Question #44071

This question tests a classic 'Lethal Collision' point in chemical equilibrium: the effect of inert gas addition.

Students often blindly apply Le Chatelier's principle, assuming that an increase in total pressure must shift the equilibrium towards the side with fewer moles (backward, in this case).

However, this only applies if the pressure increase is due to a volume decrease.

At constant volume, adding an inert gas increases total pressure but does not change the partial pressures ($p_i = n_iRT/V$) of the reacting gases, so $Q_p$ remains equal to $K_p$.

Furthermore, it tests the fundamental thermodynamic definition that $K_p$ and $K_c$ are exclusively temperature-dependent.

Students might erroneously use the mole fraction relation $K_p = K_x(P_{total})^{\Delta n_g}$ to conclude $K_p$ changes with total pressure, failing to realize that $K_x$ is the variable that adjusts, while $K_p$ remains a thermodynamic constant.