Why does the solubility of gases usually increase as temperature goes down?

Le Chatelier's principle*
position of equilibrium*
saturated solution*
Gases are usually more soluble at colder temperatures. For example, oxygen is more soluble in cold water than in hot water. The decrease in oxygen solubility with increased temperature [1] has serious consequences for aquatic life. Power plants that discharge hot water into rivers can kill fish by decreasing the dissolved oxygen concentration.

However, other gases in other solvents do not necessarily become less soluble at high temperature [2]. H2, N2, CO, He, and Ne solubility actually rises with rising temperature in common organic solvents like carbon tetrachloride, benzene, and acetone. Even in water, the solubilities of H2, N2, and He fall and then rise with rising temperature, when the water is under high pressure or at high temperatures.

Why does solubility change with temperature? Consider a beaker that contains a saturated solution of table sugar. The bottom of the beaker is covered with sugar crystals. When a tiny amount of sugar dissolves, heat is absorbed. When a tiny amount of sugar crystallizes out of solution, heat is released. We can write [3:

heat + solid sugar + water = dissolved sugar

The equation represents two processes: dissolution going left to right, and crystallization going right to left. When the sugar crystals are dissolving at exactly the same rate that sugar is crystallizing out of solution, the system is at equilibrium. The balance between dissolution and crystallization can be changed by changing the temperature of the solution. Adding heat will favor dissolution. Cooling the solution will favor crystallization.

The temperature dependence of solubility is usually explained using Le Chatelier's principle. The principle states that when a system at equilibrium is placed under stress, the equilibrium will shift in a way that relieves that stress. In this case, the "stress" is the addition of heat. Le Chatelier's principle predicts that heating the solution mixture will shift the equilibrium in favor of dissolution, to remove the added heat. This explains why sugar is more soluble in hot water than in cold.

The same sort of analysis can be applied to solutions of gases. Dissolving oxygen in water releases a small amount of heat:

gaseous O2 + nearly saturated O2 solution = saturated O2 solution + heat

Le Chatelier's principle predicts that heating the solution shifts the equilibrium to the left- less oxygen dissolves at higher temperature.

A molecular model of gas solubility. The solubility of gases, like other solubilities, can increase or decrease with temperature. A simple model can be used to explain why gases can behave either way, depending on the gas and the solvent. The heat absorbed or released when a gas dissolves in liquid has essentially two contributions [4]:

  1. Energy is absorbed to open a pocket in the solvent. Solvent molecules attract each other. Pulling them apart to make a cavity will require energy, and heat is absorbed in this step for most solvents. Water is a special case- it already contains open holes in its network of loose hydrogen bonds around room temperature. For water, very little heat is required to create pockets that can hold gas molecules.
  2. Energy is released when a gas molecule is popped into the pocket. Intermolecular attractions between the gas molecule and the surrounding solvent molecules lower its energy, and heat is released. The stronger the attractions are, the more heat is released. Water is capable of forming hydrogen bonds with some gases, while organic solvents often can't. A larger amount of heat is released when a gas molecule is placed in the pocket in water than in organic solvents.
There is usually net absorption of heat when gases are dissolved in organic solvents, because the pocket-making contribution is bigger. Le Chatelier's principle predicts that when heat is absorbed by the dissolution process, it will be favored at higher temperature. Solubility is expected to increase when temperature rises.

There is usually net release of heat when gases are dissolved in water, because the pocket-filling contribution is biggest. Solubility is expected to decrease when temperature rises.

Can you use the model to explain why N2 gas solubility in water decreases until about 70° C, and then begins to increase?

References and Notes

  1. See Predicting Oxygen Solubility for a Javascript calculator that quantitatively shows how increasing temperature decreases oxygen solubility.
  2. See K. J. Mysels, "Textbook Errors: The Solubility of Gases in Liquids", Journal of Chemical Education, 32, 399 (1955).
  3. We're adding the sugar to a nearly saturated solution, rather than to pure water. The heat absorbed or released in forming a solution varies with concentration. Only the heat of solution at saturation is relevant when applying Le Chatelier's principle to explain temperature effects on solubility. For details, see R. Fernández-Prini, "Le Chatelier's Principle and the Prediction of the Effect of Temperature on Solubilities", Journal of Chemical Education, 59, 550-553 (1982).
  4. See D. D. Eley, Transactions of the Faraday Society, 35, 1281 (1939).

Author: Fred Senese senese@antoine.frostburg.edu

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