Question

How would you separate into its different components (a) a mixture of the volatile gases propane, butane, and isopropane? (b) a solution of rubbing alcohol made up of isopropyl alcohol and water?

Short answer

Answer: (a) To separate a mixture of propane, butane, and isopropane, follow these steps: 1. Cool the mixture to condense the gases into liquids, 2. Perform fractional distillation, and 3. Collect the separated components. (b) To separate a solution of rubbing alcohol containing isopropyl alcohol and water, follow these steps: 1. Prepare the mixture, 2. Carry out liquid-liquid extraction using a separatory funnel, 3. Separate the layers, 4. Collect the separated components, and 5. Remove residual water from the isopropyl alcohol using a drying agent.

Step-by-step solution

Cooling the mixture to condense the gases into liquids

To separate the volatile gases, first cool the mixture below their boiling points to condense them into liquids. This can be done by allowing the gases to flow through a cooling system.

Fractional distillation

Perform fractional distillation on the liquefied mixture. In this process, the mixture is heated in a fractionating column, which has a temperature gradient. Each component will vaporize at different temperatures based on its boiling point. Since propane, butane, and isopropane have boiling points of -42°C, -0.5°C, and -11.7°C, respectively, they can be separated effectively.

Collecting the separated components

As each component reaches its boiling point, it will vaporize and rise through the fractionating column. The vapors are then condensed and collected separately. Propane will be the first to vaporize, followed by isopropane, and finally butane. ##Separating a solution of isopropyl alcohol and water##

Preparing the mixture

First, obtain the solution of rubbing alcohol containing isopropyl alcohol and water. Make sure the sample size is appropriate for the separation technique to be used.

Liquid-liquid extraction

Carry out liquid-liquid extraction using a separatory funnel. Add the alcohol-water mixture to the funnel, along with an organic solvent (e.g., diethyl ether) that is immiscible with water but can dissolve isopropyl alcohol. Gently mix the contents to allow the solvent to dissolve the isopropyl alcohol.

Separating the layers

Let the mixture sit in the separatory funnel until two distinct layers form. The organic solvent layer containing the isopropyl alcohol will generally be on top (depending on the solvent used), while the water layer will be on the bottom.

Collecting the separated components

Open the stopcock of the separatory funnel and carefully drain the water layer out of the funnel. Collect this separately. Next, drain the organic solvent layer containing the isopropyl alcohol into another container.

Removing residual water from the isopropyl alcohol

Add a drying agent (e.g., anhydrous sodium sulfate) to the organic solvent layer containing isopropyl alcohol to remove any traces of residual water. After adequately mixing, filter the resulting solution to isolate the pure isopropyl alcohol.

Key concepts

Fractional Distillation

Fractional distillation is a powerful technique used to separate a mixture of liquids, based on their different boiling points. Imagine you have a mixture, like one containing propane, butane, and isopropane, which first needs to be cooled to change into liquids. Then comes the magic of fractional distillation! The process involves heating the mixture in a special column known as a fractionating column. This column has a temperature gradient, which means it's hotter at the bottom and cooler at the top.

Here's what happens in this setup:

• As temperature increases, the component with the lowest boiling point—propane—turns into vapor first and rises up the column.
• Next, isopropane, with a slightly higher boiling point, evaporates and climbs up.
• Finally, butane, having the highest boiling point among them, vaporizes last.

This step-by-step separation results in each component being collected at different stages, thanks to their distinctive boiling points. Fractional distillation is excellent for separating substances that have close boiling points.

Liquid-Liquid Extraction

Liquid-liquid extraction is a technique primarily used to separate components based on their solubility in different solvents. Let's say you have a mixture with isopropyl alcohol and water; this method becomes a smart choice. You take a separatory funnel for this operation. The idea is simple: use an organic solvent that water doesn't mix with, but that can easily dissolve isopropyl alcohol.

Here’s how it works:

• First, add the alcohol-water mixture into the separatory funnel.
• Next, introduce an organic solvent, like diethyl ether, into the funnel.
• Gently shake the funnel to promote mixing, and wait for the layers to settle.

Once you have two layers formed, the isopropyl alcohol will be in the organic layer, and water in the other (thanks to their different solubilities). By carefully draining each layer, you can effectively separate the components.

Volatility of Gases

Understanding the volatility of gases is crucial in many separation and purification processes. Volatility refers to how easily a substance will vaporize. For example, different gases have different levels of volatility due to factors like molecular weight and intermolecular forces. In a mixture containing gases like propane, butane, and isopropane, their volatility plays a central role in how they are separated.

In essence:

• High volatility means a gas will vaporize at a lower temperature.
• Propane vaporizes at a lower temperature compared to isopropane and butane, due to its lower boiling point.

This variance in volatility allows methods like fractional distillation to step in and separate these components, letting each vaporize at its own pace when the mixture is gradually heated.

Boiling Points

The concept of boiling points is fundamental when discussing separation techniques like distillation. The boiling point is the temperature at which a liquid turns into vapor. When we use methods like fractional distillation to separate a mixture, we rely heavily on boiling points.

• A component with a lower boiling point will vaporize first when heat is applied.
• In mixtures like propane, butane, and isopropane, each substance has its unique boiling point (e.g., propane at -42°C).

By knowing each component's boiling point, we can predict and control when each will evaporate during the distillation process, which is key to successful separation.

Miscibility and Solubility

Miscibility and solubility are essential concepts when separating mixtures, especially with liquid-liquid extraction. Miscibility refers to the ability of two liquids to mix and form a homogeneous solution. Solubility, on the other hand, measures how much of a substance can dissolve in a given solvent.

Consider a mixture of isopropyl alcohol and water:

• Water and alcohol are completely miscible, meaning they mix thoroughly.
• However, introducing an organic solvent like diethyl ether—which is immiscible with water—enables us to separate alcohol due to its solubility in the organic layer.

Applying these concepts, one can effectively separate and purify substances based on their compatibility or non-compatibility with different solvents. This knowledge guides us in choosing proper solvents for the desired separation.