I'm struggling to interpret enthalpy of mixing data. Given this plot, enthalpy of mixing of ethanol and water from this paper, say I want to calculate the heat released from mixing 0.29 mol of 7.7% ethanol with 0.71 mol of 28.7% ethanol to get 1 mol of 22.6% ethanol.

Is it correct to say $\Delta h_m = 753 - (0.29*605 + 0.71*672) = 100 J/mol$?

Or, in other words, can I subtract the molar weighted enthalpies of mixing at the initial states from the enthalpy of mixing at the final state to get the change of enthalpy of mixing from that process?

It seems trivial, but something about it feels weird.

  • $\begingroup$ Consider it this way: You un-mix each of the two solutions, meaning you must undo the enthalpy of mixing for each. Now remix the combined solutions, getting out the enthalpy of mixing for the whole thing. And, watch you signs... $\endgroup$ – Jon Custer Aug 28 '15 at 20:47

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