In which of the following properties is an increase observed when hydrocarbon fuels undergo cracking? (A) Molecular size, (B) melting point, (C) volatility, (D) viscosity, or (E) ignition temperature.
The question mentions cracking of hydrocarbons. Cracking is a type of decomposition reaction where larger organic molecules are broken down into smaller molecules. And hydrocarbons are compounds composed only of carbon and hydrogen. Cracking reactions are performed in industry on large molecules from crude oil to produce smaller, more useful compounds, which are economically more desirable. Since cracking involves converting larger molecules to smaller molecules, we can immediately rule out answer option (A), molecular size, as we are asked what properties increase during cracking. Molecular size does not increase but decreases.
With decreasing carbon chain length or decreasing molecular size, so hydrocarbon properties tend to change in a predictable manner. For example, melting point tends to decrease as molecules get smaller. This is because larger molecules have stronger London dispersion forces of attraction between them. And so more energy is required to separate molecules during melting. Cracking tends to result in molecules with lower melting points, not increased melting points. And so we can rule out answer option (B), melting point.
The strong dispersion forces between larger molecules and the weaker dispersion forces between smaller molecules are also responsible for decreasing viscosity trends. Because large molecules are held together more tightly, they flow less easily. When in liquid form, we say they are more viscous or thick and sticky, and smaller molecules when they are in liquid form are less viscous. Since cracking produces smaller molecules, which have decreased viscosity, we can rule out answer option (D), viscosity.
However, volatility increases as molecules get smaller. Smaller molecules are more easily converted to the gas phase or more easily volatilized because of the weaker intermolecular forces. Larger molecules are less easily volatilized or converted to gases because of the strong intermolecular forces. More energy is required to convert them from a liquid to a gas. We can see that answer option (C), volatility, is a property that increases during cracking. Because of the increased volatility with smaller molecules, the forces of attraction are more easily overcome. Molecules easily gasify, separate, and can therefore mix quickly with oxygen and react and ignite, resulting in a decreasing ignition temperature trend. Finally, the property which increases as hydrocarbon fuels undergo cracking is (C), volatility.