9th Class Science Atoms and Molecules Laws of Chemical Combination

        Introduction

All the matters found in this universe are made up of small particles called atoms. The idea of divisibility of matters came into light around 500 BC. An Indian Philosopher, Maharishi Kanad, in his postulate said that if we go on dividing the padarth till it is not any more divisible, then we come across the particles which is not further divisible is called parmanu. This smallest parmanu in Greek word is called atom, which means indivisible. All this explanation was based on philosophical consideration and not on experimental work, till the end of eighteenth century, when perhaps the first scientific defination of atoms came into existence and difference between element and compound was established.

          Lams of Chemical Combination

The chemical reactions of two or more atoms to form new product is carried out by many laws.

The important laws of chemical combinations are:

• Laws of conservation of mass
• Laws of constant proportion
• Laws of multiple proportion
• Gay Lussac's law

            Laws of Conservation of Mass

This law was given by A. Lavoisier. He states that during a chemical reaction mass can neither be created nor be destroyed. For example:

\[CaC{{O}_{3}}\,\text{(rectant}\,100g)\to CaO\,(56g)+C{{O}_{2}}\,(44g)\,\text{(product)}\]

Thus we see that total mass of reactant is equal to the total mass of product. A practical example of this is: take a piece of ice cube and measure its weight. Put it in a flask and leave it to melt. When it melts completely again measure its weight. You will find the weight in both the case is approximately same. Thus we can say that the law of conservation of mass is also valid in case of physical change.

For example:

Take some amount of iron filing and sulphur powder of desired weight. Put them in a china dish and heat it. The compound so formed is iron sulphide and measure its weight. We find that the weight in both the case is approximately same. Thus we can say that law of conservation of mass is also valid for chemical change.

            Laws of Constant Proportion

It was given by Joshep Prouts in 1799. It states that in a compound the elements are always present in definite proportions by mass.

For example:

Carbon dioxide is made up of two elements, carbon and oxygen which is always present in same proportion of 12 : 32 by mass. Thus 44 g of carbon dioxide is composed of 32 g oxygen and 12 g of carbon.

Thus we see that laws of constant proportion is true, regardless of the source of the sample, which may be obtained from the natural source or prepared artificially in the laboratory.

            Laws of multiple Proportion

It states that if two elements form more than one compound between them, then the ratio of the masses of the two elements, which combine with the fixed mass of the first element, will be in the ratio of smallest whole number.

For example:

Considering two of the carbon oxides: CO and CO₂, 100 grams of carbon may react with 133 grams of oxygen to produce carbon monoxide, or with 266 grams of oxygen to produce carbon dioxide. The ratio of the masses of oxygen that can react with 100 grams of carbon is 266:133 H, which is equal to 2:1, a ratio of small whole numbers. The Law of Multiple Proportion, is just what the name suggests, is the law of multiple proportions of one constant element, within differing compounds is sharing the same type of chemical bonding.

             Gay Lussac's Law

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

Or this law can also be stated as the pressure of a sample of gas at constant volume, is directly proportional to its temperature in Kelvin.

According to Gay-Lussac's Law:

• \[\frac{{{P}_{1}}}{{{T}_{1}}}=\,\text{Constant}\,(k)\]
• After the change in pressure and temperature,
• \[\frac{{{P}_{2}}}{{{T}_{2}}}=\,\text{Constant}\,(k)\]

 Combine the two equations:

\[\frac{{{P}_{1}}}{{{T}_{1}}}=\frac{{{P}_{2}}}{{{T}_{2}}}\]

When any three of the four quantities in the equation are known, the fourth can be calculated..