Pressuredependent

Pressuredependent is a term used to describe processes whose rates or behaviors change with the total pressure of the system. In chemical kinetics, many reactions—especially unimolecular or energy-rich intermediate steps—are not governed by a single, fixed rate constant but instead by how often collisional energy transfer occurs. As a result, the observed rate constant can vary with pressure.

In the classic Lindemann–Hinshelwood mechanism, a reactant A is energized by collisions to form A*, which can either decompose to products or be deactivated back to A by collisions with a third body M. At low pressures, collisions are infrequent, so the formation of A* is rate-limiting and the overall rate scales roughly with [M]. At high pressures, deactivation is rapid and the system approaches a different, often pressure-independent high‑pressure limit for the decomposition step. The simplest expression linking these limits is k_eff = (k0 [M] k∞) / (k0 [M] + k∞), where k0 is a low-pressure limit parameter and k∞ is a high-pressure limit parameter. Real systems often show a smoother “fall-off” between these regimes and may require empirical factors.

Theoretical treatments use master equations and RRKM theory to compute pressure-dependent rate constants, sometimes supplemented by fit formulas such as the Troe expression to describe non-ideal fall-off behavior. Pressure dependence is especially important in combustion and atmospheric chemistry, where accurate modeling of unimolecular and termolecular reactions depends on pressure. Related concepts include fall-off curves, RRKM theory, master equation methods, and the use of specialized software to compute and store pressure-dependent rate constants.