Henrys Law: Factors, Applications and Limitations

Henry's Law mention that "the partial pressure applied by any gas to a liquid surface is directly proportional to its mole fraction present in the liquid solvent." Henry's Law has the following mathematical formula: –

P ∝ X, 

Where P denotes the partial pressure exerted by the gas on the liquid in the solution.

On Removing the Proportionality, X = Mole fraction of gas in liquid –

P = k_H. X

Where kH is known as Henry's law constant.

Key Terms: Henry’s law, Mole fraction, Temperature, Pressure, Solubility, Carbonated Drinks.

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Henry’s Law Definition

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Henry's law is a gas law in physical chemistry that states that the amount of dissolved gas in a liquid is directly proportional to its partial pressure above the liquid. The larger the gas's partial pressure, the greater its solubility in liquid. Henry's law constant is the proportionality factor. It was developed by William Henry, an English chemist in the early nineteenth century.

P ∝ X, 

Where P denotes the partial pressure exerted by the gas on the liquid in the solution.

On Removing the Proportionality, X = Mole fraction of gas in liquid (Mole fraction is a concentration unit defined as the number of moles of a solute divided by the total number of moles of a solution) –

P = k_H. X

Where kH is known as Henry's law constant.

There are two methods to represent Henry's law constant. 

1. When stated in terms of solubility/pressure, the constant is known as Henry's law solubility constant ('H'). 
2. If the proportionality constant is specified in terms of pressure/solubility, it is known as Henry's law volatility constant ('kH').

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Temperature and Pressure on Henry’s Law

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When the temperature rises, the solubility of the gas in liquid decreases. The relationship between temperature and gas solubility in liquid solution is T ∝ 1/ Solubility. 

If pressure is constant, the solubility of the gas in liquid decreases, and Henry's law constant value increases. The equation will look like–

 k_H ∝ 1/Solubility 

Now, the final expression can be derived as, T ∝ k_H.

Henry’s law constant will be inversely proportional to the mole fraction of the gas if the pressure remains constant. So, P ∝ X ⇒ P = k_HX

Now, pressure being constant in this equation, X ∝ 1/k_H

As the value of Henry's law constant falls, so does the value of the mole fraction of gas in the liquid. Solubility can be expressed as a mole fraction of the gas in the liquid. As a result, as Henry's law constant drops, the solute's solubility in solution increases.

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Factors Affecting Henry’s Law Constant

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The factors affecting Henry’s Law constant are–

1. The gas's composition
2. The solvent's composition
3. Pressure and temperature

Factors Affecting Henry’s Law Constant

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Applications of Henry’s Law

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Important applications of Henry’s Law in practical scenarios are as follows–

1. Under high pressure, CO₂ solubility rises in the production of carbonated drinks. As the container is exposed to air pressure, solubility decreases and gas bubbles are ejected from the liquid.
2. Hypoxia is a condition in which the oxygen content in the blood and tissues is so low that the person feels tired and unable to concentrate properly. So, climbers and people get benefited from this law at high altitudes.
3. In scuba diving the gas is inhaled at ambient pressure due to hydrostatic pressure, which rises with depth. Because the solubility of gases rises with depth, bodily tissues take up more gas with time until they are saturated for the depth, and vice versa. When a diver ascends, he or she decompresses, reducing the solubility of the gases dissolved in the tissues. If the supersaturation is excessively high, bubbles can develop and expand, causing blockages in capillaries or deformation in the more solid tissues, resulting in decompression sickness. When air is diluted with He, it allows scuba divers to breathe in high-pressure locations.

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Limitations of Henry’s Law

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Observed limitations of Henry’s Law are–

1. Henry's law does not applicable when gases are exposed to extremely high pressures.
2. This law only applies when the molecules in the system are in an equilibrium state.
3. The rule does not apply when the gas and solution are participating in chemical interactions with each other.

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Things to Remember

• Henry's Law mentions that "the partial pressure applied by any gas to a liquid surface is directly proportional to its mole fraction present in the liquid solvent.
• Henry's Law has the following mathematical formula: – P ∝ X, Where P denotes the partial pressure exerted by the gas on the liquid in the solution. 
• On Removing the Proportionality, X = Mole fraction of gas in liquid – P = k_H. X, Where k_H is known as Henry's law constant.
• When the constant is stated in terms of solubility/pressure, the constant is known as Henry's law solubility constant ('H'). 
• If the proportionality constant is specified in terms of pressure/solubility, it is known as Henry's law volatility constant ('k_H').
• The factors affecting Henry’s Law constant are–The gas's composition, The solvent's composition, and Pressure and temperature.

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