Understanding When Henry’s Law is Not Applicable

Henry’s law describes the relationship between the partial pressure of a gas above a liquid and the concentration of the gas dissolved in the liquid. It states that the solubility of a gas in a liquid is directly proportional to the partial pressure of the gas above the surface of the liquid at a constant temperature. Mathematically, this can be expressed as:

\( p = K_H \cdot c \)

Where:

• \( p \) is the partial pressure of the gas above the liquid.
• \( K_H \) is Henry’s law constant (which depends on the nature of the gas, the solvent, and the temperature).
• \( c \) is the concentration of the gas in the liquid (usually expressed as molarity or mole fraction).

However, like many laws in chemistry, Henry’s law is an ideal law and has certain conditions under which it is applicable. It deviations occur when these conditions are not met.

Conditions for Henry’s Law Applicability

Henry’s law is generally applicable under the following conditions:

• The pressure of the gas is low.
• The temperature is not too low (or high, though deviation at very high T is less common than at high P).
• The gas does not undergo any chemical reaction with the solvent.
• The gas does not undergo dissociation or association in the solvent.
• The solvent is not volatile.

When any of these conditions are significantly violated, Henry’s law may not accurately predict the solubility of the gas.

Analyzing the Given Options

Let's examine each option provided to see when Henry's law is not applicable:

1. Pressure of the gas is high: Henry’s law assumes ideal gas behavior and ideal solution behavior. At high pressures, real gases deviate significantly from ideal behavior due to intermolecular forces and the finite volume of gas molecules. Also, the solubility relationship becomes non-linear. Therefore, Henry’s law is generally not applicable at high gas pressures.
2. Gas does not react with solvent: Henry’s law is based on a physical equilibrium between the gas phase and the dissolved state. If the gas reacts with the solvent (e.g., \( \text{NH}_3 \) reacting with water to form \( \text{NH}_4\text{OH} \) or \( \text{CO}_2 \) reacting with water to form \( \text{H}_2\text{CO}_3 \)), the amount of dissolved gas is influenced by the chemical reaction in addition to the physical dissolution. Thus, Henry’s law is only applicable if the gas does not chemically react with the solvent. This option describes a condition where Henry's law *is* applicable.
3. The gas does not undergo dissociation or association: Henry’s law relates the concentration of the dissolved *species* to the partial pressure. If the gas molecules dissociate (like electrolytes) or associate (like some organic molecules in certain solvents) after dissolving, the number of particles in solution is different from the number of molecules that dissolved. For Henry’s law to hold, the dissolved gas molecules should remain in the same form as in the gas phase. This option describes a condition where Henry's law *is* applicable.
4. Temperature is high: Henry’s law relates solubility to pressure at a constant temperature. While Henry's constant \( K_H \) is temperature-dependent (solubility of gases typically decreases with increasing temperature), the law itself describes the pressure-solubility relationship *at* a given temperature. At very high temperatures, gases might become less soluble, and the assumption of constant temperature is crucial. However, the deviation from Henry's law is generally more pronounced at high pressure or when chemical reactions occur, compared to the direct effect of high temperature itself causing deviation from the law's linearity at a *given* temperature. The primary condition for deviation mentioned in standard texts regarding ideal behavior limits is high pressure.

Based on the analysis of the conditions under which Henry's law is applicable and when deviations occur, high pressure is a primary factor that causes Henry's law to become not applicable.

Conclusion

Henry's law is a limiting law applicable under ideal conditions. It deviates when the pressure of the gas is high, when the gas reacts with the solvent, or when the gas undergoes dissociation or association in the solvent. Of the given options, "Pressure of the gas is high" directly violates a key assumption for the applicability of Henry's law.

Revision Table: Henry’s Law Conditions

Additional Information on Henry’s Law Deviations

Henry’s law is similar in concept to Raoult’s law for the vapor pressure of solutions, both being applicable for ideal or dilute solutions. Real solutions and real gases deviate from ideal behavior under certain conditions, leading to deviations from these laws. For Henry’s law, deviations are most significant when the interactions between gas molecules themselves or between gas and solvent molecules are strong, which happens at high pressures or when chemical reactions occur.

The Henry's law constant, \( K_H \), is unique for each gas-solvent pair and temperature. It reflects the strength of interaction between the gas and the solvent. Gases with higher \( K_H \) values are less soluble in the solvent at a given partial pressure compared to gases with lower \( K_H \) values.

Understanding the limitations of Henry’s law is important in various applications, such as calculating the solubility of gases in natural waters, designing industrial processes involving gas absorption, and studying the behavior of dissolved gases in biological systems.