Henry's Law
Definition and meaning of Henry's Law in chemistry.
Henry's law is a physical chemistry principle that explains how gases dissolve in liquids at a constant temperature. It states that the dissolved gas concentration is directly proportional to the gas pressure above the liquid. This relationship explains how higher pressure forces more gas molecules into a solution.
In more detail
The mathematical relationship is written as C = kH·P. In this equation, C represents the final concentration of the dissolved gas. The letter P stands for the partial pressure of the gas resting above the liquid.
Finally, kH is a specific value called the Henry's law volatility constant. This constant changes for every unique gas and liquid combination. The constant also displays an inverse relationship with temperature.
As a liquid gets hotter, the dissolved gas molecules gain kinetic energy. This extra energy reduces the gas solubility and lowers the constant value. Henry's law accurately models ideal, dilute solutions.
The dissolved gas must not chemically react with the liquid or break apart into ions. Therefore, the law fails to predict the behavior of highly reactive gases. For example, carbon dioxide and ammonia react instantly with water to form new compounds.
Henry's law governs several critical biological and environmental processes. It controls the continuous exchange of oxygen and carbon dioxide inside human lungs. It also explains the slow dissolution of atmospheric oxygen into ocean ecosystems. Finally, it predicts how dangerous nitrogen gas builds up in the tissues of deep-sea scuba divers.
Key facts
| Field | Physical Chemistry |
|---|---|
| Equation | C = kH·P (Concentration equals Henry's constant times partial pressure) |
| Proposed by | William Henry, 1803 |
| Validity | Strictly valid for dilute, non-reacting gas-liquid physical mixtures |
| Temperature dependence | Gas solubility in liquids generally decreases as solvent temperature increases |
| Exceptions | Gases that chemically react or ionize in the solvent (e.g., HCl, NH3 in water) deviate significantly |
A soda factory pressurizes bottles with carbon dioxide gas during manufacturing. This high pressure forces a large amount of CO2 to dissolve into the liquid. When a consumer opens the cap, the pressure inside the bottle drops instantly. The dissolved CO2 quickly comes out of the liquid as visible bubbles. This bubbling continues until a lower equilibrium concentration is reached.
Frequently asked questions
Why does a carbonated beverage go flat if left open to the air?
Opening the container drastically lowers the pressure of carbon dioxide above the liquid. Henry's law states that this pressure drop reduces the amount of gas the liquid can hold. The excess dissolved gas escapes into the atmosphere as bubbles until it matches the low ambient carbon dioxide levels.
Does Henry's law apply to every type of gas dissolving in every liquid?
No, the law assumes ideal behavior where gas molecules simply mix into the solvent. Some gases undergo chemical reactions or break into ions when they hit the liquid. Ammonia, for instance, dissolves in water to form ammonium ions. These reacting gases heavily deviate from the mathematical prediction.
How does Henry's law relate to scuba diving and decompression sickness?
Water pressure increases as a scuba diver descends into the ocean. This increases the partial pressure of nitrogen in their breathing air. Henry's law dictates that this forces more nitrogen gas to dissolve into their bloodstream. If the diver ascends too quickly, the pressure drops rapidly. The dissolved nitrogen then forms dangerous bubbles inside their body tissues.