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Encyclopedia > Henry's law

In chemistry, Henry's law is one of the gas laws, formulated by William Henry. It states that, at a constant temperature, the amount of a given gas dissolved in a given type and volume of liquid is directly proportional to the partial pressure of that gas in equilibrium with that liquid. Chemistry (from Greek χημεία khemeia[1] meaning alchemy) is the science of matter at the atomic to molecular scale, dealing primarily with collections of atoms, such as gases, molecules, crystals, and metals. ... The gas laws are a set of laws that describe the relationship between thermodynamic temperature (T), pressure (P) and volume (V) of gases. ... William Henry William Henry (December 12, 1775—September 2, 1836) was an English chemist. ... In a mixture of ideal gases, each gas has a partial pressure which is the pressure which the gas would have if it alone occupied the volume. ...

Contents

Formula and Henry constant

A formula for Henry's Law is:

e^{p,} = e^{kc,} ,

with:

p, the partial pressure of the solute above the solution
c, the concentration of the solute in the solution (in one of its many units)
k, the Henry's Law constant, which has units such as L·atm/mol, atm/(mol fraction) or Pa·m3/mol.

Taking the natural logarithm of the formula, gives us the more commonly used formula:[1] In a mixture of ideal gases, each gas has a partial pressure which is the pressure which the gas would have if it alone occupied the volume. ... A substance is soluble in a fluid if it dissolves in the fluid. ... Dissolving table salt in water This article is about a chemical solution; for other uses of the term solution, see solution (disambiguation). ... In chemistry, concentration is the measure of how much of a given substance there is mixed with another substance. ... The natural logarithm, formerly known as the hyperbolic logarithm, is the logarithm to the base e, where e is equal to 2. ...

p = kc ,

Some values for k include:

when these gases are dissolved in water at 298 kelvin. Note that in the above, the unit of concentration was chosen to be molarity. Hence the dimensional units: L is liters of solution, atm is the partial pressure of the gaseous solute above the solution (in atmospheres of absolute pressure), and mol is the moles of the gaseous solute in the solution. Also note that the Henry's Law constant, k, varies with the solvent and the temperature. General Name, Symbol, Number oxygen, O, 8 Chemical series Nonmetals, chalcogens Group, Period, Block 16, 2, p Appearance colorless (gas) very pale blue (liquid) Atomic mass 15. ... Carbon dioxide is a chemical compound composed of one carbon and two oxygen atoms. ... This article is about the chemistry of hydrogen. ... Impact of a drop of water. ... The Kelvin scale is a thermodynamic (absolute) temperature scale where absolute zero—the lowest possible temperature where nothing could be colder and no heat energy remains in a substance—is defined as zero kelvin (0 K). ... This page refers to concentration in the chemical sense. ...


As discussed in the next section, there are other forms of Henry's Law each of which defines the constant k differently and requires different dimensional units. The form of the equation presented above is consistent with the given example numerical values for oxygen, carbon dioxide and hydrogen and with their corresponding dimensional units.


Other Forms of Henry's Law

There are various other forms Henry's Law which are discussed in the technical literature.[2][3]

Table 1: Some forms of Henry's law and constants (gases in water at 298K), derived from [3]
equation: k_{H,cp} = frac{c_{aq}}{p_{gas}} k_{H,pc} = frac{p_{gas}}{c_{aq}} k_{H,px} = frac{p_{gas}}{x_{aq}} k_{H,cc} = frac{c_{aq}}{c_{gas}}
dimension: left[frac{mol_{gas}}{L_{soln} cdot atm}right] left[frac{L_{soln} cdot atm}{mol_{gas}}right] left[frac{atm cdot (mol_{water}+ mol_{gas})}{mol_{gas}}right] left[ dimensionless right]
O2 1.3 E-3 769.23 4.259 E4 3.180 E-2
H2 7.8 E-4 1282.05 7.099 E4 1.907 E-2
CO2 3.4 E-2 29.41 0.163 E4 0.8317
N2 6.1 E-4 1639.34 9.077 E4 1.492 E-2
He 3.7 E-4 2702.7 14.97 E4 9.051 E-3
Ne 4.5 E-4 2222.22 12.30 E4 1.101 E-2
Ar 1.4 E-3 714.28 3.955 E4 3.425 E-2
CO 9.5 E-4 1052.63 5.828 E4 2.324 E-2

where: General Name, Symbol, Number oxygen, O, 8 Chemical series Nonmetals, chalcogens Group, Period, Block 16, 2, p Appearance colorless (gas) very pale blue (liquid) Atomic mass 15. ... This article is about the chemistry of hydrogen. ... Carbon dioxide is an atmospheric gas composed of one carbon and two oxygen atoms. ... General Name, Symbol, Number nitrogen, N, 7 Chemical series nonmetals Group, Period, Block 15, 2, p Appearance colorless Atomic mass 14. ... General Name, Symbol, Number helium, He, 2 Chemical series noble gases Group, Period, Block 18, 1, s Appearance colorless Atomic mass 4. ... General Name, Symbol, Number neon, Ne, 10 Chemical series noble gases Group, Period, Block 18, 2, p Appearance colorless Atomic mass 20. ... General Name, Symbol, Number argon, Ar, 18 Chemical series noble gases Group, Period, Block 18, 3, p Appearance colorless Atomic mass 39. ... Carbon monoxide, with the chemical formula CO, is a colourless, odourless and tasteless gas. ...

As can be seen by comparing the equations in the above table, the Henry's Law constant kH,pc is simply the inverse of the constant kH,cp. Since all kH may be referred to as the Henry's Law constant, readers of the technical literature must be quite careful to note which version of the Henry's Law equation is being used. The mole (symbol: mol) is the SI base unit that measures an amount of substance. ... The liter (spelled liter in American English and litre in Commonwealth English) is a unit of volume. ... In a mixture of ideal gases, each gas has a partial pressure which is the pressure which the gas would have if it alone occupied the volume. ... Standard atmosphere (symbol: atm) is a unit of pressure. ... Pressure (symbol: p or P) is the measure of the force that acts on a unit area. ... The mole fraction is one way of expressing the relative concentration of a given species. ...


It should also be noted the Henry's Law is a limiting law that only applies for dilute enough solutions. The range of concentrations in which it applies becomes narrower the more the system diverges from non-ideal behavior. Roughly speaking, that is the more chemically different the solute is from the solvent.


It also only applies for solutions where the solvent does not react chemically with the gas being dissolved. A common example of a gas that does react with the solvent is carbon dioxide, which rapidly forms hydrated carbon dioxide and then carbonic acid (H2CO3) with water. A chemical reaction occurs when vapours of hydrogen chloride in a beaker and ammonia in a test tube meet to form a cloud of a new substance, ammonium chloride A chemical reaction is a process that results in the interconversion of chemical substances [1]. The substance or substances initially involved... Carbon dioxide is a chemical compound composed of one carbon and two oxygen atoms. ... Carbonic acid (ancient name acid of air or aerial acid) has the formula H2CO3. ...


Temperature dependence of the Henry constant

When shifting temperature of the system, the Henry constant will not remain changeless. This is why some people prefer to name it Henry coefficient. There are multiple equations assessing the behaviour of the constant. A simple example is [3], which is a form of the van't Hoff equation: The Vant Hoff equation in chemical thermodynamics relates the change in temperature to the change in the equilibrium constant given the enthalpy change. ...

k_{H,cp} = k_{H,cp,Theta} cdot e^{ left[ C cdot left( frac{1}{T}-frac{1}{T_Theta}right)right]},

where T is in kelvins and the index Θ (Theta) refers to the standard temperature (298K). == // == Dimensionless temperature in transport phenomena. ...


The following table lists some values for constant C (dimension of kelvins) in the equation above:

Table 2: Values of C
Gas O2 H2 CO2 N2 He Ne Ar CO
C 1700 500 2400 1300 230 490 1300 1300

It can be seen, that the solubility of gases is decreasing with increasing temperature. While heating water (saturated with nitrogen) from 25°C to 95°C the solubility will decrease to about 43% of its initial value. This can be verified when heating water in a pot. Small bubbles evolve and rise, long before the water reaches boiling temperature. The constant C may be regarded as: General Name, Symbol, Number oxygen, O, 8 Chemical series Nonmetals, chalcogens Group, Period, Block 16, 2, p Appearance colorless (gas) very pale blue (liquid) Atomic mass 15. ... This article is about the chemistry of hydrogen. ... Carbon dioxide is an atmospheric gas composed of one carbon and two oxygen atoms. ... General Name, Symbol, Number nitrogen, N, 7 Chemical series nonmetals Group, Period, Block 15, 2, p Appearance colorless Atomic mass 14. ... General Name, Symbol, Number helium, He, 2 Chemical series noble gases Group, Period, Block 18, 1, s Appearance colorless Atomic mass 4. ... General Name, Symbol, Number neon, Ne, 10 Chemical series noble gases Group, Period, Block 18, 2, p Appearance colorless Atomic mass 20. ... General Name, Symbol, Number argon, Ar, 18 Chemical series noble gases Group, Period, Block 18, 3, p Appearance colorless Atomic mass 39. ... Carbon monoxide, with the chemical formula CO, is a colourless, odourless and tasteless gas. ...

C = frac{-Delta_{solv}H}{R} = frac{d cdot ln left(k_{H,cp}right)}{d(1/T)}

where Delta_{solv}H , is the enthalpy of solution, and R is the gas constant. Heat of solution is the quantity of heat evolved or absorbed when one mole of a solute is dissolved in a large volume of a solvent. ... The gas constant (also known as the universal or ideal gas constant, usually denoted by symbol R) is a physical constant used in equations of state to relate various groups of state functions to one another. ...


Henry's law in geophysics

In geophysics a version of Henry's law applies to the solubility of a noble gas in contact with silicate melt. One equation used is ‹ The template below has been proposed for deletion. ... The noble gases are the elements in group 18 (Group 0 IUPAC Style) of the periodic table. ... In chemistry, a silicate is a compound containing an anion in which one or more central silicon atoms are surrounded by electronegative ligands. ...

rho_m/rho_g=e^{-beta(mu_{{rm ex},m}-mu_{{rm ex},g})},

where:

  • subscripts m = melt
  • subscript g = gas phase
  • ρ = the number densities of the solute gas in the melt and gas phase
  • β = 1 / kBT an inverse temperature scale
  • kB = the Boltzmann constant
  • μex,m and μex,g = the excess chemical potential of the solute in the two phases.

Ludwig Boltzmann The Boltzmann constant (k or kB) is the physical constant relating temperature to energy. ... In thermodynamics and chemistry, chemical potential, symbolized by μ, is a term introduced in 1876 by the American mathematical physicist (Willard Gibbs and his partner Lauren Berkley), which he defined as follows: Gibbs noted also that for the purposes of this definition, any chemical element or combination of elements in given...

Henry's law versus Raoult's law

Both Henry's law and Raoult's law relate the vapor pressure of a component to its concentration. It is possible (and more convenient) for either law to write the concentration in terms of mole fractions x. Note however that the numerical value of k as well as its dimensions change when mole fractions are used rather than molarity (as seen in the Table 1). In chemistry, Raoults law states that the vapor pressure of mixed liquids is dependent on the vapor pressures of the individual liquids and the molar vulgar fraction of each present in solution. ...

Henry's law: p = k_{H,x}*x,
Raoult's law: p = p*x,

The difference is that p* is the equilibrium vapor pressure of the pure component whereas the Henry constant kH is a value that differs from p*. It must be determined experimentally from the mixtures, not the pure compound.


If the solution is ideal (which it seldom is), both components follow Raoult's law over the entire composition range. In most systems, the laws can only be applied in a very limited concentration range at extreme ends of the range. In that case, the minority component (the solute) follows Henry's law, but the solvent still follows Raoult's law. The Gibbs-Duhem equation can be used to prove that this is so. A substance is soluble in a fluid if it dissolves in the fluid. ... The Gibbs-Duhem equation in thermodynamics describes the relationship between changes in chemical potential for components in a thermodynamical system [1] : where is the number of moles of component i, the incremental increase in chemical potential for this component, the entropy, the absolute temperature, volume and the pressure. ...


See also

In chemistry, Raoults law states that the vapor pressure of mixed liquids is dependent on the vapor pressures of the individual liquids and the molar vulgar fraction of each present in solution. ... In chemistry and physics, Daltons law (also called Daltons law of partial pressures) states that the total pressure exerted by a gaseous mixture is equal to the sum of the partial pressures of each individual component in a gas mixture. ... In a mixture of ideal gases, each gas has a partial pressure which is the pressure which the gas would have if it alone occupied the volume. ... Like-A-Fish Technologies is an Israeli business that is developing a system to extract breathable air from water. ... Layers of Atmosphere (NOAA) Air redirects here. ... Impact of a drop of water. ...

References

  1. ^ University of Delware physical chemistry lecture
  2. ^ University of Arizona chemistry class notes
  3. ^ a b c An extensive list of Henry's Law constants, and a conversion tool

External links

  • www.henrys-law.org - Large compilation of Henry's law constants
  • New 'no air tanks' diving system, based on Henry's law - An article with flash presentation

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