Expressing Rates of S's Phase Transition: P1, T1, and Activity of S (Phase 1)

[Big-Daddy]

Let's say I have a general phase transition S (phase 1) ::equil:: S (phase 2). At a certain point in pressure and temperature for each phase (they can have different pressures and temperatures, we might say P₁ and T₁ for phase 1 and P₂ and T₂ for phase 2), how might we express the forward and reverse rates of this phase transition?

No doubt the forward rate will be a function of P₁, T₁ and the activity of S (phase 1); the backward rate will presumably be an analogous function of P₂, T₂ and the activity of S (phase 2). But what function? And how should we work out how much the temperature of phase 1 changes on the basis of how many moles/what activity of S (phase 1) there is, and temperature of phase 2 changes on the basis of how many moles/what activity of S (phase 2) there is.

 

[Navin A S]

Since the exact function that describes the forward and backward rates of a phase transition will vary depending on the nature of the system, it is difficult to provide a general answer. However, some basic general principles can be applied. In general, the rate of a phase transition can be described using a thermodynamic rate equation, which takes into account the pressure and temperature of each phase, along with the activity of the substance in each phase. The rate equation will also take into account any other factors that may influence the transition, such as the surface area of the material or the presence of a catalyst. The rate equation can then be used to predict how much the temperature of each phase will change for a given amount of activity of the substance. For example, if the activity of S (phase 1) increases, the rate equation can be used to determine how much the temperature of phase 1 will need to increase in order for the phase transition to occur. Similarly, if the activity of S (phase 2) decreases, the rate equation can be used to determine how much the temperature of phase 2 will need to decrease in order for the phase transition to occur.