Gas laws

The early gas laws were developed at the end of the eighteenth century, when scientists began to realize that relationships between the pressure, volume and temperature of a sample of gas could be obtained which would hold for all gases. Gases behave in a similar way over a wide variety of conditions because to a good approximation they all have molecules which are widely spaced, and nowadays the equation of state for an ideal gas is derived from kinetic theory. The earlier gas laws are now considered as special cases of the ideal gas equation, with one or more of the variables held constant.

Boyle's Law

Boyle's Law shows that, at constant temperature, the product of an ideal gas's pressure and volume is always constant. It was published in 1622. It can be determined experimentally using a pressure gauge and a variable volume container. It can also be found logically; if a container with a fixed amount of molecules inside it is reduced in volume, more molecules will hit the sides of the container per unit time causing a greater pressure.

As a mathematical equation, Boyle's law is:

Where P is the pressure (Pa), V the volume (m³) of a gas, and k₁ (measured in joules) is the constant from this equation—it is not the same as the constants from the other equations bel Charles'

Charles's Law, or the law of volumes, was found in 1787. It says that, for an ideal gas at constant pressure, the volume is proportional to the absolute temperature (in kelvins). This can be found using the kinetic theory of gases or a heated container with a variable volume (such as a conical flask with a balloon).

Where T is the absolute temperature of the gas (in kelvins) and k₂ (in m³·K⁻¹) is the constant produced.

Gay-Lussac's Law

The pressure (or Gay-Lussac's) law was found by Joseph Louis Gay-Lussac in 1809. It states that the pressure exerted on a container's sides by an ideal gas is proportional to the absolute temperature of the gas. This follows from the kinetic theory—by increasing the temperature of the gas, the molecules' speeds increase meaning an increased amount of collisions with the container walls.

As a mathematical formula, this is:

Avogadro's Law

Avogadro's Law states that the volume occupied by an ideal gas is proportional to the amount of moles (or molecules) present in the container. This gives rise to the molar volume of a gas, which at STP is 22.4 dm³ (or liters).

Where n is equal to the number of moles of gas (the number of molecules divided by Avogadro's Number).

Combined and ideal gas laws

The combined gas law or general gas equation is formed by the combination of the three laws, and shows the relationship between the pressure, volume and temperature for a fixed mass of gas:

With the addition of Avogadro's law, the combined gas law develops into the ideal gas law:

Where the constant, now named R, is the gas constant with a value of 8.314472(15) J·K^-1·mol^-1

An equivalent formulation of this law is:

where

k is the Boltzmann constant (1.381×10−23J·K−1 in SI units)

N is the number of molecules.

These equations are exact only for an ideal gas, which neglects various intermolecular effects (see real gas). However, the ideal gas law is a good approximation for most gases under moderate pressure and temperature.

This law has the following important consequences:

1. If temperature and pressure are kept constant, then the volume of the gas is directly proportional to the number of molecules of gas.
2. If the temperature and volume remain constant, then the pressure of the gas changes is directly proportional to the number of molecules of gas present.
3. If the number of gas molecules and the temperature remain constant, then the pressure is inversely proportional to the volume.
4. If the temperature changes and the number of gas molecules are kept constant, then either pressure or volume (or both) will change in direct proportion to the temperature.

Other gas laws

• Graham's law states that the rate at which gas molecules diffuse is inversely proportional to the square root of its density. Combined with Avogadro's law (i.e. since equal volumes have equal number of molecules) this is the same as being inversely proportional to the root of the molecular weight.

• Dalton's law of partial pressures states that the pressure of a mixture of gases simply is the sum of the partial pressures of the individual components. Dalton's Law is as follows:

,

OR

,

Where P_Total is the total pressure of the atmosphere, P_Gas is the pressure of the gas mixture in the atmosphere, and P_H2O is the water pressure at that temperature.

• Henry's law 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.

References

• Castka, Joseph F.; Metcalfe, H. Clark; Davis, Raymond E.; Williams, John E. (2002). Modern Chemistry. Holt, Rinehart and Winston. ISBN 0-03-056537-5.
• Guch, Ian (2003). The Complete Idiot's Guide to Chemistry. Alpha, Penguin Group Inc.. ISBN 1-59257-101-8.
• Zumdahl, Steven S (1998). Chemical Principles. Houghton Millfin Company. ISBN 0-395-83995-5.