Effect of pressure on solubility

The dissolution of anhydrous calcium chloride, CaCl2, in water is quite exothermic. Here the temperature increases from 21°C to 88°C. This dissolution process is utilized in commercial instant hot packs for quick treatment of injuries requiring heat. 

Henry s Law applies to gases that do not react with the solvent in which they dissolve (or, in some cases, gases that react incompletely). It is usually stated as follows. 

The concentration or solubility of a gas in a liquid at any given temperature is directly proportional to the partial pressure of the gas over the solution. 

Pgas is the pressure of the gas above the solution, and is a constant for a particular gas and solvent at a particular temperature. Cg s represents the concentration of dissolved gas it is usually expressed either as molarity (see Section 3-6) or as mole fraction (see Section 14-8). The relationship is valid at low concentrations and low pressures. 

Carbonated beverages can be nsed to dinstrate Henry s Law. When tbe bottle is opened, tbe eqnilibrinm is distnrbed and bubbies of CO2 foroi witbin tbe liqnid and rise to tbe surface. After some time, an equilibrium between dissolved CO2 and atmospheric CO2 is reached. 

The influence of pressure on solubility is expressed by Henry s law, which states that 

Rather than considering equation (2.17) as a means of expressing the solubility of a gas in terms of vapour pressure, we could also view it as a way of expressing the vapour pressure developed by a given concentration of dissolved gas, as explained in Box 2.2. 

In practice, Henry s law explains the often violent release of gas that occurs as a consequence of the decrease of solubility of a gas when the pressure above the gas is released suddenly, for example when the cap of a bottle of sparkling water is unscrewed quickly. 

The application of Henry s law in the calculation of the effect of pressure on the solubility of gases in liquids is illustrated in Example 2.5. 

The solubility of oxygen in water at a partial pressure of 25 torr is 8.31 mg dm at 25°C. Calculate the solubility if the partial pressure is increased to 100 torr at the same temperamre. 

If you push down on the piston, the gas volume decreases, its pressure increases, and gas particles collide with the liquid surface more often. Thus, more gas particles enter than leave the solution per unit time. In other words, higher gas pressure disturbs the balance at equilibrium. But, like any system at equilibrium. 

Henry s law expresses the quantitative relationship between gas pressure and solubility the solubility of a gas is directly proportional to the partial pres- 

SAMPLE PROBLEM 13.2 Using Henry s Law to Calculate Gas Solubility 

Problem The partial pressure of carbon dioxide gas inside a bottle of cola is 4 atm at 25°C. What is the solubility of CO2 The Henry s law constant for CO2 dissolved in water is 3.3X 10 mol/L-atm at 25°C. 

Plan We know Pco (4 atm) and the value of (3.3X10 mol/L-atm), so we substitute them into Equation 13.3 to find C02- 

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Figure 12. Effect of pressure on solubility (in units of mole fraction) of diamantane in dense (supercritical) gases at 333 K (for carbon dioxide and ethane) and at 353 K (for methane). Data from Ref. [35].

Figure 9J6 Effects of pressure on solubility of argon in silicate melts. From White et al. (1989). Reprinted with permission of The Mineralogical Society of America.

FACTORS THAT AFFECT SOLUBILITY Effect of Pressure on Solubility... 

Effect of Pressure on Solubility 203 Effect of Temperature on Solubility 204 Colligative Properties of Solutions 205 Vapor Pressure 205 Boiling Point 207 Freezing Point 208 Osmotic Pressure 209 Colloids 212 Review Questions 213... 

When you open a carbonated drink, you can observe the effect of pressure on solubility. Figure 8.13 shows this effect. Inside a soft drink bottle, the pressure of the carbon dioxide gas is very high about 400 kPa. When you open the bottle, you hear the sound of escaping gas as the pressure is reduced. Carbon dioxide gas escapes quickly from the bottle, since the pressure of the carbon dioxide in the atmosphere is much lower only about 0.03 kPa. The solubility of the carbon dioxide in the liquid soft drink decreases greatly. Bubbles begin to rise in the liquid as gas comes out of solution and escapes. It takes a while for all the gas to leave the solution, so you have time to enjoy the taste of the soft drink before it goes flat. ... 

Do you crack your knuckles The sound you hear is another example of the effect of pressure on solubility. Joints contain fluid. When a joint is suddenly pulled or stretched, the cavity that holds the fluid gets larger. This causes the pressure to decrease. A bubble of gas forms, making the sound you hear. You cannot repeatedly crack your knuckles because it takes some time for the gas to re-dissolve. 

The discussion in the preceding paragraph may now be used to examine the effect of pressure on solubility that is to say on conditions away from the eutectic point. We now have crystals of 2 in equilibrium with a solution consisting of 1 and 2. We need now only consider the second equation of (22.24). Under isothermal conditions... 

The first experimental determinations of the effect of pressure on solubility were made by E. von Stackelberg, who obtained values which are in qualitative agreement with Braun s law and the theorem of Le Chatelier, as the following table shows —... 

Eq. (1.43) is the most general form of the solubility equation. In most situations (though not all) the effect of pressure on solubility is negligible so that the last term on the right-hand side of the equation can be dropped. In addition, the heat capacity term can also usually be dropped from the equation. This yields... 

Bottlers use the effect of pressure on solubility in producing carbonated beverages, which are bottled under a carbon dioxide pressure greater than 1 atm. When the bottles are opened to the air, the partial pressure of CO2 above the solution decreases. Hence, the solubility of CO2 decreases, and C02(g) escapes from the solution as bubbles (4 FIGURE 13.16). 

The effect of pressure on solubility was calculated independently by F. Braun and J. J. Thomson. ... 

Pouring root beer into a glass illustrates the effect of pressure on solubility. The escaping CO2 produces the foam. 

The effect of pressure on solubility plays a prominent role only where gases are involved and has little effect in the cases of liquids and solids. 

Le Chatelier s principle was introduced in Section 12.3, where it was used to determine the effect of pressure on solubility of gases in liquids. 

The phase behaviour of water and carbon dioxide has been studied by various authors, so literature data in wide ranges of temperature and pressure is available. Selected data for the relevant pressure and temperature conditions are presented in Fig. 15.9 [15,16]. CO2 has the lowest solubility in water barely exceeding 6 wt% at 313 K and 20.3 MPa. The effect of pressure on solubility weakens above 5 MPa as depicted by the near vertical isotherm progress. The composition of the light phase shows around 4 wt% water vapour at 1.1 MPa and 373 K. 

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