Behaviour of Real Gases — Prelims Strategy

Prelims Strategy

To excel in NEET questions on 'Behaviour of Real Gases', a multi-faceted strategy is essential. Firstly, solidify the conceptual foundation by clearly understanding the assumptions of the ideal gas model and precisely why real gases deviate.

Memorize that real gases approach ideal behavior at high temperatures and low pressures, and show maximum deviation at low temperatures and high pressures. Understand the roles of finite molecular volume and intermolecular forces in causing these deviations.

Secondly, master the compressibility factor (Z). Know its definition ($Z = PV/nRT$) and how its value ($Z=1$, $Z<1$, $Z>1$) indicates ideal behavior, dominance of attractive forces, or dominance of molecular volume, respectively. Practice interpreting Z vs. P graphs for different gases, noting the unique behavior of H2 and He. For numerical problems, ensure correct unit conversions and careful substitution into the formula.

Thirdly, understand the van der Waals equation and the physical significance of its constants 'a' and 'b'. 'a' relates to attractive forces (higher 'a' means stronger attraction), and 'b' relates to molecular volume (higher 'b' means larger molecules). Be prepared to compare 'a' and 'b' values for different gases based on their structure and polarity. While direct derivation is not usually tested, knowing the corrected terms is vital.

Finally, memorize and practice critical constant formulas. The relationships $T_c = \frac{8a}{27Rb}$, $P_c = \frac{a}{27b^2}$, and $V_c = 3b$ are frequently tested. Practice deriving one constant from others or calculating 'a' and 'b' from given critical constants.

Pay close attention to units. For conceptual questions, avoid common traps like confusing the roles of 'a' and 'b' or misinterpreting the conditions for ideal behavior. Always read the question carefully to identify whether it's asking about ideal or real gas behavior and the specific conditions.