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I came across a question asking to indicate the enthalpy of solution for a solution of 1-pentanol in 1-butanol.

The options were the following:

  1. $\Delta H < 0$
  2. $\Delta H \approx 0$
  3. $\Delta H > 0$

I chose 3, which was wrong, as I thought that the $\Delta H_\text{solution}$ would be positive since the intermolecular forces in 1-pentanol would be slightly greater due to the extra methyl group, and as such, it would require more energy to break the IMFs between the 1-pentanol molecules than the energy released from the forming of IMFs between the 1-pentanol and 1-butanol. Which one should be the correct answer?

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  • $\begingroup$ Your reasoning is good whether wrong or right , it might depend on the relative amounts. read my comment below $\endgroup$
    – jimchmst
    Commented Aug 28, 2023 at 22:09
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    $\begingroup$ Dissolving a "big" molecule like 1-pentanol in 1-butanol is equivalent to suppressing a small proportion of the hydrogen bridges between neighboring ROH. The changes in van der Wals effects due to alkyl groups are negligible. So, because of the small changes in hydrogen bridges, the mixture should be a little endothermic. But I agree with Poutnik, the heat effect must be tiny, and near zero. $\endgroup$
    – Maurice
    Commented Aug 30, 2023 at 16:12
  • $\begingroup$ There most likely is a positive entropy of mixing, at equilibrium H =TS so both have the same sign. The reaction is endothermic whether one can measure it accurately is their problem not the test takers. A poor test question. $\endgroup$
    – jimchmst
    Commented Aug 30, 2023 at 18:03

1 Answer 1

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It is more about careful reading of the task info than about knowledge of chemistry.

Both molecules are almost identical, it may be hard to guess the enthalpy sign. But whatever sign the mixing enthalpy has, it will be close to zero.

Therefore. the right answer is $\Delta H \approx 0$, regardless of if it is positive, negative or if its difference from zero is statistically insignificant.

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  • $\begingroup$ This is another question that if one actually knows the answer one is doomed to get it wrong Either 1 or 3 MUST be correct but the answer is two because the test writer is ignorant. $\endgroup$
    – jimchmst
    Commented Aug 28, 2023 at 22:02
  • $\begingroup$ Option 2 will be correct in any case too $\endgroup$
    – Poutnik
    Commented Aug 29, 2023 at 3:50
  • $\begingroup$ No option 2 is politically correct The question is just inappropriate. $\endgroup$
    – jimchmst
    Commented Aug 29, 2023 at 21:30
  • $\begingroup$ If the confidence interval of Delta H covers 0...... $\endgroup$
    – Poutnik
    Commented Aug 30, 2023 at 4:08

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