Gay-Lussac's law

The expression Gay-Lussac's law is used for each of the two relationships named after the French chemist Joseph Louis Gay-Lussac and which concern the properties of gases, though it is more usually applied to his law of combining volumes, the first listed here. One law relates to volumes before and after a chemical reaction while the other concerns the pressure and temperature relationship for a sample of gas.

Law of combining volumes

The law of combining volumes states that, when gases react together to form other gases, and all volumes are measured at the same temperature and pressure:

The ratio between the volumes of the reactant gases and the products can be expressed in simple whole numbers.

This reflects the fact that (by Avogadro's law) equal volumes of gas contain equal numbers of molecules (at the same temperature and pressure), and also that in chemical reactions, the molecules combine in a ratio of whole numbers - this is known as the stoichiometry of the chemical reaction and is expressed via the chemical equation for the reaction.

Pressure-temperature law

Gay-Lussac's other law, discovered in 1802, states that:

The pressure of a fixed mass and fixed volume of a gas is directly proportional to the gas's temperature.

Simply put, if a gas's temperature increases then so does its pressure, if the mass and volume of the gas are held constant. The law has a particularly simple mathematical form if the temperature is measured on an absolute scale, such as in Kelvin. The law can then be expressed mathematically as:

or

where:

P is the pressure of the gas.

T is the temperature of the gas (measured in Kelvin).

k is a constant.

This law holds true because temperature is a measure of the average kinetic energy of a substance; as the kinetic energy of a gas increases, its particles collide with the container walls more rapidly, thereby exerting increased pressure.

For comparing the same substance under two different sets of conditions, the law can be written as:

Charles's Law was also known as the Law of Charles and Gay-Lussac, because Gay-Lussac published it in 1802 using much of Charles' unpublished data from 1787. However, in recent years the term has fallen out of favor since Gay-Lussac has the second but related law presented here and attributed to him. This related form of Gay-Lussac's Law, Charles's Law, and Boyle's law form the combined gas law. The three gas laws in combination with Avogadro's Law can be generalized by the ideal gas law.

Further reading

• 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. 
• Mascetta, Joseph A. (1998). How to Prepare for the SAT II Chemistry. Barron's. ISBN 0-7641-0331-8.