Non-Ideal Gases and Limits of the Ideal Model

Non-Ideal Gases and Limits of the Ideal Model

An ideal gas is like a simplified first version of reality.

Real molecules have size and can attract one another.

When those effects become important, the ideal model needs correction.

Non-Ideal Gases and Limits of the Ideal Model

Definition

A non-ideal or real gas is a gas whose behaviour differs appreciably from the ideal-gas model because molecular size and intermolecular interactions can no longer be neglected.

Two Main Ideal-Gas Assumptions That Fail

The ideal model is an approximation.

Real molecules occupy finite volume.

They can also attract or repel each other.

At sufficiently low density, these effects may be small.

But deviations become important particularly when molecules are crowded or intermolecular interactions become significant.

1. Negligible molecular volume

Real particles occupy space.

2. Negligible intermolecular forces

Real molecules interact.

Why the Ideal Model Still Matters

A simplified model gives a baseline.

Scientists can then understand real systems by examining how and why they depart from the baseline.

This is one of the ways the ideal-gas model helped develop statistical mechanics. The current curriculum explicitly uses ideal gases as the starting point for understanding non-ideal behaviour. ([Scribd][2])

Formula / Key Relation

Ideal baseline:

**pV=NkT**

Real behaviour:

pV ≠ NkT exactly under all conditions

Non-Ideal Gases and Limits of the Ideal Model — Advanced

Real-gas deviations arise from excluded volume and intermolecular potential energy. Statistical mechanics allows more realistic interaction models to be added systematically.