Gases in Solution

1. Gases in Solution • In general, the solubility of gases in water increases with increasing mass. • Larger molecules have stronger dispersion forces.

2. Pressure Effects • The solubility of liquids and solids does not change appreciably with pressure. • The solubility of a gas in in any solvent is directly proportional to its pressure.

3. Effect of pressure on gas solubility Dynamical equilibrium is achieved when the rate at which the gas enters the solution equals the rate at which it leaves the solution If we apply pressure on the gas above the solution As the pressure increases, rate at which the gas molecules strike the solution surface to enter the solution phase would increase. The solubility of the gas will increase until the dynamical equilibrium is again established. The solubility of a gas increases in direct proportion to it’s partial pressure above the solution according to the following equation Henry's Law

4. Henry’s Law Where, Sg is the solubility of the gas; k is the Henry’s law constant, it is different for each solute-solvent pair. It also varies with temperature Pg is the partial pressure of the gas above the liquid.

5. The solubility of N2 gas in water at 25 oC and 0.78 atm pressure is An Example: Calculate the Henry’s constant and predict the solubility of N2 gas in water at 1.56 atm pressure At 25 oC and P = 1.56 atm (pressure is doubled) Or When the pressure of the gas is doubled, the solubility is doubled

6. Applications of Effects of Pressure on Solubility of Gases What is going on? Bottles and cans of carbonated soft drinks and beer are sealed under carbon dioxide pressure greater than a 1 atm. When a bottle is opened to the air, the partial pressure of CO2 gas above the solution is reduced. Hence, the solubility of CO2 decreases and the CO2 gas bobbles out of the solution.

7. Deep see diving is fun but it could be dangerous if you are not aware of the chemistry and the rules!

8. The solubility of gases increases with increasing pressure so divers who breathe compressed air have to be concerned about the solubility of gases in their blood. Deep-sea divers must ascend slowly to prevent dissolved gases from being released rapidly from blood and other fluids in the body. These bubbles affect nerve impulses and give rise to the affliction known as decompression sickness or “the bend “, which is painful and can be fetal. Nitrogen gas is the main problem because it has the highest partial pressure and it can only be removed through the respiratory system whereas oxygen , in contrast, is consumed in metabolism Sometimes divers replace nitrogen gas with helium? Helium gas is much less soluble in biological fluid than nitrogen

9. Temperature Effects on Solubility of Ionic Compounds in Water Generally, the solubility of solid solutes in liquid solvents increases with increasing temperature. Very slight change exception Solubilities of several ionic compounds in water as a function of temperature

10. Temperature Effects on Gas Solubility in Water • Solubility of gases in water decrease with increasing temperature • Carbonated soft drinks are more “bubbly” if stored in the refrigerator whereas they go flat as they are allowed to warm • Warm lakes have less O2 dissolved in them than cool lakes (Thermal pollution of lakes). Variation of gas solubility with temperature

11. Gases are emitted from the cooking pot surfaces as it is heated. • Dissolved gases are less soluble in solution as temperature increases. • Water molecules begin to enter the gas phase to stimulate boiling. • Boiling actually begins on a small scale at temperatures below the boiling point.

12. Gases are emitted from the cooking pot surfaces as it is heated. • Dissolved gases are less soluble in solution as temperature increases. • Water molecules begin to enter the gas phase to stimulate boiling. • Boiling actually begins on a small scale at temperatures below the boiling point.

13. Ways of Expressing Concentrations of Solutions Concentration of solution can be expressed either qualitatively or quantitatively Qualitatively Dilute solution: a relatively small amount of solute is dissolved in a solvent Concentrated solution: a large amount of solute is dissolved in solvent • Quantitatively: • The most common ways of expressing concentration of solutions are • Mass Percentage, ppm, and ppb • Mole Fraction, Molarity, and Molality

14. mass of A in solution total mass of solution Mass Percentage, ppm, and ppb Mass Percentage Mass % of A = The mass percentage of a component in a solution is given by  100 If we have a solution of hydrochloric acid that is 36% HCl by mass, how many grams of HCl in a 100 g of solution? Answer: 36 g

15. Determine the mass percentage of hexane in a solution containing 11 g of butane in 110 g of hexane. 9.0 % 10. % 90.% 91 %

16. Answer 9.0 % 10. % 90.% 91 % mass of component in solution =  mass % of component 100 total mass of solution Thus, 110 g (110 g + 11 g)  100 = 91%

17. mass of A in solution total mass of solution mass of A in solution total mass of solution Parts per Billion (ppb) For even more dilute solutions, we define the concentration as ppb (number of grams of solute per billion (109) grams of solvent)  109 ppb = Parts per Million (ppm) and Parts per Billion (ppb) Parts per Million (ppm) For very dilute solutions, we define the concentration as ppm (number of grams of solute per million (106) grams of solvent) ppm =  106

18. Useful Unit Conversion Most of pollutants are measured in units of ppm