Question

Absolute alcohol (100\% alcohol) is prepared by 6 distilling rectified spirit over (a) \(\mathrm{Mg}\left(\mathrm{OC}_{2} \mathrm{H}_{5}\right)_{2}\) (b) \(\mathrm{CaCl}_{2}\) (c) \(\mathrm{Na}\) (d) \(\mathrm{Mg}\)

Short answer

Option (a) \(\mathrm{Mg}\left(\mathrm{OC}_{2} \mathrm{H}_{5}\right)_{2}\) is used to prepare absolute alcohol from rectified spirit.

Step-by-step solution

Understanding Rectified Spirit

Rectified spirit is typically a mixture consisting of 95% ethanol and 5% water. To create absolute alcohol, which is 100% ethanol, we need to sufficiently remove the water content from the mixture.

Assessing the Drying Agents

We need a drying agent that can effectively remove water and leave ethanol untouched. Consider the options provided: \(\mathrm{Mg}\left(\mathrm{OC}_{2} \mathrm{H}_{5}\right)_{2}\), \(\mathrm{CaCl}_{2}\), sodium (\(\mathrm{Na}\)), and magnesium (\(\mathrm{Mg}\)). Each has different properties and applications, influencing their effectiveness in dehydrating rectified spirit.

Evaluating Each Option

- **\(\mathrm{Mg}\left(\mathrm{OC}_{2} \mathrm{H}_{5}\right)_{2}\):** This is magnesium ethoxide and it is known to react specifically with water to form alcohols and magnesium hydroxide, effectively removing water.- **\(\mathrm{CaCl}_{2}\):** It is a common drying agent but in this case, it has limited effectiveness due to the presence of both ethanol and water.- **\(\mathrm{Na}\):** Sodium reacts vigorously with water but also reacts with alcohols, producing undesirable by-products such as sodium ethoxide and hydrogen gas.- **\(\mathrm{Mg}\):** While magnesium is reactive, it is less efficient and selective in removing only water without altering the alcohol.

Choosing the Best Option

Out of the options, \(\mathrm{Mg}\left(\mathrm{OC}_{2} \mathrm{H}_{5}\right)_{2}\) is the most suitable for drying rectified spirit to obtain absolute alcohol. It selectively reacts with water, allowing ethanol to remain undisturbed.

Key concepts

Rectified Spirit

When you hear the term 'rectified spirit,' you are essentially looking at a high-concentration alcohol, specifically a mixture that contains about 95% ethanol and 5% water. This is commonly used in various industries and applications. However, to create absolute alcohol, this remaining 5% water must be completely removed. It's not an easy process since ethanol and water form a special mixture known as an azeotrope that boils as if it were a single substance. To break this bond, special agents and processes are required to draw out the last traces of water, and that's where drying agents come into play.

Drying Agents

Drying agents are crucial in the dehydration process, especially when aiming to achieve absolute alcohol from rectified spirit. These agents work by absorbing or chemically binding with water molecules, thereby extracting them from the mixture. Different drying agents have varying levels of effectiveness based on their chemical properties and reactivity.

• Calcium Chloride (128046128046): Often used as a drying agent, but not ideal for removing water from ethanol because it interacts with both components in the mixture.
• Magnesium (1217267;): Reactive but not selective enough, making it a less suitable choice.
• Sodium (12234;): Combine this with alcohol and you get unwanted by-products, including sodium ethoxide and hydrogen gas.

Thus, the drying agent must effectively target and remove water without altering the ethanol itself, which brings us to magnesium ethoxide.

Magnesium Ethoxide

Magnesium ethoxide (312;(3ca05;2;2)) stands out as the most effective drying agent for producing absolute alcohol from rectified spirit. It operates by specifically targeting water molecules in the mixture. This compound reacts with water to produce alcohol and magnesium hydroxide, completely removing the unwanted water without any detrimental effects on the ethanol. By using magnesium ethoxide, you can reliably perform the dehydration process, producing absolute or 100% alcohol. This specificity and effectiveness are what make magnesium ethoxide the preferred choice over other potential drying agents.

Dehydration Process

The dehydration process is essentially about removing water from a mixture to obtain a purer form of the desired substance—in this case, to convert rectified spirit into absolute alcohol. Typically, this involves distillation, but due to the azeotropic nature of ethanol-water mixtures, merely distilling the mixture is not sufficient. Specialty drying agents like magnesium ethoxide are employed to ensure that the water is removed.
Once magnesium ethoxide reacts with the water, it forms magnesium hydroxide and frees the ethanol, allowing for the complete conversion of rectified spirit to absolute alcohol. This method ensures that you achieve 100% alcohol, enabling its use where maximum purity is required, such as in laboratory or pharmaceutical applications.