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# Heat of Solution

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Because heating decreases the solubility of a gas, dissolution of gases is exothermic. Consequently, as a gas continues to dissolve in a liquid solvent, temperature will decrease, while the solution continues to release energy.

I'm confused, if heating decreases the solubility of a gas... and if the temperature is continuing to decrease, how the heck is it releasing energy? According to the AAMC, this was a 'difficult' concept. I thought about it for quite a bit and got the question related to it wrong. Grrr.

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You have the concept wrong, but you should be glad to know the blame is squarely aimed at that horrendous "explanation" you were given.

X(gas) -> X(aq) + heat

So gas going into liquid, i.e. dissolution of a gas is exothermic

X(aq) + heat -> X(gas)

This is essentially what they mean when they say heat decreases the solubility of a gas (it makes it more likely for a gas to go out of solution), which is endothermic.

Or in other words, dissolution of gases = increasing the solubility of a gas.

So their stupid explanation is essentially:

because heating decreases the solubility of a gas and is therefore endothermic, then increasing the solubility of a gas (dissolution) must be exothermic.
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Ah thank you so much. That makes sense.

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I'm confused, if heating decreases the solubility of a gas... and if the temperature is continuing to decrease, how the heck is it releasing energy? According to the AAMC, this was a 'difficult' concept. I thought about it for quite a bit and got the question related to it wrong. Grrr.

Breaking bonds uses energy while forming bonds release energy. When a solute dissolves in solvent, you break solute solute bonds (almost nonexistent for a gas) and form solvent solute bonds. Therefore, the process of dissolving gas is exothermic. There are two factors which contribute to gas solubility. The enthalphy/entrophy of the reaction, and the gas gradient that initially exists between the atmosphere and the liquid (with no gas in it). At first, the enthalpy (exothermic), the entrophy (always positive for producing mixtures) and the solubility gradient all point towards exothermic and a negative gibbs free energy (exogonic as well). As more gas is dissolved, it is harder and harder for the gas molecules to find appropriate solvent bonds, so the enthalphy becomes less negative, but both the entrophy and the gradient can push it on (though less exothermic, is still exogonic overall). Eventually, there will be a point where the enthalphy change becomes positive (endothermic, so temperature will drop), but the entrophy is still positive and the gradient might be high enough so that the process is still exogonic overall for gibbs free energy. the dissolving of gas will stop only when the net gibbs free energy is 0 (from a increasing enthalpy and decreasing gradient). This is the same reasoning for why a endothermic salt dissolving in water can still proceed despite lowering temperature. The entropy and the solubility gradient means a net negative gibbs free energy (exogonic overall, which is all that matters for a spontaneous reaction, NOT exothermic)

Hope that makes sense!

Princeton instructor to the rescue!

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