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10.5: Henry's Law

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    521801

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    Learning Objectives
    • Describe the effect of pressure on the solubility of gases in water and apply this concept in health-related conditions.

    Solubility of a Gas in Water

    Earlier you were introduced to the effect of intermolecular attractive forces on solution formation. The chemical structures of the solute and solvent dictate the types of forces possible and, consequently, are important factors in determining solubility.

    For example, under similar conditions, the water solubility of oxygen is approximately three times greater than that of helium.

    The solubility of oxygen (nonpolar) in the liquid hydrocarbon hexane, C6H14(nonpolar), is approximately 20 times greater than it is in water (polar).

    Solubility of a Gas and Temperature

    Temperature is another factor that affects gas solubility. Gas solubility decreases as temperature increases. This is one of the major impacts resulting from the thermal pollution of natural bodies of water. When the temperature of a river, lake, or stream is raised abnormally high, usually due to the discharge of hot water from some industrial process, the solubility of oxygen in the water is decreased. Decreased levels of dissolved oxygen may have serious consequences for the health of the water’s ecosystems and, in severe cases, can result in large-scale fish kills.

    Henry's Law (Solubility of a Gas and Pressure)

    The solubility of a gaseous solute is also affected by pressure. Gas solubility increases as the pressure of the gas increases. Carbonated beverages provide a nice illustration of this relationship. The carbonation process involves exposing the beverage to a relatively high pressure of carbon dioxide gas and then sealing the beverage container, thus saturating the beverage with CO2 at high pressure. When the beverage container is opened, a familiar hiss is heard as the carbon dioxide gas pressure is released, and some of the dissolved carbon dioxide is typically seen leaving solution in the form of small bubbles (Figure \(\PageIndex{1}\)). With time, the dissolved carbon dioxide concentration will decrease to its equilibrium value and the beverage will become “flat.”

    A dark brown liquid is shown in a clear, colorless container. A thick layer of beige bubbles appear at the surface of the liquid. In the liquid, thirteen small clusters of single black spheres with two red spheres attached to the left and right are shown. Red spheres represent oxygen atoms and black represent carbon atoms. Seven white arrows point upward in the container from these clusters to the bubble layer at the top of the liquid.
    Figure \(\PageIndex{1}\): Opening the bottle of carbonated beverage reduces the pressure of the gaseous carbon dioxide above the beverage. The solubility of CO2 is thus lowered, and some dissolved carbon dioxide may be seen leaving the solution as small gas bubbles. (Credit: modification of work by Derrick Coetzee)

    Decompression Sickness (“The Bends”)

    Decompression sickness (DCS), or “the bends,” is an effect of the increased pressure of the air inhaled by scuba divers when swimming underwater at considerable depths. In addition to the pressure exerted by the atmosphere, divers are subjected to additional pressure due to the water above them, experiencing approximately 2 atm at 10 m of depth. Therefore, the air inhaled by a diver while submerged contains gases at the corresponding higher pressure, and the concentrations of the gases dissolved in the diver’s blood are proportionally higher per Henry’s law.

    As the diver ascends to the surface of the water, the atmospheric pressure decreases and the dissolved gases becomes less soluble. If the ascent is too rapid, the gases escaping from the diver’s blood may form bubbles that eventually bursts capillaries and cause a variety of symptoms ranging from rashes and joint pain to paralysis and death. To avoid DCS, divers must ascend from depths at relatively slow speeds (10 or 20 m/min) or otherwise make several decompression stops, pausing for several minutes at given depths during the ascent. This allows excess dissolved gases in the blood to be exhaled. If stops or slow speeds are not an option, then the diver should exhale rapidly during the ascent.

    Divers may also use a special mixture of helium and oxygen. Nitrogen in air is replaced by helium. He gas is not as soluble in the blood serum, so the effects of DCS is minimized. Under similar conditions, the water solubility of nitrogen is greater than that of helium.

    When these preventive measures are unsuccessful, divers with DCS are often provided hyperbaric oxygen therapy in pressurized vessels called decompression (or recompression) chambers.

    Henry’s Law

    Henry's law is one of the gas laws formulated by William Henry in 1803. It states: "At a constant temperature, the amount of a given gas that dissolves in a given type and volume of liquid is directly proportional to the partial pressure of that gas in equilibrium with that liquid." An equivalent way of stating the law is that the solubility of a gas in a liquid is directly proportional to the partial pressure of the gas above the liquid.

    To explain this law, Henry derived the equation:

    \[ C =k P_{gas} \nonumber \]

    where

    Henry’s Law tells us that the greater the pressure of gas above the surface of a liquid, the higher the concentration of the gas in the liquid. Also, Henry’s law tells us that gases diffuse from areas of high gas concentration to areas of low gas concentration.

    Applicability of Henry's Law
    • Henry's law only works if the molecules are at equilibrium.
    • Henry's law does not work for gases at high pressures (e.g., \(N_{2\;(g)}\) at high pressure becomes very soluble and harmful when in the blood supply).
    • Henry's law does not work if there is a chemical reaction between the solute and solvent (e.g., \(HCl_{(g)}\) reacts with water by a dissociation reaction to generate \(H_3O^+\) and \(Cl^-\) ions).

    Application of Henry's Law: Scuba diving

    Our respiratory systems are designed to maintain the proper oxygen concentration in the blood when the partial pressure of O2 is 0.21 atm, its normal sea-level value. Below the water surface, the pressure increases by 1 atm for each 10.3 m increase in depth; thus a scuba diver at 10.3 m experiences a total of 2 atm pressure pressing on the body. In order to prevent the lungs from collapsing, the air the diver breathes should also be at about the same pressure.

    Five scuba divers swim underwater, surrounded by blue water and air bubbles, with one diver in the foreground wearing purple fins.
    Diagram showing sea level at 1 atm pressure, water depth of 10.3 m adding 1 atm, and total pressure of 2 atm at 10.3 m below the surface.
    Figure \(\PageIndex{2}\): Scuba Diving actively takes into account both Henry's and Dalton's Laws

    But at a total pressure of 2 atm, the partial pressure of \(O_2\) in ordinary air would be 0.42 atm; at a depth of 100 ft (about 30 m), the \(O_2\) pressure of 0.8 atm would be far too high for health. For this reason, the air mixture in the pressurized tanks that scuba divers wear must contain a smaller fraction of \(O_2\). This can be achieved most simply by raising the nitrogen content, but high partial pressures of N2 can also be dangerous, resulting in a condition known as nitrogen narcosis. The preferred diluting agent for sustained deep diving is helium, which has very little tendency to dissolve in the blood even at high pressures.

    Career Focus: Respiratory Therapist

    Certain diseases—such as emphysema, lung cancer, and severe asthma—primarily affect the lungs. Respiratory therapists help patients with breathing-related problems. They can evaluate, help diagnose, and treat breathing disorders and even help provide emergency assistance in acute illness where breathing is compromised.

    Most respiratory therapists must complete at least two years of college and earn an associate’s degree, although therapists can assume more responsibility if they have a college degree. Therapists must also pass state or national certification exams. Once certified, respiratory therapists can work in hospitals, doctor’s offices, nursing homes, or patient’s homes. Therapists work with equipment such as oxygen tanks and respirators, may sometimes dispense medication to aid in breathing, perform tests, and educate patients in breathing exercises and other therapy.

    Because respiratory therapists work directly with patients, the ability to work well with others is a must for this career. It is an important job because it deals with one of the most crucial functions of the body.

    Hyperbaric Chamber

    Hyperbaric (high pressure) oxygen treatment in another application of Henry's Law. This is an air chamber that is at two to three times the atmospheric pressure. The solubility of oxygen increases with an increase in pressure. A patient placed in a hyperbaric chamber has a higher concentration of oxygen dissolved in blood. The higher concentration of oxygen is toxic to many strains of bacteria. Therefore the hyperbaric chambers are used to treat burn patients, gangrene, tetenus, in surgeries, and to treat some cancers.

    The hyperbaric chambers are also used to treat carbon monoxide (CO) poisoning because the higher concentration of oxygen in the chamber can displace the CO bound with hemoglobin faster than atmospheric oxygen does.

    Another use of hyperbaric chambers is to treat scuba divers suffering from the bends. If a diver ascends too quickly, the nitrogen dissolved in blood makes bubbles in the vessels that block the blood flow, a condition called bends. The divers suffering from the bends are placed in a hyperbaric chamber at high pressure, and then the pressure is slowly decreased to atmospheric pressure. The nitrogen dissolved in blood under higher pressure and slowly diffuses out through the lungs as the pressure is gradually decreased.

    Two photos are shown. The first shows two people seated in a steel chamber on benches that run length of the chamber on each side. The chamber has a couple of small circular windows and an open hatch-type door. One of the two people is giving a thumbs up gesture. The second image provides a view through a small, circular window. Inside the two people can be seen with masks over their mouths and noses. The people appear to be reading.

    Figure \(\PageIndex{2}\): (a) US Navy divers undergo training in a recompression chamber. (b) Divers receive hyperbaric oxygen therapy.

    Summary

    The solubility of a gas in a liquid is proportional to the pressure of the gas over the liquid. The higher the pressure of a gas above a liquid, the greater its solubility.

    Contributions & Attributions

    This page titled 10.5: Henry's Law is shared under a CC BY license and was authored, remixed, and/or curated by Deboleena Roy (American River College) (Cañada College) .