I know this is more of a chemistry question but it's come to this I'm afraid. When calculating entropy changes in chemical reactions sometimes we students of chemistry use standard molar entropies of the reactants and products. But this surely doesn't account for the mixing effect. e.g. N2 + 3 H2 --> 2 NH3 calculating the entropy change reaction for this using standard molar entropies would surely yield an entropy change too large because the reactants would in fact start off mixed, thus at a higher entropy than just the N2 and 3 H2 together.