In a particular solvent, the relationship between the pressure and solubility of gas was given by William Henry in 1803. This generalization is known as Henry’s law, which states that when a gas is dissolved in a solvent at a constant temperature, the concentration of the gas is directly proportional to the pressure of the gas.
The gas solubility in a solvent depends on the pressure and temperature. When a gas is enclosed in a container over its saturated solution, an equilibrium is established between the molecules of gas and the gas in the solution.
Gas ⇌ Gas in solution
When the pressure is increased in the system, according to Le Chatelier Principle, this results in shifting the equilibrium to the direction where pressure is decreased. The pressure can be further decreased by dissolving more gas in the solvent. However, the concentration of a gas in a solvent can be increased by increasing the pressure.
Henry’s law constant
Mathematically, Henry’s law may be expressed as,
C ∝ P
C = k x P
- C = concentration of the gas in a solution
- k = proportionality constant
- P = pressure of the gas
The above expression is also known as Henry’s law equation
Factors affecting Henry’s law constant
Henry law constant is represented by “k” and its value depends on the following factors which are:
- Nature of the gas
- Nature of the solvent
- Units of pressure
- Units of concentration
- Temperature and pressure of the system
The nature of gas and solvent has a great effect on Henry’s law constant. This is a reason, different solvents and gases have different values of Henry’s law constant.
Applications of Henry’s law
These are the following applications and uses of Henry law:
- In the production of carbonated drinks
- In the service of cask-conditioned beer
- In respiration and blood oxygenation
- In underwater diving
Production of carbonated drinks
Hendry’s law is used to dissolve pure carbon dioxide (CO2) in carbonated drinks. The pressure of carbon dioxide (CO2) is usually more than the standard pressure in the bottles of carbonated drinks.
However, when the bottles are opened, carbon dioxide comes out with pressure and makes a hissing sound. This results in an instant decrease in the partial pressure of carbon dioxide (CO2) in the atmosphere in the drink’s bottle. According to henry’s law, the solubility of CO2 also decreased. This is the reason carbon dioxide (CO2) moves to the surface of the drink in the bottle and forms bubbles and is released into the atmosphere.
When the carbonated drink is left open for several minutes, an equilibrium is established between the carbon dioxide (CO2) in the atmosphere and the carbon dioxide (CO2) in the drink. Due to this, carbonated drinks lose their fizzy taste.
Respiration and blood oxygenation
This principle plays an important role in respiration. When air is inhaled, it increased the partial pressure of oxygen (O2) in the alveoli of the lungs. This is due to the interaction between deoxygenated blood and oxygen-rich air molecules. Similarly, during exhalation, the partial pressure of carbon dioxide (CO2) is increased.
These are the following gas exchange reactions:
- Deoxygenated blood has a lower amount of dissolved oxygen while alveoli have a larger amount of oxygen that is came from the air. Therefore, the partial pressure of oxygen in the alveoli is high. However, oxygen moves from the alveoli to the deoxygenated blood.
- Similarly, the amount of carbon dioxide (CO2) is very high in the deoxygenated blood and lower in the alveoli. This results in an increase in the partial pressure of carbon dioxide in the deoxygenated blood. So, carbon dioxide moves from the deoxygenated blood to the alveoli of the lungs.
According to henry’s law, increased pressure means solubility of more gases. Therefore, at the depth of the sea, divers consume more gases because the pressure is more at the bottom of the sea as compared to the surface of the sea.
When divers breathed compressed air into their lungs, both the oxygen and nitrogen dissolve in the bloodstream. When they ascend to the surface where pressure is normal, nitrogen gas bubbles out of the solution and causes painful symptoms of decompression sickness in the body of divers.
Henry’s law applies only to those gases that have nearly ideal behaviors. These are the limitations of Henry’s law, it is applicable when:
- The temperature and pressure of the system are moderate.
- The solubility of the gas in a solvent is low.
- Gas does not react with the solvent to form a new species.
- Association and dissociation of the gas do not take place.
- The system is in equilibrium.
What are the conditions for using Henry’s Law?
These are the conditions of henry’s law:
- There is an equilibrium established between molecules of gas and molecules of gas in the liquid.
- Pressure should not very high or low.
What does Henry’s law constant depend on?
The value of henry’s law constant depends on the nature of the gas and solvent, and units of concentration and pressure.
What is the unit of Henry’s law?
The unit of henry law constant is mol m−3 Pa−1.
Why Henry’s law is called limiting law?
Henry law is a limiting law because of its limitations. It is only applicable in the state of equilibrium. It does not work well under high pressure.
Which gas does not obey Henry’s law?
It depends on the nature of the solvent and gas. For example, oxygen is dissolved in water while ammonia (NH3) does not dissolve in water. However, those gases that are not dissolved in liquids are not obeyed henry’s law.
How is Henry’s law different from Raoult’s law?
Henry’s law states at a constant temperature, the amount of dissolved gas in a liquid is directly proportional to the pressure of the gas while Raoult’s law states partial pressure of every component of the mixture of liquid is equal to the mole fraction that is multiplied by the vapor pressure.