Need help with Heat of Solution...

[jayel]

I'm having trouble understanding why the heat of solution is exothermic for the release of dissolved CO2 in a can of cold soda being opened at room temp.

I thought that the heat of solution would be endothermic [CO2(aq) + heat <--> CO2(g), equation I came up with] since there is a threshold of KE needed to escape the intermolecular forces on the CO2 molecules by the water and other soda related molecules, so it can become part of the air.

However, the AMCAS MCAT guide(2010 version, p.92) says that "the solubility of CO2 in water decreases as temperature increases. It would be best concluded from this statement that the dissolution of CO2 in water is exothermic..." I get the rationale, and thinking about the formula with the LeChatlier's principle makes it seem to me the explanation is proving that it's endothermic, not exothermic!

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[startswithb]

I think a little bit of intuition helps with this one. Think about a can of soda and what happens when you open it. You know the CO2 escapes because you can hear it. If it was endothermic, that would require heat being added, and where would that come from?

You can also check yourself by using the Gibbs free energy equation.

G=H-TS. The reaction is spontaneous and S is + because the gas is released from solution. For G to remain -, H would have the also be - unless the process was at a high T, which it is not (room temp). -H means exothermic.

(-) = (?) - (T) (+)

Hope this helps.

 

[Phantastic]

I thought that the heat of solution would be endothermic [CO2(aq) + heat <--> CO2(g), equation I came up with] since there is a threshold of KE needed to escape the intermolecular forces on the CO2 molecules by the water and other soda related molecules, so it can become part of the air.

If they were in equilibrium, disrupting equilibrium would require the input of energy. However, the CO2 has been "forced" into the solution, much like a spring that has been compressed. Equilibrium exists inside the can before you open it, but conditions favor evolution of CO2 gas from the can once opened.

I think this has something to do with Dalton's law of partial pressures...but I forget.

 

[jayel]

I get it now, thanks for the help. It always helps when someone else explains it!