Entropy Export and Chemical Reactions in Plasmas

ABSTRACT Is plasma truly a “fourth state of matter”? And if not, does this change how we would approach its chemical reactivity? In equilibrium systems, temperature ( T ) is the main control parameter for chemical reactions. Here, we argue that in non‐equilibrium plasmas the main driving quantity is the entropy change (Δ S ) associated with the externally supplied energy. Therefore, plasma‐chemical activation is analyzed for (i) low and (ii) highly collisional plasmas, and (iii) plasmas with significant heating. In all cases, the governing parameter is the energy available per molecule, reflecting the entropy exported through electron‐impact and thermally driven reactions. Based on this, analytical expressions of Arrhenius‐ and Saha‐like form are derived that describe plasma gas conversion, for example, for CO 2 and CH 4 .