Chromatography is used for separations of volatile or reasonably volatile organic liquids and solids.

What is elution?

Elution is a term used in analytical and organic chemistry to describe the process of extracting one material from another by washing with a solvent

How can you get the partial pressure of a liquid in a mixture?

For mixtures of liquids that obey Raoult's Law, the partial pressure of each the components of a solution is equal to the product of the vapor pressure of the pure component at a particular temperature and the mol fraction (X) of the component. Mol fraction is equal to the number of moles a component divided by the total number of moles of all components.

Why do chemists use fractional distillations instead of simple distillations?

Fractional distillation improves separation of components of a mixture beyond that which is possible in a simple distillation. The key to an efficient fractional distillation is the number of vaporization-condensation cycles provided by the apparatus. Each cycle is equivalent to a separate simple distillation. A simple distillation gives a single vaporization-condensation cycle.

Why is it preferable to allow cold water to enter at the bottom of a condenser and exit at the the top rather than vice versa?

If water enters from the bottom of the condenser, it will always be completely filled with cold water which ensures efficient cooling. During distillation, vapors are formed in the heated distillation flask. The condenser cools these vapors condensing them back to liquid droplets that flow down the condenser into the receiver flask. A completely filled condenser provides maximum cooling therefore allowing for maximum recovery of the purified liquid during distillation. 2nd Answer: It is quite simple. If the water enters from the bottom and exits at the top, the jacket is full of water during the process so the heat of condensation can be transferred to the water. If the water entered the top and left at the bottom, the jacket would never be full of water so the distillation process would be inefficient at best.

What is distillation?

It is a process used to purify liquids.

Distillation of a pure liquid occurs at the boiling point of a liquid. What is the boiling point defined as?

It is defined as the temperature at which vapor pressure above the solution is equal to the atmospheric pressure. Normal boiling point is defined as the boiling point of a pure liquid when the atmospheric pressure is one atmosphere (760 torr).

What is a simple distillation?

It is the condensation of vapors from a boiling liquid and collection of the condensed vapors in a receiving vessel (Hickman still).

What is the relationship between volatility and the vapor pressure of a solvent?

More volatile liquids, such as diethyl ether, exhibit higher vapor pressures at all temperatures than less volatile liquids.

Why is simple distillation not the best type of distillation?

Simple distillation is generally unsatisfactory unless the components have vastly different boiling points (about 100 C difference). A mixture of cyclohexane and toluene cannot be separated into pure cyclohexane and pure toluene by simple distillation because the boiling points of these compounds are too close together. Better separation can be achieved by fractional distillation.

How would an increase in the flow rate of the carrier gas effect retention times?

Slower flow rates of the carrier gas decrease elution rates of components, which can improve separations. An incr

How are the components separated in gas-liquid chromatography (GC)?

The components are partitioned between a liquid coating on the column (the stationary phase) and an inert gas (the mobile phase). A small volume of a mixture of volatile substances (usually dissolved in a solvent) is injected by syringe onto a heated column through which an inert carrier gas is flowing. The heat applied, as well as the gas flow, help the molecules from the sample travel through the column. Smaller, more volatile molecules generally emerge first from the opposite end of the column through an exit port and are detected. The detector is connected to a recording device, which shows a deflection when a sample passes the detector in proportion to the amount of sample detected.

What is partial pressure?

The contributions of each component to the total pressure is called partial pressure. Dalton's Law of Partial Pressure states that the partial pressures are additive. (1.1)

How does a typical liquid molecule move in the distilling flask?

The molecule circulates in the solution until it passes into the vapor state due to heating. Once in the vapor state, the molecule returns to the solution in the flask and travels up through the still head and over through the condenser to the receiving vessel. Distillation of ocean water to furnish pure water is an example of a commercial application of simple distillation.

What is retention time?

The retention time of a solute is taken as the elapsed time between the time of injection of a solute and the time of elution of the peak maximum of that solute.

What is a Hickman still?

This is a glass tube that has a collar to catch condensed distillate as it flows from the top of the still downward toward the distilling flask. Some Hickman stills have a small exit port near the collar that is opened to allow convenient removal of distillate at appropriate intervals.

What is the purpose of adding boiling chips or a spin vane?

To reduce bumping (?). Never add a boiling chip to a solution that is already boiling. Doing so will cause the liquid to bump violently.

A pure liquid, such as acetic acid, exhibits a vapor pressure that is temperature dependent. At ambient temperature and pressure, the aroma of acetic acid can be detected due to molecules in the vapor state above the surface of the liquid. What happens to the vapor pressure of a liquid when it is being heated?

Upon heating, the vapor pressure increases slowly and then more rapidly near the boiling point.

What is a volatile liquid?

Volatile liquids are liquids which have a very high tendency to convert to the gas phase. This is dependent on the vapor pressure of the liquid. The pressure at which different substances can stay at equilibrium with their liquid phase and their gas phase is different. If for the same temperature, a liquid has a higher vapor pressure than another, it is considered to be more volatile. If the vapor pressure for a liquid is more than the surrounding atmospheric pressure for a particular temperature it has a tendency to convert to gas phase at that temperature. Volatile liquids have a vapor pressure higher than the normal atmospheric pressure at normal room temperature.

What is a volatile liquid? Definition 2

Volatile liquids are liquids with low boiling points and high vapor pressure, in other words, they tend to evaporate more easily at room temperature. Substances have different pressures in which they can maintain equilibrium between their liquid and gas phases. The higher a liquid's vapor pressure, the more volatile it is. Because of their low boiling points, volatile liquids convert to the gas phase at a lower temperature and the gas molecules can diffuse faster than the molecules of a non-volatile liquid. This is the reason why perfumes use volatile liquids so that their odor can disperse more quickly into the air. Some other examples of volatile liquids include acetone, ammonia, dichloromethane, and diethylether.