Video Transcript
Which of the following is not a use of alkenes? (A) Manufacturing plastics, (B) manufacturing rubber, (C) cracking, (D) producing industrial chemicals, or (E) burning as a fuel.
An alkene is a hydrocarbon that contains a carbon-carbon double bond. A common reaction of alkenes is polymerization. Polymerization is a reaction in which molecules, called monomers, join together to form long-chain molecules, called polymers. For example, styrene can be polymerized to produce polystyrene. Note that I have used these lines to indicate that the polymer would actually be longer than this and that the structure shown is repeated.
Polystyrene is a type of plastic. It’s often used in packaging and disposable cups. We now know that alkenes can be used to manufacture plastics. Since the question asks us which of the following is not a use of alkenes, we can eliminate option (A).
Another example of alkenes being used in polymerization reactions is in styrene-butadiene. Styrene-butadiene is a synthetic rubber and is often used in car tires. So we know that alkenes can be used to manufacture rubber. Therefore, option (B) cannot be the solution to this question.
The possible answer given in (C) is cracking. Cracking is a process by which less useful long-chain hydrocarbons, more specifically long-chain alkanes, are broken down into more useful short-chain hydrocarbons, more specifically short-chain alkenes and alkanes. We can see that alkenes are not used in cracking because they are in fact the product of the process. Therefore, the answer to the question “Which of the following is not a use of alkenes?” is (C) cracking.
But to confirm this, let’s check out options (D) and (E). Option (D) is producing industrial chemicals. Alkenes are an important chemical feedstock. For example, ethene is regularly used in industry to make ethanol and ethane. Thus, we can confirm that (D) is not the answer to this question.
Lastly, the option for (E) is burning as a fuel. Alkenes do combust. They burn in oxygen to produce carbon dioxide and water, generating energy. For example, one mole of ethene would react with three moles of oxygen to produce two moles of carbon dioxide and two moles of water. But it should be noted that alkanes are preferred to alkenes as a fuel, as they’re more cost effective and produce a less sooty flame when burned. However, alkenes can be used to burn as a fuel. Thus, option (E) cannot be the answer to this question.
So now we can be sure that the answer to the question “Which of the following is not a use of alkenes?” is (C) cracking.
