partial pressure and temperature

[riseNshine]

Hi guys, can someone clarify the relationship between partial pressure, temperature, and solubility.

For gases, as you increase the temperature, vapor pressure increases so solubility decreases. However, an increasing partial pressure for gases means increasing solubility. Does this mean partial pressure increases as temperature decreases?

Also, how does this apply to liquids since the partial pressure of liquid increases as temperature increases?

Thanks

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[ClemsonFirst]

Another way to think of it is this: solids and liquids are fairly stationary. Their molecules are vibrating around in place and heating them up does not change their speed very much. They will still be contained in solution if thermodynamically favorable. Gas molecules are not as stationary and flying around in space at a much greater speed. Since they are moving faster, it is more difficult to trap them in solution. If you heat them up, you are just giving the gas more energy to move faster making it more difficult to trap resulting in decreased solubility.

Temperature would increase partial pressure. Dissolving a gas is an exothermic process, so the addition of heat will push the equilibrium to the reactants (out of solution). My guess is while partial pressure does increase, the rise in temperature has a greater effect on the equilibrium than the partial pressure, which results in a shift out of solution.

 

[mehc012]

I'm fairly certain that partial pressure and temperature are two unrelated entities. As temperature increases, partial pressure is unaffected because the total number of moles for that particular gas doesn't change. Likewise, if you increase the partial pressure of a gas, ideally the temperature doesn't change, and that is how they should present it in most problems.

I think what you meant to say was solubility of liquids. This is why we have two separate rules. If you dissolve a solid or liquid in a liquid, the solubility goes up as temperature increases because it is easier for those molecules to disperse in the solvent. The gas does not have as strong intermolecular forces, so when you heat it up, it is much easier for the gas to escape the liquid solvent. This is the reason why increasing temperature increases solubility of solids and liquids and decreases solubility of gases.

Another way to think of it is this: solids and liquids are fairly stationary. Their molecules are vibrating around in place and heating them up does not change their speed very much. They will still be contained in solution if thermodynamically favorable. Gas molecules are not as stationary and flying around in space at a much greater speed. Since they are moving faster, it is more difficult to trap them in solution. If you heat them up, you are just giving the gas more energy to move faster making it more difficult to trap resulting in decreased solubility.

Make sure you learn the two rules as separate and good luck!

How would partial pressure not increase if temperature were raised? You can think of it two different ways - partial pressure is the pressure the gas would have if it were alone in the container, or partial pressure is the mole fraction times the total pressure of the container - and either way, if temperature increases while volume stays the same, partial pressure would increase.

I think the OP is asking about the relative magnitude of the effects of temperature on solubility, temperature on vapor pressure, vapor pressure on solubility, solubility on partial pressure, etc...which I have no good answer for, as not all of them go in the same direction.

 

[Endure]

Hi guys, can someone clarify the relationship between partial pressure, temperature, and solubility.

For gases, as you increase the temperature, vapor pressure increases so solubility decreases. However, an increasing partial pressure for gases means increasing solubility. Does this mean partial pressure increases as temperature decreases?

Also, how does this apply to liquids since the partial pressure of liquid increases as temperature increases?

Thanks

The scenario you're describing needs to be looked at with caution. What you're essentially talking about is the conflict between Henry and Raoult's Law, which is valid.

In the first scenario you describe, you are correct when referring to the solvent - that is, the component present in excess in your system. If you increase the temperature of the solvent, you increase its vapor pressure in accordance with the Clausius-Clapeyron relation. When the gas is the solute, its concentration in solution increases with the partial pressure of that solute over the solution (as predicted by Henry's law).

Think about it: will more gas particles be present in solution when it's compressed or when it's open? Similarly, if you increase the temperature of your solvent, are more or less molecules escaping from solution? All Raoult's Law is saying is that the more solvent you have relative to solute, the more your vapor pressure above the solution will reflect your solvent's (pure) vapor pressure.

I'm fairly certain that partial pressure and temperature are two unrelated entities. As temperature increases, partial pressure is unaffected because the total number of moles for that particular gas doesn't change. Likewise, if you increase the partial pressure of a gas, ideally the temperature doesn't change, and that is how they should present it in most problems.

Increasing temperature absolutely increases partial pressure. You're mixing up the questioned scenario with what happens when an additional gas is introduced to a mixture of gases.