Henry’s Law

Henry’s law was established during the 19th century by the chemist William Henry. It states that at a constant temperature, the quantity of gas dissolved in a particular volume of liquid is directly relative to the pressure of the gas when it is in equilibrium with the liquid. This phenomenon is observed when the temperature remains constant. The constant proportionality in the law equation is known as Henry’s law. 

Henry’s law constant can be described in two different ways:

•  If the constant is expressed in solubility and pressure, it is called Henry’s law formula solubility constant. 

• On the other hand, if this constant of proportionality is expressed concerning pressure and solubility, it is known as Henry’s law volatility constant.

Mathematically Henry’s law expressed  as 

P ∝ XA

P = KH x XA

Where P is pressure of gas and XA is mole fraction of gas which is dissolved in liquid , KH is Henry’s constant

The more the gas’s partial pressure, the more its solubility in the liquid.

Examples of Henry’s law

Here are some examples that show applications of Henry’s law:

Carbonated drinks

The gas above the closed bottle of aerated drinks is mainly pure carbon dioxide; it remains slightly above standard atmospheric pressure. Due to Henry’s law, carbon dioxide is highly soluble in soft drinks.

The pressurised CO2 comes out into the atmosphere when the bottle is opened. The hissing sound produced is the pressurised carbon dioxide released into the air. 

Due to Henry’s law, the partial pressure of the carbon dioxide above the bottle gradually reduces, thereby decreasing the ability of the gas to dissolve in the drink. The remaining dissolved carbon dioxide comes to the drink’s surface as tiny bubbles, thereby escaping into the air.

Keeping the carbonated drinks open for a long time reduces the amount of carbon dioxide. As a result, the drink’s carbon dioxide attains a balance or equilibrium. Thus, the drink becomes flat. 

Respiration and oxygenation of the blood

During the respiration process, the partial pressure of oxygen in the lungs increases.

When deoxygenated blood in the body comes in contact with the oxygenated air present in the lungs, the following gaseous interchanges occur due to Henry’s law: 

• Due to high partial pressure of oxygen in the lungs and lower amount of oxygen in the blood, the gas gets transferred from the alveoli into the deoxygenated blood.

• The partial pressure of carbon dioxide in the lungs is relatively low. As dissolved carbon dioxide is high, the gas tends to move from the blood toward the lungs. Later on, the carbon dioxide is released from the body through exhalation. Hence, this law plays a vital role in the respiration of many living organisms.

Mountaineers and the state Henry’s law

While climbing mountains, the air automatically decreases. In the high altitudes, the pressure of oxygen becomes less than the ground level. As a result, people staying in high altitudes face low oxygen levels in their blood and tissues.This occurs in keeping with the functioning of gases as described in Henry’s law. All these symptoms happen due to a condition known as anoxia. 

Aspects that affect the Henry’s Law constant

The constant of a gas depends on three main factors:

• The characteristic features of the gas

• The characteristics of the solvent

• Temperature and pressure

Thus, different glasses come with different Henry’s law constants for different solvents. 

Conclusion: 

According to the above study we can understand that those gases which are extremely low in solubility, its aqueous solution is in accordance with Henry’s law, namely, its solubility is conditional on gases partial pressure and Henry’s constant. On one hand, partial pressure has a crucial influence on its solubility. In the high altitudes, the pressure of oxygen becomes less than the ground level. As a result, people staying in high altitudes face low oxygen levels in their blood and tissues.This occurs in keeping with the functioning of gases as described in Henry’s law.

Limitations

• State Henry’s law applies only when the molecules present in the system are in equilibrium.

• Henry’s law will not work when gases are kept under too much high pressure.

• Similarly, the law does not apply when the gas and the solution participate in the chemical reaction.

What is the relation of Henry’s law constant with temperature?

We know that maximising the temperature, solubility of gas especially in the liquid reduces. As per Henry’s law, if the pressure remains constant and as the value of the Henry’s law constant increases, the solubility of the gases in liquid decreases.