Henry's Law

Introduction

Henry's law is one of the important gas rules which was proposed by the English physician and chemist William Henry in the year 1830. The name is given as an honour to him. And is applicable only for dilute solutions. This law combined with other theories has witnessed many applications in Chemistry. The law focuses on the dissolution of gases on a particular liquid at a particular volume. It will deal with the weight of gases dissolved in liquids. It relates the relationship between the gas dissolved and partial pressure. According to this law, they are directly proportional at a constant temperature.

What is Henry’s Law?

Henry's law is a gas rule since it deals with the properties of gases. At constant temperature for a fixed volume of liquid, the quantity of gas decomposed in it is proportional to the partial pressure of gas present above the liquid. This means the gas and liquid are at an equilibrium condition. A gas dissolved means that it deals with the solubility of gases in liquids. The law can be represented mathematically as,

$$\mathrm{C\alpha\:P_{gas}}$$

$$\mathrm{C\:=\:KP_{gas}}$$

Where,

$\mathrm{C\:=\:solubility\:of\:gas(ml,gas/L)}$

$$\mathrm{K\:=\:Henry's\:law\:constant\:(M/atm)}$$

$$\mathrm{P_{gas}\:=\:partial\:pressure\:of\:the\:gas(atm)}$$

Henry's law also states that the partial pressure of any gas is directly equal to the mole fraction. So, we can give another mathematical formula for Henry's law. And is,

$$\mathrm{P_{gas\:\varpropto\:X}}$$

$$\mathrm{P_{gas}\:=\:KX}$$

Where,

$$\mathrm{X\:=\:mole\:fraction}$$

What is Mole Fraction?

Mole fraction or amount fraction deals with the solution. It is the number of moles of a solute in a solution split up by the total number of moles present in a solution. That is,

$$\mathrm{Mole\:Fraction\:=\:number\:of\:moles\:of\:solute\:/\:total\:number\:of\:moles\:present\:in\:the\:solution}$$

Graphically the relation between Mole fraction and partial pressure can be represented as,

When Pressure is Constant?

Henry's law will vary when pressure is kept constant. From Henry's law, the partial pressure and mole fraction are directly related,

$$\mathrm{P_{gas}\:\varpropto\:X\:or}$$

$$\mathrm{P_{gas\:=\:KX}}$$

If pressure is constant, the equation will change too,

$$\mathrm{X\:\varpropto\:1/K}$$

That is the mole fraction of a solution and Henry's law constants are inversely related. Mol fraction also deals with solubility so the increase of Henry's law constant will reduce the solubility of a gas in a fluid.

$$\mathrm{k\:\varpropto\:1/Solubility}$$

Relation of Henry’s Law Constant with Temperature

The dissolution of a gas in a liquid is an exothermic reaction that is the reaction liberates heat. So, the addition of extra heat or increasing the temperature will have a negative impact. Temperature and solubility of gases are inversely related. That is the temperature rise will decrease the solubility of the gas in the liquid. Mathematically,

$$\mathrm{T\:\varpropto\:1/Solubility}$$

But

$$\mathrm{K\:\varpropto\:1/Solubility}$$

So, the equation becomes,

$$\mathrm{t\:\varpropto\:K}$$

The temperature and Henry's law constants are directly proportional. The increasing temperature increases the value of the constant also.

Factors affecting Henry’s Law Constant

Some of the aspects that affect the value of Henry's law constants are,

• The temperature of the system and the temperature rise will heighten the value of Henry's law constant.

• It will depend on the nature of a gas that the value will be different for different gases.

• It will depend on the nature of the solvent involved in the dissolution.

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

The pressure and solubility of gases are directly proportional. The effect of pressure on solubility according to Henry's law is different for different gases. The graph below shows the effect of pressure on solubility.

Applications of Henry’s Law

There are numerous real-life applications of this law. Some of them are,

• The dissolution of oxygen helps in the breathing of people who live in areas where the partial pressure is low.

• The production of soft drinks especially carbonated drinks works based on this law.

• The breathing of scuba divers is based on this law.

• This love also explains why marine organisms live better in cold conditions.

• The concentration of the solution and dilute solutions obey Henry's law.

Limitations of Henry’s Law

Even though it has many applications some limitations also exist. They are,

• It is only favourable when the system is at equilibrium.

• It is not considered the chemical change taking place during the dissolution of a gas over a liquid.

• It depends on the nature of gas, that is the gases which do not react with the liquid or the gases that has less solubility.

• A particular temperature and pressure are necessary for the validation of this law.

Conclusion

Henry's law is one of the important laws of gases that has many applications in real life. According to this law, the solubility of a certain gas in a particular liquid at its fixed volume and a constant temperature is directly equal to the partial pressure of the gas and liquid at equilibrium. That is the solubility of gases on a liquid change with changing temperature and pressure. The rise in pressure will heighten the solubility of gases but the temperature rise will reduce the solubility of gases. There exist some other aspects that are the character of gas, the character of a solvent, etc. Even though it has explained many phenomena it has some limitations too.

FAQs

1. Which gases do not obey Henry's Law?

Ammonia and Carbon dioxide gas will not obey Henry's law when dissolved in water. And is because of the reactivity of Ammonia and Carbon dioxide in water. Their solubility will be high in such cases.

2. How does Henry's Law relate to respiration?

The process of respiration that is happening in our body can be explained with the help of Henry's law. That is the quantity of Oxygen gas decomposed in the blood is directly related to the partial pressure of gas in the alveoli.

3. Does an ideal solution obey Henry Law?

Ideal gases will obey Henry's law and is because of the reason that the solute- solute and solute-solvent relations are distinct in the case of ideal gases.

4. What is Raoult's Law?

Raoult's law asserts that the partial vapour pressure of a solution is equal to the product of the mole fraction of a solvent and the partial pressure of a solvent. It deals with solutions.

5. Why is the solubility of Nitrogen more than Helium in blood?

The comparison of a partial pressure will explain the process, that is the nitrogen gases have more partial pressure compared to helium. And according to Henry's law solubility and partial pressure are related so nitrogen becomes more soluble than Helium. And is the reason why Helium is used in the Oxygen cylinders during scuba diving.