Chlorophile
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Why does the solubility of gasses (specifically carbon dioxide) in water increase with decreasing temperature? I've not found a better answer than "carbon dioxide has a greater affinity for water at lower temperatures" which obviously isn't a great explanation, but I would really like to understand it.
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RoyalBlue7
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This is more of a chemistry question than a physics one. We learn about the coefficient of solubility (if I recall correctly) in the equilibrium process where a particular gas dissolves in a solvent (usually water), in unit 4 Edexcel Chemistry.

Depending on the reaction (the number and nature of bond breaking and bond forming), the forward reaction (dissolving) may be exothermic or endothermic. Thus the position of equilibrium may shift with a change in temperature.

Whether it actually dissolves or not would depend on the total entropy change. The rate of the forward would depend on the enthalpy change and the temperature.

If I'm correct (correct me if I m wrong) the dissolving of oxygen in water is exothermic. So an increase in temperature would favour the backward reaction; the oxygen concentration would decrease with temperature. I guess this can be used as evidence for global warming, after measuring oxygen levels over a period of time or in layers of ice in the poles.

+ grain of salt
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Chlorophile
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(Original post by RoyalBlue7)
This is more of a chemistry question than a physics one. We learn about the coefficient of solubility (if I recall correctly) in the equilibrium process where a particular gas dissolves in a solvent (usually water), in unit 4 Edexcel Chemistry.

Depending on the reaction (the number and nature of bond breaking and bond forming), the forward reaction (dissolving) may be exothermic or endothermic. Thus the position of equilibrium may shift with a change in temperature.

Whether it actually dissolves or not would depend on the total entropy change. The rate of the forward would depend on the enthalpy change and the temperature.

If I'm correct (correct me if I m wrong) the dissolving of oxygen in water is exothermic. So an increase in temperature would favour the backward reaction; the oxygen concentration would decrease with temperature. I guess this can be used as evidence for global warming, after measuring oxygen levels over a period of time or in layers of ice in the poles.

+ grain of salt
Since CO2 solubility increases with decreasing temperature, it would certainly make sense that the process is exothermic. I would be very interested, however, in understanding why it's exothermic when the process of dissolving virtually all ionic compounds is endothermic.

I've not heard of actually measuring oxygen levels as a proxy for temperature, especially since I'm fairly certain the annual oscillation in oxygen concentration because of photosynthesis will be greater than any effect of greater solubility. However, oxygen isotopes can be used as a proxy for temperature. Oxygen-18 rich carbonates indicate greater ice coverage.
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RoyalBlue7
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(Original post by Chlorophile)
Since CO2 solubility increases with decreasing temperature, it would certainly make sense that the process is exothermic. I would be very interested, however, in understanding why it's exothermic when the process of dissolving virtually all ionic compounds is endothermic.

I've not heard of actually measuring oxygen levels as a proxy for temperature, especially since I'm fairly certain the annual oscillation in oxygen concentration because of photosynthesis will be greater than any effect of greater solubility. However, oxygen isotopes can be used as a proxy for temperature. Oxygen-18 rich carbonates indicate greater ice coverage.
Yes, my teacher made it clear that exothermic dissolving is not very common for ionic compounds. But for gases - most (I guess all) do dissolve better in cold water than warm water. This explains why the increase in concentration of CO2 in the atmosphere facilitates a positive feedback as it thermally pollutes (both air and sea) - and causes CO2 to dissolve out of the warmer water. This happens more in the tropics than in the polar regions. In the polar regions the oceans acts as carbon sinks - the colder water absorbs more CO2 from the atmosphere and then transports it down by convection (colder water is more denser).

I think its clear why the dissolving OUT of gases is endothermic (rather than exothermic in the case of ionic solids). The dissolved gas forms stronger bonds with the water molecules than they do with their own molecules (its easy to realise why*). So the breaking of these bonds to free the gas surely requires energy.


*The reason as to why CO2 molecules has more affinity to the water molecules than to their own molecules could be easily explained by the stronger inter-moleculer bonds they form with water. Water is more polar than CO2.

Well, you're right with this oxygen case. I confused it with temperature. It is more of an indicator to the rate of photosynthesis in water bodies, despite the small changes due to temperature changes. (When studying eutrophication).
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charco
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(Original post by Chlorophile)
Since CO2 solubility increases with decreasing temperature, it would certainly make sense that the process is exothermic. I would be very interested, however, in understanding why it's exothermic when the process of dissolving virtually all ionic compounds is endothermic.

I've not heard of actually measuring oxygen levels as a proxy for temperature, especially since I'm fairly certain the annual oscillation in oxygen concentration because of photosynthesis will be greater than any effect of greater solubility. However, oxygen isotopes can be used as a proxy for temperature. Oxygen-18 rich carbonates indicate greater ice coverage.
You can't compare dissolving gases (molecular) with ionic crystals (giant structures).

When an ionic substance dissolves a huge amount of energy is needed to break the giant lattice. Most of this is compensated for by the high hydration energy of the ions. The deciding factor is the increase in entropy from ionic crystal to solution.

However, all of the above is generalisation and we know that some ionic substances are insoluble.

Gases do not have any interparticulate forces to overcome. They lose a large amount of entropy going from gas to solution.

Both processes are governed by Gibbs free energy.

ΔG = ΔH - TΔS

For something to be soluble ΔG must be negative, the more negative the greater the solubility.

In the case of ionic solids ΔS is positive and increasing temperature makes ΔG more negative.

In the case of gases ΔS is negative and increasing temperature makes ΔG less negative (more positive)
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Chlorophile
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(Original post by charco)
You can't compare dissolving gases (molecular) with ionic crystals (giant structures).

When an ionic substance dissolves a huge amount of energy is needed to break the giant lattice. Most of this is compensated for by the high hydration energy of the ions. The deciding factor is the increase in entropy from ionic crystal to solution.

However, all of the above is generalisation and we know that some ionic substances are insoluble.

Gases do not have any interparticulate forces to overcome. They lose a large amount of entropy going from gas to solution.

Both processes are governed by Gibbs free energy.

ΔG = ΔH - TΔS

For something to be soluble ΔG must be negative, the more negative the greater the solubility.

In the case of ionic solids ΔS is positive and increasing temperature makes ΔG more negative.

In the case of gases ΔS is negative and increasing temperature makes ΔG less negative (more positive)
But if gasses are more stable dissolved as a liquid, why won't all of the gas dissolve immediately until saturation point?
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charco
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(Original post by Chlorophile)
But if gasses are more stable dissolved as a liquid, why won't all of the gas dissolve immediately until saturation point?
At any given temperature the gas reaches an equilibrium with its solution. The rate of attainment of equilibrium depends on kinetic factors.

If the gas is left in contact with the liquid it does dissolve up to saturation, which is the maximum solubility of the gas at that temperature.
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RoyalBlue7
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(Original post by charco)
At any given temperature the gas reaches an equilibrium with its solution. The rate of attainment of equilibrium depends on kinetic factors.

If the gas is left in contact with the liquid it does dissolve up to saturation, which is the maximum solubility of the gas at that temperature.


If its a system of dynamic equilibrium, then would it be acceptable to explain the change in concentration of the gas in the solvent with temperature using the concept that a change in temperature would shift the position of equilibrium?
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charco
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(Original post by RoyalBlue7)
If its a system of dynamic equilibrium, then would it be acceptable to explain the change in concentration of the gas in the solvent with temperature using the concept that a change in temperature would shift the position of equilibrium?
Don't see why not ...
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