Properties of Solutions - Henry’s Law – Effect of Pressure on Solubility

Henry’s Law – Effect of Pressure on Solubility

Let us suppose we have a volume of water (or any solvent) in a container containing air and nitrogen gas N₂ of partial pressure P₁. The nitrogen exerts  a certain partial pressure to the container and to water, which is a measure of the concentration of nitrogen in the gas (shown with blue arrows in Fig. I.1, Dalton’s Law of Partial pressures). This pressure causes some of the nitrogen to get dissolved in water. If the concentration of N₂ would be increased in the container, the partial pressure of N₂ would be increased proportionally and consequently more gas would get dissolved.

The dissolving process for gases is an equilibrium (Fig. I.1). At equilibrium the number of molecules leaving the gas phase to enter the solution equals the number of gas molecules leaving the solution. In fact, the concentration of dissolved nitrogen in the water would be directly proportional to the partial pressure of nitrogen (P_N2) in the gas. This is known as Henry’s law.

Fig. I.1: N₂ equilibrium in the gaseous and liquid (dissolved) phase. The dissolving process for gases is an equilibrium. The solubility of a gas depends directly on the gas partial pressure. When the N₂ partial pressure is doubled (P₂ = 2 * P₁) the concentration of  dissolved N₂ ([N₂]liquid) is also doubled.

Henry’s law states that the amount of gas dissolved in solution varies directly with the partial pressure of that gas over the solution. The solubility of a gas is directly proportional to its partial pressure at any given temperature

C = [concentration of gas in solution] = k_H . P_gas       (Henry's law)      (1)

where, C is the concentration of gas in solution (mol/l),

k_H Henry’s law constant (depends on gas, temperature and solvent) ( mol/l . atm)

P_gaspartial pressure of gas (atm)

 

Note:

i) Henry’s law is only valid for low dissolved gas concentrations. Gases should not dissociate in or react with the solvent.

ii) The atmospheric partial pressures of N₂, NH₃, N₂O, NO, NO₂ and HNO₂ are all low enough and therefore Henry’s law applies. In contrast, it does not correctly represent the behaviour of gaseous such as hydrogen chloride in water because of the dissociation reaction:

HCl_(g)  ------->   H⁺_(aq) + Cl^-_(aq)

iii) Henry’s law constant depends on temperature. It is important to recognize that this constant is actually a strong, nonlinear function of temperature.

iv) A common problem with Henry’s constants is caused by the wide variety of possible units of measure. If proper care is not taken, it can be easy to lead to serious errors.

 

Carbonated soft drinks show how Henry’s law works. These beverages are packed under pressure in a chamber filled with CO₂ gas. Due to the pressure CO₂ exerts on the liquid dissolves in it. When the can or bottle is opened the partial pressure of CO₂ drops and the dissolved CO₂ will gradually escape from the liquid.

Henry’s law also explains the acute decompression sickness (e.g. the “Bends”) experienced mainly by divers. Divers have more N₂ gas dissolved in their blood due to the increased pressure from the water on their bodies during a dive. When they return to the surface the excess dissolved N₂ gas is released due to the lower pressure causing the acute decompression sickness.

 

Example I.1

Pick the conditions that would yield the highest concentration of N₂(g) in water.

(A) partial pressure of gas = 1.0 atm; temperature of water = 25°C

(B) partial pressure of gas = 0.50 atm; temperature of water = 55°C

(C) partial pressure of gas = 2.0 atm; temperature of water = 25°C

(D) partial pressure of gas = 2.0 atm; temperature of water = 85°C

(E) partial pressure of gas =1.0 atm; temperature of water = 85°C

 

Answer: From Henry's law (equation 1) the concentration C of the dissolved gas is directly proportional to its partial pressure - the highest the partial pressure of the gas the highest the concentation of the gas dissolved in solution. The given partial pressure of the nitrogen gas is highest in answers (C) and (D). Henry's law constant is proportional to temperature. The highest the temperature the highest the k_H would be. Therefore, answer (D) is correct.

 

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Relevant Posts

Dalton's law - Law of Partial Pressures

Solutions and Concentration

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References

1. J. Kotz, P. Treichel, J. Townsed, “Chemistry and Chemical Reactivity”, 7th Edition,  Cengage Learning, 2008
2. W. J. Moore, “Physical Chemistry”, 4th Edition, Englewood Cliffs, NJ: Prentice – Hall, 1972
3. P. Atkins, J. Paula, “Physical Chemistry”, 9th Edition, W.H. Freeman, 2009
4. J. J. Caroll, Chem. Eng. Progress, 87, 48, (1991)

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Key Terms

partial pressure, concentration, Henry's law, Dalton's law of partial pressures, equilibrium, gas phase, Henry's constant, dissolved gas concentrations,

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