Under what conditions would you expect a real gas to be (a) more compressible than an ideal gas; (b) less compressible than an ideal gas?

The Ideal Gas Law is a fundamental equation in chemistry and physics, represented as \( PV = nRT \), where \( P \) stands for pressure, \( V \) for volume, \( n \) for the number of moles, \( R \) for the universal gas constant, and \( T \) for temperature in Kelvin. This law assumes that gas particles are in constant, random motion and that they do not exert any forces on each other (neither attraction nor repulsion). Furthermore, it's assumed that the volume of the individual gas molecules is so small compared to the total volume the gas occupies that it can be considered negligible. These simplifications allow for easy calculations of one state variable when the others are known.

However, when dealing with real gases, we notice deviations from the Ideal Gas Law, particularly under extreme conditions such as high pressure and low temperature. This is due to the law's neglect of intermolecular forces and the volume of the gas molecules themselves. To account for these factors, additional corrections are often applied, leading to more complex equations like the van der Waals equation.

Intermolecular forces are the attractive and repulsive forces between molecules. They play a crucial role in determining the physical properties of substances, including gases. The strength and type of intermolecular forces vary depending on the nature of the molecules involved, but they are generally weaker than the bonds within molecules (intramolecular forces).

In the context of gas compressibility, intermolecular forces can significantly affect how a real gas behaves under compression. When a real gas is subjected to high pressure or low temperature, the molecules are forced closer together, allowing attractive forces (such as London dispersion forces, dipole-dipole interactions, and hydrogen bonds) to become more significant. This increased attraction can make the gas more compressible than predicted by the Ideal Gas Law. Conversely, at very low pressures or high temperatures, repulsive forces become predominant when the molecules are squeezed too close together, making the gas less compressible.

When we refer to the volume of gas molecules, we are talking about the actual space that the molecules occupy, rather than the volume of the container they are in. In the ideal gas model, it's assumed that this volume is negligible relative to the container's volume. However, in reality, gas molecules do have a finite size, and their volume becomes significant under certain conditions.

As pressure increases, gas particles are compressed to the point where their actual volume is no longer negligible when compared to the container volume. This starts to limit how much further the gas can be compressed. This concept is particularly important when considering high-pressure situations where the volume of individual molecules can impede compression, leading to the real gas being less compressible than expected according to ideal gas predictions.