A simplification of gas clathrate hydrate thermochemistry using the Thermodynamic Difference Rule (TDR). Part 1. Generation of particularly simple forms for standard thermodynamic parameters for specific hydrates

The Thermodynamic Difference Rule (TDR) adopts a particularly simple form when the enthalpy of formation of the parent atom/molecule within a clathrate hydrate is in its standard reference state at 298 K and 101.3 kPa (1 atm). In this case the simple form: Delta H-f(o)=kn emerges for the standard formation data, where k is a numerical constant (=Theta(Hf)(H2O, s-g)) and n is the number of moles of water present in the formula unit of the clathrate hydrate. The specific form is are: Delta H-f(o)(MpXq center dot nH(2)O, s)/kJ mol(-1) approximate to Theta(Hf)(H2O, s-g)n=-296.1 n which is capable of offering predictions for standard thermodynamic data for a whole host of clathrate hydrates for which there isnt much or any experimental values. Moreover these data are predicted almost to within experimental error. Another new result emerges, not previously reported, in that linear relationships with n are also found to exist for Delta H-diss for the dissociation process: MpXq center dot nH(2)O(s)-> nH(2)O(l)+MpX(q)(g) and Delta H-diss' for the process: MpXq center dot nH(2)O(s)-> nH(2)O(s,ice)+MpXq(g) such that: Delta H-diss/kJ mol(-1) approximate to 10.3 n and Delta H-diss/kJ mol(-1) approximate to 4.3 n. Since no data exist for Delta(f)G(o)(MpXq center dot nH(2)O,s) or S-298(o)(MpXq center dot nH(2)O,s) in the literature, we further conjecture that, in accordance with previous studies, the equations: Delta(f)G(o)(MpXq center dot nH(2)O,s)/kJ mol(-1) approximate to Theta(Gf)(H2O,s-g)n=-242 n and S-298(o)(MpXq center dot nH(2)O,s)/JK(-1) mol(-1) approximate to Theta(So)(H2O,s-g)n=41 n. could reasonably be anticipated to predict these unknown values for which no data are yet reported. The ability of TDR to tease out simple empirical relationships which have not been noticed previously amounts to a considerable simplification of this area of thermochemistry and provides the means of quick access estimation of (accurate) standard thermodynamic parameters for clathrate hydrates in the category studied here. (C) 2017 Elsevier Ltd.