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Law of conservation of energy.






Physical principle that the total amount of energy in a system remains constant, although energy can be changed from one form to another or transferred from one object to another. (Other formulation: energy can be converted from one form to another, but can be neither created nor destroyed).

We will defi ne energy as the capacity to do work or to produce heat.

The law of conservation of energy can be seen in these everyday examples of energy transference: water can produce electricity. Water falls from the sky, converting potential energy to kinetic energy. This energy is then used to rotate the turbine of a generator to produce electricity. In this process, the potential energy of water in a dam can be turned into kinetic energy which can then become electric energy.

 

Laws of defi nite and multiple proportions.

Joseph Proust (1754–1826), showed that a given compound always contains exactly the same proportion of elements by mass. For example, Proust found that the substance copper carbonate is always 5.3 parts copper to 4 parts oxygen to 1 part carbon (bymass). The principle of the constant composition of compounds, originally called “Proust’slaw, ” is now known as the law of defi nite proportion. Proust’s discovery stimulated John Dalton (1766–1844), an English schoolteacher, to think about atoms as the particles that might compose elements. Daltonreasoned that if elements were composed of tiny individual particles, a given compoundshould always contain the same combination of these atoms. This concept explained whythe same relative masses of elements were always found in a given compound. But Dalton discovered another principle that convinced him even more of the existence of atoms. He noted, for example, that carbon and oxygen form two different compounds that contain different relative amounts of carbon and oxygen.

Dalton noted that compound II contains twice as much oxygen per gram of carbon as compound I, a fact that could easily be explained in terms of atoms. Compound I might be CO, and compound II might be CO2.* This principle, which was found to apply to compounds of other elements as well, became known as the law of multiple proportions: When two elements form a series of compounds, the ratios of the masses of the second element that combine with 1 gram of the fi rst element can always be reduced to small whole numbers.

Avogadro’s law

 

Avogadro’s law, a statement that under the same conditions of temperature and pressure, equal volumes of different gases contain an equal number of molecules. This empirical relation can be derived from the kinetic theory of gases under the assumption of a perfect (ideal) gas. The law is approximately valid for real gases at sufficiently low pressures and high temperatures.

The specific number of molecules in one gram-mole of a substance, defined as the molecular weight in grams, is 6.02214129 × 1023, a quantity called Avogadro’s number, or the Avogadro constant. For example, the molecular weight of oxygen is 32.00, so that one gram-mole of oxygen has a mass of 32.00 grams and contains 6.02214129 × 1023 molecules.

The volume occupied by one gram-mole of gas is about 22.4 L at standard temperature and pressure (0°C, 1 atmosphere) and is the same for all gases, according to Avogadro’s law. It called molar volume of gas.

The law was first proposed in 1811 by Amedeo Avogadro, a professor of higher physics at the University of Turin for many years, but it was not generally accepted until after 1858, when an Italian chemist, Stanislao Cannizzaro, constructed a logical system of chemistry based on it.

From the Avogadro's law follows an important consequence: the mass ratio of equal volumes of the two different gases is constant for these gases. This constant is called the relative density of the gas and marked D. Because the molar volumes of all gases are identical, the ratio of the molar masses of each pair of gases is also equal to this constant:

where М1 and М2 – molar masses of two gaseous substances.


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