Henry's Law | Solutions Class 12 Chemistry

Henry's Law

Solubility of Gases in Liquids | Solutions Class 12

1. Statement of Henry's Law

Statement: The partial pressure of the gas in vapor phase ($p$) is directly proportional to the mole fraction of the gas ($\chi$) in the solution.

$$ p = K_H \cdot \chi $$

Where:

• $p$ = Partial pressure of the gas above the liquid surface.
• $\chi$ = Mole fraction of the gas in the solution (Solubility).
• $K_H$ = Henry's Law Constant.

2. Characteristics of Henry's Constant ($K_H$)

• Nature of Gas: $K_H$ depends on the nature of the gas. Different gases have different $K_H$ values at the same temperature.
• Temperature: $K_H$ increases with an increase in temperature.
• Solubility Relation: From the formula $\chi = p / K_H$, it is clear that Solubility is inversely proportional to $K_H$ (at constant pressure).
  Higher $K_H$ $\rightarrow$ Lower Solubility.

Example: Aquatic species are more comfortable in cold water because at low temperature, $K_H$ is low, and thus dissolved Oxygen is high.

3. Applications of Henry's Law

1. Soft Drinks: To increase the solubility of $CO_2$ in soft drinks and soda water, the bottle is sealed under high pressure. When opened, pressure drops, solubility decreases, and bubbles form.
2. Scuba Diving: At depth (high pressure), atmospheric gases ($N_2, O_2$) dissolve in blood. When the diver comes up, pressure decreases, releasing $N_2$ bubbles in capillaries, causing a painful condition called "Bends". To avoid this, tanks are filled with air diluted with Helium (low solubility).
3. High Altitude: At high altitudes, partial pressure of $O_2$ is less. This leads to low oxygen concentration in blood, causing weakness and unclear thinking, known as Anoxia.

4. Limitations

Henry's Law is valid only when:

• Pressure is not too high.
• Temperature is not too low.
• The gas is not highly soluble (e.g., $NH_3$ and $HCl$ react with water and do not obey Henry's law well).
• The gas does not undergo chemical reaction or dissociation/association in the solvent.

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