Question

Describe two ways in which the octane number of a gasoline consisting of alkanes can be increased.

Short answer

To increase the octane number of a gasoline consisting of alkanes, one can use two primary methods. First, include more branched-chain alkanes, such as isooctane, during fuel production, as they have higher resistance to knocking and burn more smoothly than straight-chain alkanes. Second, add octane-enhancing additives like aromatic hydrocarbons or oxygenates, which increase the gasoline's resistance to knocking and improve fuel performance.

Step-by-step solution

1. Using Branched-Chain Alkanes

One way to increase the octane number of a gasoline containing alkanes is by including more branched-chain alkanes. Branched-chain alkanes have a higher resistance to knocking because they burn more smoothly than straight-chain alkanes. When branched-chain alkanes are used in gasoline, they prevent the air-fuel mixture from detonating prematurely, which ultimately leads to a higher octane number. For example, isooctane, a branched-chain alkane with the chemical formula \(C_8H_{18}\), has an octane number of 100, which is used as the reference fuel for measuring the octane rating of other fuels. In contrast, heptane, which is a straight-chain alkane, has an octane number of 0 and is more likely to cause knocking. To produce gasoline with a higher octane number, it is essential to include a higher percentage of branched-chain alkanes during fuel production. This can be achieved through various processes, such as hydrocracking, which breaks down larger molecules into smaller branched-chain alkanes.

2. Adding Octane-Enhancing Additives

A second method to increase the octane number of a gasoline consisting of alkanes is by adding specific additives that enhance the fuel's resistance to knocking. These additives work by either altering the combustion process to reduce the likelihood of knocking or by increasing the gasoline's chemical stability. Some popular octane-enhancing additives include: 1. Tetraethyllead (TEL) – Tetraethyllead was a widely used additive until it was phased out due to concerns about its toxicity and its harmful effects on the environment. It worked as an anti-knock agent by raising the gasoline's octane rating, thus helping to prevent premature detonation. 2. Aromatic hydrocarbons – Aromatic hydrocarbons, like benzene, toluene, and xylene, can significantly increase gasoline's octane rating. However, they can also increase the levels of harmful tailpipe emissions. As a result, their use in gasoline is usually limited to a specific percentage. 3. Oxygenates – Oxygenates, such as alcohol (ethanol, methanol) and ethers (MTBE, ETBE), can also effectively increase the octane number of gasoline. Adding oxygenates to the gasoline not only raises the octane rating but also helps reduce harmful tailpipe emissions, making it a popular choice for modern gasolines. By utilizing these two methods, including more branched-chain alkanes and adding octane-enhancing additives to gasoline, the octane number of a gasoline containing alkanes can be increased, improving fuel performance and reducing the likelihood of engine knocking.