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In terms of energy per gram of reactants, the evaporation of water at -2.2 kJ/g, is much higher than any chemical endothermic reaction that I have seen demonstrated. At extreme temperatures there is no real limit, but for reactions that occur at a reasonable rate at room-temperature it seems water is hard to beat. And it isn't even a chemical reaction.

I have reason to think water is hard to beat: the entropy of gases is much higher than liquids. The strong hydrogen bond network of water makes the liquid phase lower entropy than non-polar liquids because it structures the liquid. Finally, water is a very light molecule so you get many moles of gas per kg of liquid. Despite this huge entropy gain, the phase-transition proceeds sluggishly because it takes so much dry air to shift the equilibrium forward, if it took much more energy it wouldn't happen at a meaningful rate.

Can any chemical reaction approach this at room temperature? If so, can any non-gas-evolving reaction come close?

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    $\begingroup$ Most, largest, biggest type questions over all chemicals aren't particularly interesting. First you end comparing apples and oranges. Second, there is always something else to discover. $\endgroup$
    – MaxW
    Commented Mar 12, 2020 at 3:10
  • $\begingroup$ Why is evaporation not a chemical reaction? Or do you just assume phase changes have nothing to do with chemistry? Seems odd... $\endgroup$
    – Jon Custer
    Commented Mar 17, 2020 at 16:06

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Don't know if these are quite in the spirit of what you are looking for since, with the exception of the first reaction, the entropy change alone is not sufficient to drive them (i.e., at room temp., their equilibria lie far to the left) (and, for ozone formation, the entropy change is actually unfavorable). Furthermore, ozone formation and photosynthesis aren't single reactions, but multi-step processes. But they all do take place at room temperature (or below):

Sublimation of ice (yes, snow can evaporate!): 2.8 kJ/g

Formation of ozone (O$_3$) from O$_2$ (photochemical, occurs in stratosphere): 3.0 kJ/g

Photosynthesis: 7.5 kJ/g

Electrolysis of water: 15.9 kJ/g

The values I've listed are all enthalpy changes (at constant pressure, the heat flow is equal to the change in enthalpy; at constant volume, the heat flow equals the change in energy). In addition, by convention, the sign of the enthlapy for endothermic reactions is positive, since heat flows into the system, so the enthalpy is increased (not negative, as you've used).

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One endothermic low-temperature reaction that gets a lot if use, seasonally, is salting of icy or snowy surfaces. The sodium chloride combines with the ice to form a liquid solution. Sodium chloride has little heat of solution either way with liquid water, but here the solid ice has to be converted to the liquid. Therefore we have a reaction that's roughly as endothermic as the latent heat of water melting (80 calories per gram), but does not require directly warming the temperature of the ice.

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