Le Chatelier's principle
position of equilibrium
However, other gases in other solvents do not necessarily become less soluble at high temperature . 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 sugarThe 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 + heatLe 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 :
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?
Author: Fred Senese firstname.lastname@example.org
Copyright © 1997-2010 by Fred Senese
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Last Revised 08/17/15.URL: http://antoine.frostburg.edu/chem/senese/101/solutions/faq/print-temperature-gas-solubility.shtml