Unit – II C

Solubility of Gases in Liquids:

Factors Affecting Solubility of Gases in Liquids:

Nature of gas and nature of solvent:

  • The gases which are easily liquefiable are relatively more soluble than dihydrogen and dioxygen.
  • The gases which are capable of undergoing a chemical reaction with the water are relatively more soluble in water than other solvents.

Effect of temperature:

  • The solubility of gases in liquids decreases with the rise in temperature. When dissolved, the gas molecules are present in the liquid phase and the process of dissolution can be considered similar to condensation and heat is evolved in this process.
  • We know that dissolution process involves dynamic equilibrium and thus must follow Le Chatelier’s Principle. As dissolution is an exothermic process, the solubility should decrease with the increase of temperature.

Effect of pressure:

  • The solubility of gases in liquids is greatly affected by pressure and temperature. The solubility of gases increases with the increase of pressure.
  • For a solution of gases in a solvent, consider a system as shown in following fig (a). The lower part is the solution and the upper part is the gaseous system at pressure p and temperature T. Assume this system to be in a state of dynamic equilibrium, i.e., under these conditions rate of gaseous particles entering and leaving the solution phase is the same.

  • Now increase the pressure over the solution phase by compressing the gas to a smaller volume [fig. (b)]. This will increase the number of gaseous particles per unit volume over the solution and also the rate at which the gaseous particles are striking the surface of the solution to enter it. The solubility of the gas will increase until a new equilibrium is reached resulting in an increase in the pressure of a gas above the solution and thus its solubility increases.

Henry’s Law:

  • Henry was the first to give a quantitative relation between pressure and solubility of a gas in a solvent which is known as Henry’s law.
  • The law states that at a constant temperature, the solubility (S) of a gas in a liquid is directly proportional to the pressure (P) of the gas.

S α  P

∴  S = KP
Where K = Henry’s constant

  • Dalton, a contemporary of Henry, also concluded independently that the solubility of a gas in a liquid solution is a function of partial pressure of the gas.
  • If we use the mole fraction of a gas in the solution as a measure of its solubility, then it can be said that the mole fraction of gas in the solution is proportional to the partial pressure of the gas over the solution. The most commonly used form of Henry’s law states that “the partial pressure of the gas in the vapour phase (p) is proportional to the mole fraction of the gas (x) in the solution” and is expressed as:

p  = KHx

Where KH is Henry’s law constant.

  • If we draw a graph of the partial pressure of the gas versus mole fraction of the gas in solution, then we should get a plot of the type as shown.

Solubility Graph

  • Different gases have different KH values at the same temperature. This suggests that KH is a function of the nature of the gas. From above equation, we can conclude that higher the value of KH at a given pressure, the lower is the solubility of the gas in the liquid.

Applications of Henry’s Law:

Henry’s law finds several applications in industry and explains some biological phenomena.

  1. To increase the solubility of CO2 in soft drinks and soda water, the bottle is sealed under high pressure.
  2. Scuba divers must cope with high concentrations of dissolved gases while breathing air at high pressure underwater. Increased pressure increases the solubility of atmospheric gases in the blood. When the divers come towards the surface, the pressure gradually decreases. This releases the dissolved gases and leads to the formation of bubbles of nitrogen in the blood. This blocks capillaries and creates a medical condition known as bends, which are painful and dangerous to the life. To avoid bends, as well as, the toxic effects of high concentrations of nitrogen in the blood, the tanks used by scuba divers are filled with air diluted with helium (11.7% helium, 56.2% nitrogen and 32.1% oxygen).
  3. At high altitudes, the partial pressure of oxygen is less than that at the ground level. This leads to low concentrations of oxygen in the blood and tissues of people living at high altitudes or climbers. Low blood oxygen causes climbers to become weak and unable to think clearly, symptoms of a condition known as anoxia.

Limitations of Henry’s Law:

Henry’s law is applicable only under following conditions.

  1. The pressure of the gas in not too high.
  2. The temperature is not too low.
  3. The gas should not undergo any chemical reaction with solvent.
  4. The gas should not undergo dissociation in the solvent.

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