Greetings, I am doing a question with a solution I don\'t understandconceptually

Question

Greetings,

I am doing a question with a solution I don't understandconceptually. I will leave some of the information out of thequestion because it is very long and you won't need it as I justwant an explanation of the solution I will post broken down intosimpler terms.

The question states: Which best concludes the relationship betweenthe solubility of a gas and the partial pressure of the gas giveninformation in a passage. And which determines the sign of the heatof solution of CO2 in water?

The solution states: Equilibrium is disturbed by thedecrease in the CO2 partial pressure above the carbonated water andthat, therefore, some of the dissolved CO2 bubbles out of thecarbonated water. It would be best concluded from thisview that as the pressure of gas decreases, the solubilitydecreases, so the solubility of gas is directly proportional toPressure of gas.

^^ I don't understand how that is derived from the firstsentence--that is bolded, to determine that it is a directproportional relationship?

Next, it says for the next part of the question that the solubilityof CO2 in water decreases as the temperature increases. It would bebest concluded from this statement that the dissolution of CO2 inwater is exothermic with negative delta H of solution. I don'tunderstand this as I thought it would be endothermic since temp isincreasing---wouldn't that mean that it is taking in heat(endothermic)?

Thanks so much to someone who can break down the solution intosimpler terms and possibly give an example of some sort to relateto this. I will rate lifesaver.

Explanation / Answer

Remember Le'Chatlier's principle? The equilibrium isdisturbed, so the system responds in such a way to minimize thedistrubance; to offset the change. That's what's happening when theequilibrium is disturbed. Solubility of gasses is based onpressure, as other things are based on concentration, like when youwrite the law of mass action for something it's molarity, when youare talking about gasses it's written in terms of pressure (bars).So, the relationship between solubility and partial pressure is thesame as the relationship between solubility and concentration (of,an aqueous solution, for example). for the reaction Ksp=[CO2]
Dalton's Law ofPartial Pressures was formulated to calculate individual gaspressures when the pressure is known for a mixture of gasses! Is itliquid water in this problem??? I'm going to assume it is anypartial pressure here is used to calculate volume of gas collectedOVER water. In this case, you also have to use the pressure exerted by thewater vapor: PT=PCO2+PH2O In this case, you also have to use the pressure exerted by thewater vapor: PT=PCO2+PH2O The sign of the heat of solution should be related topressure by PV=nRT, so the relationship between pressureand temperature is direct. Think of a aerosol can, when you sprayit, it feels cold; as the pressure decreases,the temperature decreases.   Another thing isthe CO2 gas dissolution is based on pressure, not necessarilythe temperature. One more thing, remember molecular collisiontheory? If there is more pressure, does the chance of a molecularcollision increase? Anything that increases the incidence ofmolecular collision, increases solubility. Like temperature! Hope this helps. I'm not really sure why they're using partialpressures like this.

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