[Chemistry] Laws of Chemical Combination

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INTRODUCTION

  • As we have argued before that chemistry is the science of matter.
  • Matter is always transforming from one state to another state. These transformations occur as a result of interaction among different types of matter.
  • This interaction among various forms of matter is termed as a chemical reaction

CHEMICAL REACTION:

DEFINITION:

A process by which one or more substances (reactants) interact with each other leading to the formation of one or more different substances (products).

    • Chemical reaction occurs by the re-arrangement of constituent atoms and molecules of the reactants.
    • Chemical reaction results in the formation of a product that is chemically different than the reactants.
  • In simple words chemical reaction brings about a chemical change.

EXAMPLE:

2H2 + O2 —————————————> 2H2O

Reactants product

LAWS OF CHEMICAL COMBINATION:

The combination of different elements to form various compounds is based upon certain laws known as the laws of chemical combination. There are five laws of chemical combinations.

LAW OF CONSERVATION OF MASS:

BACKGROUND:

This law was coined by Antoine Lavoisier in 1789 and was based upon the data he collected from studying various combustion reactions.

STATEMENT:

This law states that

Mass can neither be created nor destroyed. It can only be converted from one form to another form.

EXPLANATION:

When a chemical reaction takes place, the sum of masses of reactants is the same as that of the products.

In simple terms

Mass of the reactants = Mass of the product.

EXAMPLE:

  • Heating 10 grams of calcium carbonate (CaCo3) results in the formation of 4.4 grams of carbon dioxide (CO2) and 5.6 grams Calcium oxide (CaO)

CaCO3 ————————————-> CO2 + Cao

10 gms ———————————-> 4.4 gms + 5.6 gms

10 grams of reactants = 10 grams of the product

  • Since the mass of the reactants is equal to the sum of masses of the products, this argument is in agreement with the law of conservation of mass.

LAW OF DEFINITE PROPORTIONS:

BACKGROUND:

The law of definite proportion, also known as Proust’s Law, was put forward by a French chemist, Joseph Proust in 1779.

STATEMENT:

This law states that

A chemical compound always consists of the same elements present in the exact same proportion irrespective of the source, origin and quantity

Explanation:

  • A chemical compound is made up of the same elements irrespective of the source and quantity of the compound.
  • The percentage composition of elements by weight within a compound also remains unchanged

EXAMPLE:

In order to understand the law of constant proportion we take the reaction of water as an example

2H2 + O2 ———————–> 2H2O

4 units + 32 units = 36 units

1 unit : 8 units

  • From the above reaction we can see that 1 unit of hydrogen combines with 8 units’ oxygen to form water.
  • Therefore, the proportion of hydrogen to oxygen in water is 1:8.
  • This proportion of hydrogen and oxygen in the water will always remain the same whether the water is collected from a river or tap

LAW OF MULTIPLE PROPORTIONS:

BACKGROUND:

The law of multiple proportions was put forward by John Dalton in 1803.

STATEMENT:

This law states that,

If two elements form more than one chemical compound, the masses of the second element which combines with a fixed mass of the first element are always in ratios of simple whole numbers.

EXPLANATION:

  • Same elements under different conditions can combine to form different compounds i.e. hydrogen and oxygen can form water as well as hydrogen peroxide
  • The mass of hydrogen is always the same in both compounds
  • The mass of oxygen changes among the two compounds
  • The ratio among the masses of oxygen within the two compounds will always be in the whole number such as 1:2. In simple words the ratio can never be in decimals

EXAMPLE:

  • Carbon and oxygen under different circumstances can combine to form two compounds
  1. Carbon monoxide: 12 parts of carbon combines with 16 parts of oxygen
  2. Carbon Dioxide: 12 parts of carbon combines with 32 parts of oxygen.
  • From the above example you can see that the same elements i.e. carbon and oxygen react to form two different compounds.
  • The mass of carbon in both reactions remain constant i.e. 12 parts
  • The mass of oxygen in each reaction changes i.e. 16 parts and 32 parts.
  • When the ratio is derived from the concentrations of oxygen i.e. 32/16 = 2 : 1, this ratio is always in whole numbers and can never be in decimal numbers

GAY-LUSSAC’S LAW OF GASEOUS VOLUMES:

BACKGROUND:

This law was coined by the French Chemist, Joseph Gay-Lussac in 1808 based on his observations.

STATEMENT:

This law states that

When gases combine or are produced in a chemical reaction they do so in a simple ratio by volume, provided all the gases involved in the reaction are at the same temperature and pressure.

EXPLANATION:

This law is similar to the law of definite proportions. The only difference is that Gay-Lussac’s law is stated in terms of volume whereas the law of Definite proportions is stated in terms of mass.

EXAMPLE:

  • In order to understand Gay-Lussac’s law, we take the reaction of nitrogen and hydrogen leading to the formation of ammonia as an example

N2 + 3H2 —————————————–> 2NH3

1 vol + 3 Vol —————————————–> 2 vol

  • When the temperature and pressure are kept constant, 1 volume of nitrogen will always react with 3 volumes of hydrogen to give 2 volumes of ammonia.
  • the proportion of hydrogen and nitrogen required for ammonia is 1:3 which is in simple whole number.

POINTS TO PONDER:

  • when all the reactants and products are not in gaseous form then only the volumes of gases will be in a simple whole-number ratio i.e. this law only applies to the volume of gases.
  • Gay-Lussac’s law of gaseous volumes should not be confused with Gay-Lussac’s law of pressure-temperature.

AVOGADRO’S LAW:

BACKGROUND:

This law was derived by Amedeo Avogadro in 1811.

STATEMENT:

This law states that

At the same temperature and pressure, equal volumes of all gases will contain the same number of molecules.

EXAMPLE:

Suppose under conditions of constant temperature and pressure we have equal volumes of CO2 and NH3.

  • Since the volumes of gases are equal i.e. 1 litre of CO2 and NH3
  • The temperature and pressure is constant
  • Therefore, both volumes of gases contain an equal number of particles

MATHEMATICAL EXPLANATION:

  • Suppose 1 litre of gas contains x no of particles.
  • Then two litres of the same gas will have 2x no of particles

1 litre of gas = x number of particles

2 litres of gas = 2x number of particles

½ litre of gas = 0.5x number of particles and so on

From the above discussion we can derive that,

Whereas k is the constant for given temperature and pressure

This is the mathematical presentation of Avogadro’s law

 

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