National Council of Examiners for Engineering and Surveying (NCEES) Fundamentals of Engineering (FE) Chemical Practice Exam

The equilibrium constant for gas phase reactions is expressed as the ratio of product partial pressures to reactant partial pressures. This is due to the fact that gas phase reactions involve the interaction of gaseous species, and their behavior can be accurately described using partial pressures according to Dalton's law of partial pressures.

At equilibrium, the equilibrium constant (K_p) is determined using the expression:

\[ K_p = \frac{P_{\text{products}}}{P_{\text{reactants}}} \]

where \(P\) represents the partial pressures of the gases involved in the reaction. This formulation allows for a clear understanding of how the equilibrium state is achieved in terms of the concentration of gases present, reflecting the ratios of the pressures exerted by the gaseous species at a given temperature.

In contrast, while concentrations can also be related to equilibrium constants in the context of gaseous reactions (using molar concentrations), the specific relationship for gas phase reactions is more accurately described through partial pressures, especially in ideal gas behavior.

The other forms mentioned do not correctly represent how equilibrium constants are defined for gas phase reactions, as they either do not distinguish between products and reactants or describe relationships that do not contribute to the equilibrium constant definition itself.