Lows of Chemical Combinations

Laws of Chemical Combinations

The combination of elements to form compounds is governed by the following six basic laws:

1. Law of conservation of mass (Lavoisier, 1774)

This law states that during any physical or chemical change, the total mass of the products is equal to the total mass of reactants. It does not hold good for nuclear reactions.

2. Law of definite proportions (Proust, 1799)

According to this law, a chemical compound obtained by different sources always contains same percentage of each constituent element.

3. Law of multiple proportions (Dalton, 1803)

According to this law. if two elements can combine to form more than one compound. the masses of one element that combine with a fixed mass of the other element, are in the ratio of small whole numbers, e.g., in NH3 and N2H4, fixed mass of nitrogen requires hydrogen in the ratio 3 : 2.

4. Law of reciprocal proportions (Richter, 1792)

According to this law, when two elements (say A and 13) combine separately with the same weight of a third element (say C), the ratio in which they do so is the same or simple multiple of the ratio in which they (A and H) combine with each other.

Law of definite proportions, law of multiple proportions and law of reciprocal proportions do not hold good when same compound is obtained by using different isotopes of the same element. e.g H2O and D2O

5. Gay Lussac’s law of gaseous volumes

It states that under similar conditions of temperature and pressure. whenever gases react together. the volumes of the reacting gases as well as products (if gases) bear a simple whole number ratio.

6. Avogadro’s hypothesis

It states that equal volumes of all gases under the same conditions of temperature and pressure contain the same number of molecules.

Dalton’s Atomic Theory (1803)

This theory was based on laws of chemical combinations. It’s basic postulates are

1. All substances are made up of tiny. indivisible particles, called atoms.

2. In each element, the atoms are all alike and have the same mass. The atoms of different elements differ in mass.

3. Atoms can neither be created nor destroyed during any physical or chemical change.

4. Compounds or molecules result from combination of atoms in some simple numerical ratio.