Henrys law

Henry’s law is a gas law according to which the amount of gas that is dissolved in a liquid is directly proportional to the partial pressure of that gas over the liquid when the temperature is kept the same. The constant of proportionality for this relationship is called Henry’s law constant (usually denoted by ‘kH‘). Henry’s law mathematical formula :

P ∝ C (or) P = kH.C

Where,

‘P’ refers to  the partial pressure of the gas in the atmosphere above the liquid.

‘C’ refers to  the concentration of the dissolved gas.

‘kH’ refers to  the Henry’s law constant of the gas.

This law was formulated in the early 19th century by the English chemist William Henry. We can notice that Henry’s law constant can be expressed in two different ways. If we  define the constant as solubility/pressure, it is referred to as the Henry’s law solubility constant (denoted by ‘H’). On the other hand,  in terms of pressure/solubility,proportionality. It is called Henry’s law volatility constant (denoted by ‘kH’).

Henry’s Law

Henry’s law itself defines  the relationship between the solubility of a gas in a liquid and the partial pressure of that gas in the atmosphere above the liquid (as dictated by Henry’s law) is provided above. The Greater the partial pressure of the gas, the higher the solubility in the liquid.

Examples of Henry’s Law are 

If we consider Pepsi and other Carbonated Drinks as an example 

Henry’s law comes into play every time a bottle of Pepsi (or any other carbonated drink) is opened. The gas above the unopened carbonated drink is usually pure carbon dioxide, kept at a pressure which is slightly above the standard atmospheric pressure. And because of Henry’s law, the solubility of carbon dioxide in the unopened drink will also be higher.

When the bottle is opened, the pressurized CO2 comes out  into the atmosphere (which is usually accompanied by a hissing sound). As the partial pressure of CO2 in the atmosphere above the drink rapidly decreases, the solubility of the carbon dioxide in the drink also decreases (due to Henry’s law). Due to this the dissolved CO2  comes to the surface of the drink in the form of tiny bubbles and escapes into the atmosphere.

If the carbonated drink is left open for the long time, the concentration of carbon dioxide in the drink will reach the state of an equilibrium with the concentration of carbon dioxide in the atmosphere (~0.05%), causing it to go flat (the drink will lead to ‘fizzy’ taste).

Respiration and the Oxygenation of Blood

In the process of respiration, inhalation is accompanied by an increase in the partial pressure of oxygen in the alveoli. When deoxygenated blood interacts with the oxygen-rich air in the alveoli, the following gas-exchanges take place as a consequence of Henry’s law:

Since the partial pressure of oxygen in the alveoli is high and the amount of dissolved oxygen in the deoxygenated blood is low, oxygen flows from the alveoli into the deoxygenated blood.

The partial pressure of carbon dioxide in the alveoli is very low (CO2 constitutes approximately 0.05% of the atmosphere). Since the concentration of dissolved CO2 in the deoxygenated blood is very high, the gas moves from the blood into the alveoli. This carbon dioxide is thrown out from the body via exhalation.

Thus, Henry’s law plays an essential role in the respiration of many organisms.

Factors Affecting the Henry’s Law Constant

The value of the Henry’s law constant depends upon the following factors mentioned below_

The nature of the gas

The nature of the solvent

Temperature & pressure

Therefore, dissimilar gases have unlike Henry’s laws constant in different solvents

Limitations of Henry’s Law

This law will be applicable only when the molecules of the system are in a state of equilibrium.

Henry’s law will not  hold true when gases are placed under extremely high pressure.

 when the gas and the solution participate in chemical reactions with each other,then the law will not be applicable.

Conclusion 

We can conclude with this that Henry’s law is a gas law that states that the amount of gas that is dissolved in a liquid is directly proportional to the partial pressure of that gas above the liquid keeping the temperature constant.

We should also keep in mind that gases like NH3 and CO2 do not obey Henry’s law.  This is because these gases react with water, and also Due to reactions of the gases like NH3,CO2(g), they have greater solubilities than expected by Henry’s law.