C4H10 is a gas at room temperature.
C8H18 is another alkane but is a liquid at room temperature. Explain why C4H10 is a gas but C8H18 is a liquid at room temperature.
The Correct Answer and Explanation is:
The difference in the physical states of butane (C4H10) and octane (C8H18) at room temperature can be attributed primarily to their molecular structure and size, which affect their intermolecular forces, specifically van der Waals forces (or London dispersion forces).
Molecular Structure and Size
Butane (C4H10) is a smaller alkane with four carbon atoms, while octane (C8H18) contains eight carbon atoms. As the number of carbon atoms increases in alkanes, the molecular size and weight also increase, leading to changes in physical properties.
Intermolecular Forces
Intermolecular forces are the forces of attraction between molecules. In alkanes, the predominant type of intermolecular force is van der Waals forces. These forces arise due to temporary dipoles that occur when electrons move around a molecule. Larger molecules, like octane, have more electrons and a greater surface area, resulting in stronger van der Waals forces. As a result, octane molecules experience stronger attractions to each other, making it more likely to be in a liquid state at room temperature.
In contrast, butane has fewer carbon atoms and a smaller molecular size, resulting in weaker van der Waals forces. The lower molecular weight and smaller surface area of butane lead to less interaction between the molecules. Consequently, butane does not require as much energy to overcome these weaker forces, allowing it to exist as a gas at room temperature.
Boiling Points
The boiling points of these alkanes further illustrate this difference: butane has a boiling point of approximately -1 °C, while octane has a boiling point of about 126 °C. The significantly higher boiling point of octane indicates that it requires more energy to convert from liquid to gas compared to butane, reinforcing the conclusion that the larger octane molecules are held together by stronger intermolecular forces.
In summary, the differences in molecular size and the resultant strength of intermolecular forces explain why butane is a gas and octane is a liquid at room temperature.