What are the conditions for steam cracking?

Steam cracking is a petrochemical process used to break down larger hydrocarbons into smaller, more useful molecules, such as ethylene and propylene. The process generally requires high temperatures and the presence of steam.

The correct choice states that steam cracking occurs at 550°C with steam. These conditions are effective for ensuring that the reaction proceeds efficiently. The presence of steam helps to prevent the formation of unwanted by-products, facilitating the conversion of long-chain hydrocarbons into smaller alkenes, which are key building blocks in the production of plastics and other chemicals.

When steam is introduced, it serves not only to provide the right environment for the thermal decomposition of hydrocarbons but also to carry hydrogen away, which can help improve the yield of desirable products. The temperature of 550°C is optimal as it provides enough energy to break the carbon-hydrogen bonds without reaching a point where excessive thermal degradation occurs.

In contrast, other temperature and reactant combinations listed in the alternatives would not provide the efficient cracking of hydrocarbons as steam at high temperatures does. For example, using air could lead to combustion rather than cracking, while lower temperatures or the absence of steam would not effectively promote the necessary reactions for producing valuable alkenes.