Henry's Law

What is Henry’s Law?

Henry law explains the solubility of a gas in liquid solution by partial pressure and mole fraction of the gas in liquid. 


Henry’s Law states that “the partial pressure applied by any gas on a liquid surface is directly proportional to its mole fraction present in liquid solvent.”

The Mathematical Formula of Henry’s Law is as Follows –

P ∝  X

Where P = partial pressure applied by the gas on liquid in the solution

X = Mole fraction of gas in liquid 

On Removing the Proportionality –

P = kH.X

Where kH is called Henry’s law constant. 


What is Mole Fraction? 

Mole fraction can be defined as a unit of concentration which is equal to the number of moles of a solute divided by the total number of moles of a solution. It is also called amount fraction or molar fraction. Thus, mole fraction of a gas (solute) in a solution of gas in liquid can be written as follows –

Mole fraction of Gas in Liquid (X) = [Moles of gas (solute)] / [Total number of moles (solution)]

As mole fraction is a ratio, it is a unitless expression. 


When Pressure is Constant 

When pressure remains constant, according to henry’s law henry constant will be inversely proportional to mole fraction of the gas. It can be represented as follows –

P ∝ X (Henry’s Law)

P = kHX

If P is constant, from above equation can be written as –

X ∝ \[\frac{1}{k_{H}}\]

It means if the value of Henry's law constant decreases, the value of mole fraction of gas in liquid increases. Mole fraction of the gas in liquid can be taken as solubility. Thus, as henry’s law constant decreases, solubility of the solute in solution increases.


Relation of Henry’s Law Constant with Temperature 

As we know, increasing the temperature solubility of the gas in liquid decreases. Thus, we can write a relation between temperature and solubility for a gas in liquid solution as follows –

T ∝ \[\frac{1}{Solubility}\]  - - - - - - (1)

According to henry’s law, we have explained above that if pressure is constant, as the value of henry’s law constant increases, solubility of gas in liquid decreases. It can also be expressed as follow –

\[k_{H}\] ∝ \[\frac{1}{Solubility}\] - - - - - - (2)

From equation (1) and (2), we can write –

T ∝ kH


Factors affecting Henry’s Law Constant 

Following Factors Affect the Henry’s Law of Constant of a Gas –

  • Nature of the gas. This is the reason different gases have different henry’s law constants in the solvent.

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  • Nature of the solvent 

  • Temperature and Pressure of the system. This is the reason kH value is given differently for a gas at different temperatures. 


Graph Between Solubility and Pressure (for Gas in Liquid Solution) According to Henry’s Law

Mathematical Expression of Henry’s Law can be Written as Follows –

P = kH.X _ _ _ _ _(3)

Where,

P = Partial pressure of the gas 

X= mole fraction of gas in liquid

kH = Henry’s law constant

Equation (3) is the same as Y = MX which is a straight-line equation. So, graph can be drawn as –

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Applications of Henry’s Law 

  • Production of carbonated beverages is based on Henry’s law. 

  • Hypoxia (concentration of oxygen is low in blood and tissues) can be explained by Henry’s law.

  • It is applied in deep sea diving as well.

  • We can understand by Henry’s law that aquatic life survives better in cold water. 


Limitations of Henry’s law 

  • It is applicable when the system is in equilibrium. 

  • It is applicable for those gases only which do not react with water(solvent). 

  • Gas shouldn’t cause any chemical change in the solution.

  • Henry’s law is appropriate only when pressure is not high, temperature is not too low and gas is extremely soluble.

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