Positive deviation from ideal behaviour takes place because of

A Molecular attraction between atoms and PV/nRT>1

Molecular attraction between atoms and PV/nRT

Finite size of atoms and PV/nRT>1

Finite size of atoms and PV/nRT

To understand why positive deviation from ideal behavior occurs in gases, we need to delve into the concepts of molecular attraction and the finite size of atoms. The ideal gas law, represented as PV = nRT, assumes that gas particles do not interact and occupy no volume. However, real gases often deviate from this ideal behavior under certain conditions. Let's break this down step by step.

Molecular Attraction and Its Impact

In an ideal gas, we assume that there are no intermolecular forces acting between the gas molecules. However, in reality, molecules do experience attractive forces, especially when they are close together. This attraction can lead to a positive deviation from ideal behavior under certain conditions.

• Attraction Between Molecules: When gas molecules are attracted to each other, they tend to stick together rather than move freely. This attraction can cause the pressure of the gas to be lower than what the ideal gas law predicts, leading to a negative deviation.
• High Temperatures and Low Pressures: At high temperatures, the kinetic energy of the molecules increases, which can overcome intermolecular attractions. In this scenario, the gas may behave more ideally, but at low temperatures or high pressures, these attractions become significant.

Understanding PV/nRT > 1

When we say that PV/nRT > 1, we are indicating that the actual behavior of the gas is deviating positively from what the ideal gas law would predict. This can occur due to the following reasons:

• Increased Molecular Interactions: If the attractive forces between molecules are strong enough, they can lead to a situation where the volume occupied by the gas is less than expected, resulting in a higher pressure than predicted by the ideal gas law.
• Real Gas Behavior: In real gases, especially at high pressures, the volume occupied by the gas molecules themselves becomes significant. This finite size of the molecules means that they cannot be compressed indefinitely, leading to deviations from ideal behavior.

The Role of Finite Size of Atoms

The finite size of gas molecules also plays a crucial role in the deviation from ideal behavior. In the ideal gas model, we assume that gas particles are point-like and have no volume. However, in reality, gas molecules do occupy space.

• Volume of Gas Molecules: When we consider the actual volume occupied by gas molecules, it becomes clear that at high pressures, the volume available for the gas to expand is reduced. This can lead to an increase in pressure, causing PV/nRT to exceed 1.
• Implications for Gas Laws: As the size of the molecules increases, the deviations from ideal behavior become more pronounced, especially in gases like carbon dioxide or ammonia, where molecular size and intermolecular forces are significant.

Real-World Examples

To illustrate these concepts, consider a gas like carbon dioxide. Under standard conditions, it behaves relatively ideally. However, if we increase the pressure significantly, the molecules are forced closer together, enhancing intermolecular attractions and leading to a positive deviation from the ideal gas law. Similarly, at low temperatures, the finite size of the molecules becomes more relevant, as they cannot be compressed indefinitely, further contributing to deviations.

In summary, positive deviations from ideal gas behavior can be attributed to the molecular attractions between gas particles and the finite size of those particles. Understanding these factors helps explain why real gases do not always conform to the ideal gas law, especially under varying temperature and pressure conditions.