21. Preparation of Ethyne

Summary

This educational video demonstrates the preparation and examination of ethyne (C2H2), a colorless gas with a pungent odor. Ethyne is synthesized from calcium carbide, which often contains impurities like calcium sulfide and phosphide. The resulting ethyne gas is purified by passing it through acidified copper sulfate to remove contaminants. The video outlines safety precautions, such as avoiding contact with calcium carbide and using no open flames. It also covers tests for ethyne's properties, including its insolubility in water, solubility in organic solvents, and its reactivity with bromine and potassium permanganate, which indicate the presence of a carbon-carbon triple bond.

Takeaways

• 🔬 Ethine (C2H2) is prepared by reacting water with calcium carbide (CaC2), also known as calcium dicarbide.
• ⚠️ Calcium carbide is often impure, containing contaminants like calcium sulfide, phosphide, and nitride, which can affect the purity of ethine.
• 🌬️ The ethine produced may contain impurities such as hydrogen sulfide, phosphine, and ammonia, which are removed by passing the gas through acidified copper(II) sulfate.
• 👐 Handle calcium carbide with care, using a spatula to avoid initiating the reaction with moisture from hands.
• 🚫 Keep naked flames away from the gas preparation apparatus due to the risk of explosion when ethine mixes with air.
• 🔗 Ensure the gas washing bottle is correctly connected to prevent impure gases from being collected.
• 💧 Add water slowly to calcium carbide to control the rate of ethine production.
• 🧪 Ethine is a colorless gas that may have an unpleasant smell if impurities are present, and it is insoluble in water but soluble in organic solvents.
• 🔥 Ethine burns with a luminous, smoky flame and produces soot, indicating the presence of unburned carbon.
• 🌐 The addition of bromine water to ethine decolorizes the solution, demonstrating the reaction of the carbon-carbon triple bond with bromine.
• 🟣 Acidified potassium permanganate solution also decolorizes in the presence of ethine, showing another test for unsaturation due to the reactive triple bond.

Q & A

What is ethine and what is its chemical formula?

Ethine is a compound with the chemical formula C2H2, also known as acetylene.

What is the primary reactant used to prepare ethine in the described experiment?

The primary reactant used to prepare ethine in the experiment is calcium carbide (CaC2), also known as calcium dicarbide.

Why is calcium carbide often contaminated with other substances?

Calcium carbide is often contaminated with substances like calcium sulfide, calcium phosphide, and calcium nitride because it cannot be obtained in a pure state.

What are the safety precautions when handling calcium carbide?

When handling calcium carbide, one should not touch it with bare hands to avoid initiating the reaction with moisture, and a spatula should be used for transferring it. Also, no naked flame should be brought near the gas preparation apparatus due to the risk of explosion.

How are impurities in ethine gas removed during the experiment?

Impurities in ethine gas are removed by bubbling the gas through acidified copper(II) sulfate.

What is the purpose of the gas washing bottle in the experiment setup?

The gas washing bottle is used to ensure that the ethine gas is clean and free of impurities before it is collected and tested.

What physical properties of ethine are mentioned in the script?

Ethine is described as a colorless gas that is insoluble in water but soluble in organic solvents.

How is the combustion test for ethine performed, and what is observed?

The combustion test is performed by bringing a lit taper to the mouth of a gas jar containing ethine. It burns with a luminous, smoky flame, and a great deal of soot is formed due to unburned carbon.

What happens when ethine reacts with bromine water, and why?

When ethine reacts with bromine water, the yellow color disappears because the very reactive carbon-carbon triple bond in ethine reacts readily with the bromine.

What is the purpose of adding acidified potassium permanganate to ethine, and what is observed?

Adding acidified potassium permanganate to ethine is used to test for unsaturation. The purple color of the potassium permanganate disappears, indicating a reaction with the carbon-carbon triple bond in ethine.

Why is it important to perform tests on ethine away from the apparatus?

Tests on ethine should be performed away from the apparatus because ethine gas forms an explosive mixture with air, and conducting tests near the apparatus could be dangerous.