1
$\begingroup$

Is an endothermic reaction always endothermic at all conditions of pressure, temperature etc.? Similarly for exothermic... I mean is sign of ∆H always the same as sign of ∆H°?

This might seem to be a vague doubt but I just feel that there can be a reaction with trends of energies of formation of reactants and products such that endothermic reaction changes to exothermic or vice versa subject to change in concentrations of substances or external factors like temperature, pressure etc.

This is just a random thought of mine and even an example in this direction would be helpful.

Thanks!

| improve this question | | | | |
$\endgroup$
  • $\begingroup$ There are no known example of reactions changing from exothermic to endothermic reaction when changing the conditions of pressure or temperature. $\endgroup$ – Maurice Mar 7 at 20:34
  • 1
    $\begingroup$ Depends on what one considers a reaction, or more precisely is it a single specific reaction or whole process. $\endgroup$ – Mithoron Mar 8 at 1:19
  • 2
    $\begingroup$ @Maurice That's a bold claim. Do you have anything to support it? $\endgroup$ – theorist Mar 8 at 6:50
3
$\begingroup$

Because $\Delta H$ is temperature-, pressure-, and concentration-dependent, it's certainly theoretically possible for $\Delta H$ for a chemical reaction to change sign in response to changes in any of those intensive properties.

I don't have an example of such a chemical reaction handy. However, there are direct experimental measurements showing enthalpies of solution that change sign in response to both temperature and concentration changes:

For instance, the enthalpy of solution of polystyrene in decalin has been shown to change from ~ –10 J/g at ~20 C to ~ +6 J/g at ~100 C (the ~'s were needed because I'm eyeballing these from a plot). Reference:

Tager, A. A., et al. "The temperature and concentration dependence of enthalpy, free energy and entropy of mixing polystyrene solutions with an upper and lower critical temperature of mixing." Polymer Science USSR 12.6 (1970): 1497-1511.

And the enthalpy of solution of $\ce{LiNO_3}$ has been shown to change from negative to positive as the concentration increases. Reference:

Wolf, G., V. Vacek, and V. Pekarek. "Enthalpies of solution and of crystallization of lithium nitrate and of lithium nitrate trihydrate in water at 25° C." Journal of Solution Chemistry 19.10 (1990): 1029-1039.

| improve this answer | | | | |
$\endgroup$
  • $\begingroup$ Thank you for the examples. Does this also have any useful implications other than this factual information? $\endgroup$ – AbsoluteZero Mar 8 at 6:47
  • $\begingroup$ It could have practical implications in process chemistry, since the sign and magnitude of $\Delta H$ determine the temperature-dependence of the equilibrium constant. So it's useful to know that, within certain concentration or temperature ranges, increasing the temperature will favor the forward reaction while, at other concentrations or temperatures, increasing the temperature will favor the reverse reaction. $\endgroup$ – theorist Mar 8 at 7:25

Not the answer you're looking for? Browse other questions tagged or ask your own question.