Why Gases Aren't Always "Ideal"
The ideal gas law (PV=nRT) is a great starting point, but real gases don't always follow it perfectly. They "deviate" from ideal behavior, especially under certain conditions, because:
- Molecules have volume: Ideal gas law assumes gas particles have no size, but real gas molecules take up space.
- Molecules attract each other: Ideal gas law assumes no forces between particles, but real gas molecules have weak attractions.
- High pressure conditions: When gas is squeezed, molecules are closer, so their volume and attractions become more noticeable.
- Low temperature conditions: When gas is cold, molecules move slower, making their attractions more significant.
The Van der Waals Equation: A Better Model
The van der Waals equation is one of the first and most common ways to describe real gas behavior. It adds corrections to the ideal gas law to account for the non-ideal aspects:
- 'b' parameter (molecular volume): This term corrects the volume, acknowledging that gas molecules themselves occupy space.
- 'a' parameter (intermolecular attractions): This term corrects the pressure, accounting for the attractive forces between gas molecules that pull them closer.
- Pressure correction term: The `an²/V²` part is added to the pressure to account for the reduced impact of collisions due to attractions.
- Volume correction term: The `nb` part is subtracted from the volume to account for the actual space available for molecules to move.
The Redlich-Kwong Equation: An Even Better Fit
The Redlich-Kwong equation is another popular real gas equation that often provides more accurate results than the van der Waals equation, especially over a wider range of temperatures and pressures. It improves upon the van der Waals model by:
- Better temperature dependence: It includes a temperature term in the attraction part, making it more accurate for how
- More accurate at high pressures
- Modified attraction term
- Better critical point prediction
Critical Properties
At the critical point:
- Liquid and gas phases become indistinguishable
- Used to determine equation parameters
- Define reduced properties
- Important for corresponding states