실제 가스: 반 데르 발스 방정식을 유도하는 분자간 힘 및 분자 부피의 효과

Real gases deviate from ideality at high pressures because gas particle density increases, making the combined volume of particles significant relative to container volume. Additionally, intermolecular attractive forces become more pronounced when particles are closer together. These factors violate kinetic molecular theory assumptions that particles occupy negligible volume and exert no attractive forces on each other.

The volume correction factor nb accounts for the actual volume occupied by gas molecules themselves. At high pressures, the combined molecular volume becomes significant and must be subtracted from the container volume to determine the volume available for particle movement. The constant b is gas-specific and experimentally determined, with units of L/mol.

Intermolecular attractive forces reduce the pressure exerted by real gases compared to ideal gases. When particles interact, they spend more time with neighboring molecules and less time colliding with container walls, decreasing collision frequency. The van der Waals equation corrects for this using the term an²/V², where a represents the strength of intermolecular attraction for a specific gas.

The van der Waals equation reduces to the ideal gas law PV = nRT when volume is relatively large and the number of moles is relatively small, corresponding to low-pressure conditions. Under these circumstances, both correction terms become negligible: molecular volume is insignificant relative to container volume, and particles are too far apart for intermolecular forces to matter.

For one mole of an ideal gas, the ratio PV/RT equals one regardless of pressure. When this ratio deviates from one, it indicates non-ideal behavior. At low pressures, real gases approach this ideal value, but as pressure increases, the ratio diverges significantly, with different gases showing different deviations based on their molecular properties.

At high temperatures, gas particles possess high kinetic energy relative to intermolecular attractive forces, allowing them to overcome attractions and move quickly. At lower temperatures, particles move more slowly and have lower kinetic energy, making intermolecular forces more significant. This is why deviations from ideal behavior are more pronounced at low temperatures and high pressures.

The constant a represents the strength of intermolecular attractive forces for a specific gas, with units of L²·atm/mol². The constant b represents the size of gas molecules and the volume they occupy, with units of L/mol. Both are experimentally determined values that vary for different gases and are essential for correcting real gas behavior.