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Henry's Law (Gas Solubility)
Relates the partial pressure of a dilute gas above a liquid to its mole fraction dissolved in the liquid using Henry's constant.
Interpretation
Henry's law: P_i = H · x_i for gas solubility in liquid. H is Henry's constant. Example: CO₂ in water, H≈3000 atm·mole fraction, x=0.001 → P=3 atm.
Variables
| Symbol | Quantity | Unit |
|---|---|---|
| P_i | Partial pressure of gas i | atm |
| H | Henry's law constant | atm |
| x_i | Mole fraction of gas dissolved in liquid |
What it means
Henry’s law relates the partial pressure of a gas above a liquid to its mole fraction in the liquid, for dilute solutions. It is expressed as P_i = H_i x_i, where P_i is the partial pressure of the gas in the vapour phase, x_i is the mole fraction of the dissolved gas in the liquid, and H_i is the Henry’s constant (which depends on temperature and the gas‑liquid pair). The law is valid when the solution is dilute and the gas does not react chemically with the solvent. Henry’s law is used in many environmental and chemical engineering applications: predicting gas absorption (e.g., CO₂ capture), oxygen transfer in bioreactors, and volatilisation of pollutants from water. It is also the basis for the design of stripping and aeration processes. Henry’s constants are available in handbooks and increase with temperature for most gases, meaning solubility decreases as temperature rises. Understanding Henry’s law is crucial for managing gas‑liquid mass transfer in industrial processes.
Worked example
Henry's Law – Two Examples
Real‑World| Parameter | Value |
|---|---|
| H | 1640 atm |
| xᵢ | 0.0001 |
| Parameter | Value |
|---|---|
| H | 29.4 atm |
| xᵢ | 0.001 |
Common mistakes
- Henry’s constant H: Strongly temperature‑dependent; check the reference temperature. Units must be consistent with P and x.
- Mole fraction x_i: In the liquid phase.
- Applicability: Henry’s law is valid for dilute solutions of gases in liquids; at high concentrations, deviations occur.
- Gas partial pressure: P_i is the partial pressure of the gas above the solution.
- Different forms: Some use H in atm·m³/mol; ensure consistency.
Applications
Henry's law, P_i = H·x_i, relates the partial pressure of a gas above a liquid to its mole fraction dissolved in the liquid, with H being the Henry's constant. This law is crucial for gas absorption, stripping, and aeration processes. Engineers use it to design scrubbers, carbonation systems, and oxygen‑transfer equipment. In environmental engineering, it models the exchange of volatile contaminants between air and water. Henry's constants vary with temperature and depend on the gas‑liquid pair. By applying Henry's law, engineers can calculate the solubility of gases, the driving force for mass transfer, and the efficiency of gas‑liquid contactors. It is also used in the design of beverage carbonation and in the analysis of natural water bodies.
- Design of gas absorption columns (e.g., CO₂ capture, SO₂ removal)
- Sizing of strippers and degassers
- Environmental modelling of volatile organic compound (VOC) emissions
- Design of aeration systems for wastewater treatment
- Carbonation processes in food and beverage industry
Frequently Asked Questions
Henry's law describes the solubility of a gas in a liquid at dilute concentrations: P_i = H · x_i, where P_i is the partial pressure of the gas above the liquid, x_i is the mole fraction of the dissolved gas, and H is the Henry's law constant (units of pressure). It is valid for dilute solutions and low pressures.
H can be expressed in various units:
- atm (or bar) per mole fraction.
- Pa·m³/mol (using concentration).
- Atm·L/mol.
- Applying it outside the dilute range where it is no longer linear.
- Using the wrong Henry constant for the given temperature – H is strongly temperature‑dependent.
- Confusing the dimensionless Henry constant (H_c) with the pressure‑based H.
- Not accounting for the presence of other solutes (salting‑out effect).
Generally, H increases with temperature for most gases, meaning solubility decreases as temperature rises. The dependence can be modelled with the van 't Hoff equation: d(ln H)/d(1/T) = ΔH_sol/R.
Henry's law is the limiting law for the solute at low concentration, while Raoult's law is the limiting law for the solvent at high concentration. They are complementary.
Given P_i and H, x_i = P_i / H. The moles of dissolved gas can then be calculated from the total moles of liquid.
- Design of gas‑liquid contactors (absorbers, strippers).
- Predicting oxygen solubility in water.
- Carbonation of beverages.
- Environmental modelling (volatilisation of contaminants).