Avogadro's law

The Avogadro's law, even the law of Avogadro, Avogadrosches principle or set of Avogadro, is a historic, Amedeo Avogadro 1811 -positioned law according to which all gases at the same temperature and pressure equal volumes of the same number of particles (atoms with rare gases and metals or molecules in polyatomic gases ) included. The average distance of the particles from each other must be so large relative to the range of their interactions that interaction is negligible. In this case we speak of an ideal gas. If the interactions are not negligible, one obtains a real gas.

Avogadro initiated this law from from those found by Gay -Lussac law-governed relations across the compounds of gaseous substances. He distinguished atoms and molecules, and also stressed that the transition of the elements in the gas state, these often do not dissolve only in molecules which still consist of several individual atoms but atoms.

Various formulations

Gases consist of molecules or atoms. When atoms and molecules are grouped together as the " smallest particle", the following applies:

• " Equal volumes of all gases contain at the same temperature and pressure equal numbers of the smallest particles. "

• " The molar gas constant has the same value for all gases. "

From the gas laws of Gay- Lussac and Boyle follows for a homogeneous gas, that is a type of gas for the temperature and pressure in the volume are the same everywhere:

The set of Avogadro now states that this constant takes the same value for all gases, it is the universal gas constant or R. This is not, of course, because it means:

• " The same number of particles of two different gases at the same temperature and exercises are always included in the same volume from the same print "

Different gases have different molar masses large, that is, the particles have different weights. Heavier particles move at the same temperature but slower, their speed is so low. Therefore, it is expected that the number of collisions per unit area on a vessel wall for heavier molecules is smaller, but also that the consideration transferred in a shock pulse is correspondingly larger. Here, the result of the collisions resulting pressure is equal, that is, the different influences alike straight.

It follows that the gas- Eq

For all sufficiently dilute gases is considered. It is a general gas equation applies regardless of the particular molecule or molecular weight and thus is independent of material.

Application

From Avogadro's law indirectly follows that the density of different gases is proportional to the molecular weight at the same pressure and temperature. This also applies to molecules of the same gas, which consist of different isotopes of the elements. This fact comes in uranium enrichment for use.

Another important application of the law is the determination of molecular or molar masses ( often incorrectly called " molecular weights ") is prepared by weighing a known volume of gas.

Importance

Historical Significance

More accurate weighings it was possible at the end of the 18th century, to determine the density of gases. With the advent of electrolysis, the water could be converted into two types of gas: oxygen and hydrogen.

After Lavoisier are all chemical substances from the elementary substances, the elements constructed. The then known metals such as silver, copper, lead, tin were filed by Lavoisier as elements. These elements could with the gas the air - form compounds, which composite materials such as lead oxide, tin oxide or copper oxide formed - Oxygène.

What substances were now elements and substances which were composite materials? These are the questions a chemist employed in the subsequent period. The gases were present the key to the determination of the elements.

Since differed oxygen and hydrogen from water vapor, the water had that in an oxyhydrogen explosion of oxygen and hydrogen was to be a composite material.

Avogadro initiated this law from from those found by Gay -Lussac law-governed relations across the compounds of gaseous bodies. Avogadro now turned to the law, that the same number of gas particles are present in an equal volume at the same pressure and temperature. He used the names élémentaires Molecules (atoms ) and molecules integrantes (molecules). Even for a compound gas was the law. Avogadro assumed that the elements are put together. Each molecule of an element in the gas phase should consist of two atoms of the element.

Three years after Avogadro's publication appeared a paper by André- Marie Ampère. Back then used amps for molecules the term particles. However Ampère had different views on the particles, it was assumed that they would have to be at least eight atoms. 1833 Marc Antoine Gaudin has corrected the atomic theory of Ampère and took as Avogadro two atoms of an elementary gas on. For other substances, such as mercury, he postulated monatomic particles in the gas phase, for he took six sulfur atoms in the gas phase. Since the theories of the molecules in the gas phase were quite complex, they soon fell into oblivion and the majority of chemists around 1845 did not know the ideas of Avogadro.

Jean Baptiste Dumas used the vapor density to determine the atomic mass of a variety of substances. Charles Frédéric Gerhardt formulated from vapor densities formulas for hydrogen chloride, water, ammonia, carbon dioxide. He compared the specific atomic masses with atomic masses of Berzelius and then noted differences. Gerhardt moved the atomic mass of hydrogen, used with H = 1 Berzelius as a reference oxygen, with O = 100 In support of the variations in the atomic masses Gerhardt assumed that an organic molecule in the gas phase requires two parts by volume.

Stanislao Cannizzaro was only rediscovered the work of Avogadro. Important was the realization that certain gas molecules can transform at higher temperature in the elements and thus distort measurements. By determination of the density of diethyl zinc, which has been shown for the first time by Edward Frankland, succeeded in the correct interpretation of the molecules in the gas phase. He concluded that hydrogen in the gaseous state would not exist as atomic gas rather than H2 molecule. Other gases such as oxygen and nitrogen, had to be in the molecular weight and not in atomic state. He also concluded that the atomic mass of the metals had to be given twice as high as before.

Through the knowledge of the molecular masses Cannizzaro many volatile organic compounds could be determined in the subsequent period, so that the structure elucidation of substances significantly improved.

Avogadro's views only first for almost half a century after its first formulation for validity. Since the molar masses of fundamental importance, they won by this Act, a secure foundation for the further development of chemistry. Avogadro's law was thus of great importance, especially for the chemistry in general. But it is also important for the physics, especially for the kinetic theory of gases, which was further developed by James Clerk Maxwell.

Today's meaning

Nowadays molecular weights are determined almost exclusively by means of the mass spectrometer, so that the law is now no longer used for this purpose. However, it has an educational value and is in the thermal equation of state of ideal gases

- Albeit hidden - included ( here in the form of the general gas equation).

Literature sources

• Thermodynamics