Hydrocarbons — Part 2: Alkenes and alkynes (ethene and ethyne): visual lesson, ICSE Class 10 Chemistry (Science Paper 2)

Lesson

Bonds that can open up

Part 2 — Ethene (C₂H₄, the first alkene) has a C=C double bond; ethyne (C₂H₂, the first alkyne, also called acetylene) has a C≡C triple bond. These unsaturated hydrocarbons are made by removing small molecules from saturated compounds, and they react by adding molecules across the multiple bond.

In this lesson you will

Write equations with conditions for preparing ethene from bromoethane and from ethanol
Write equations with conditions for preparing ethyne from calcium carbide and from 1,2-dibromoethane
Write the addition products of ethene and ethyne with hydrogen and each halogen
Use bromine solution to distinguish saturated from unsaturated hydrocarbons, and give uses of each hydrocarbon

Concept

Ethene from bromoethane: dehydrohalogenation

Boiling bromoethane with a concentrated solution of potassium hydroxide in ethanol (alcoholic KOH) removes one H atom and one Br atom from neighbouring carbon atoms as HBr. This is dehydrohalogenation (here dehydrobromination). Note: aqueous KOH gives ethanol instead, not ethene.

\ce{C2H5Br + KOH ->[{alcohol, heat}] C2H4 + KBr + H2O}

Alcoholic KOH removes H and Br from neighbouring carbons.

Concept

Ethene from ethanol: dehydration

Ethanol is heated with excess concentrated sulphuric acid at about 170 °C. The acid is a dehydrating agent: it removes a molecule of water from each ethanol molecule. The gas is passed through sodium hydroxide solution to remove the impurities sulphur dioxide and carbon dioxide, and is collected over water.

C₂H₅OH → C₂H₄ + H₂O (conc. H₂SO₄, 170 °C)

Cookies