real gas
Summary
A real gas is a gas whose behavior departs from PV = nRT because its particles take up volume and attract one another. The deviation is generally largest at high pressure and low temperature, and smallest for small, nonpolar gases such as He and H₂.
Detail
Meaning: The ideal gas model treats particles as points with no attractions. Real particles have finite size and attract each other. Attractions pull particles away from the walls, so the measured pressure is lower than the ideal value. Finite particle volume means the free space is less than the container volume. At very high pressure, volume effects dominate and PV/nRT rises above 1. At moderate pressure, attractions dominate and PV/nRT falls below 1. Low temperature makes attractions matter more because particles move slowly, and the gas may condense. Large or polar molecules (CO₂, NH₃) deviate more than He or H₂.
Equation: (P + an²/V²)(V − nb) = nRT (van der Waals). P is pressure in Pa, V is volume in m³, n is moles in mol, T is temperature in K, R = 8.314 J/(mol·K), a is the attraction constant in Pa·m⁶/mol², b is the excluded volume per mole in m³/mol.
On the MCAT: A passage may show a plot of PV/nRT against pressure for several gases, or list a and b values. You predict which gas is most ideal, which condenses more easily (larger a), or how conditions change the deviation.
Common confusion: Students think heating a gas makes it less ideal. The opposite is true: high temperature and low pressure make real gases act nearly ideally. Also, the corrections go in opposite directions: P is increased by an²/V², and V is reduced by nb.
Sources
- Chemistry: The Central Science
- Chemistry (Zumdahl)
- Chemistry: The Molecular Nature of Matter and Change
Written for the MCAT and checked before it was published. For study reference only. Spotted an error? Let us know.