Stoichiometry (Quantitative relationship between reactants and products)

July 16, 2018 Sushil Humagain Chemistry 0

  • Stoichiometry (Greek; ‘Stoicheion’ meaning ‘element’ and ‘metron’ meaning ‘measurement’) means to measure elements.
  • Thus, stoichiometry is the branch of science (Chemistry) that deals with the quantitative relationship between the reactants and the products in a chemical reaction.
  • Dalton’s atomic theory is based on this study.
  • There are five laws of stoichiometry or laws of chemical combination which are as follows:
  1. The law of conservation of mass
  2. The law of constant proportion
  3. The law of multiple proportion
  4. The law of reciprocal proportion
  5. The law of gaseous volumes

The law of conservation of mass:

  • This law was enunciated by Lavoisier in 1789.
  • According to this law, there is no detectable change in mass during a chemical reaction in a closed system.
  • The total mass of reactant undergoing a chemical reaction is always equal to the total mass of the product in a chemical change.
  • Suppose substance A reacts with substance B to give the products C and D.

A + B —–> C + D

  • If mass ‘a’ of A and ‘b’ of B react to ‘c’ of C and ‘d’ of D, then according to the law,

a + b = c + d

  • Example:

2HCl               +             CaO      ——>     CaCl2                  +          H2O

2×36.5 (73)                  40+16 (56)           40+35.5×2 (111)           2+16 (18)

  • In this balanced equation, the total mass of HCl and CaO undergoing chemical reaction equals the total mass of the product, i.e. 73+56=111+18=129

The law of constant proportion:

  • This law was put forward by Prout in 1799.
  • This law states that a pure chemical compound always contains the same elements combined together in the same proportion by weight whatever may be the method of its preparation or whatever may be its source.
  • Let a compound AB is prepared by two methods. In the first method, let ’a’ gm. of A combines with ‘b’ gm. of B to form AB.
  • Similarly, let ‘x’ gm. of A and ‘y’ gm. of B combine to form AB in the second method.
  • Then according to the law of constant or definite proportion, the ratio a:b is always equal to the ratio x:y.

a : b = x : y

  • Example:

Preparation of carbon dioxide by the action of HCl with CaCO3 is shown below:

CaCO3 + 2HCl ——> CaCl2 + H2O + CO2

Similarly, Carbon dioxide can also be prepared by heating coal in atmospheric air.

C + O2 —–> CO2

  • In these, balanced chemical equations, the gas carbon dioxide always contains the elements carbon and oxygen combining together 12 parts by weight of carbon and 32 parts by weight of oxygen. i.e. the ratio by weight is always the same and is equal to 12:32.

The law of multiple proportions:

  • This law was put forward by Dalton in 1803.
  • This law states that if two elements combine to give two or more different chemical compounds, then the weight of one of the elements, which combine with the fixed weight of the other, always show numerical ratio to one another.
  • Example:

2H2 +O2 —–> 2H2O

Here, 2 parts by weight of hydrogen combines with 16 parts by weight of oxygen in one water molecule.

H2 + O2 —–> H2O2

Here, 2 parts by weight of hydrogen combines with 32 parts by weight of oxygen in one hydrogen peroxide molecule.

  • The different weights of oxygen i.e. 16 and 32 which combine with the fixed 2 parts by weight of hydrogen show a simple numerical ratio, 16:32 or 1:2.

The law of reciprocal proportion:

  • This law was put forward by Richter in 1792.
  • The law states that two or more elements, which combine separately with the fixed weight of another element, are either the same or the simple multiples of the weights of the elements when they combine among themselves.
  • Example:

Carbon and oxygen combine separately with hydrogen to produce corresponding compounds methane and water respectively.

C+ 2H2 —–> CH4

O2+ 2H2 —–> 2H2O

  • In the given balanced chemical equations, 12 parts by weight of carbon and 32 parts by weight of oxygen combine separately with the fixed weight 4 parts by weight of hydrogen, to produce methane and water respectively.
  • We know 12 parts by weight of carbon and 32 parts by weight of oxygen combine to form carbon dioxide, which proves the law.

The law of gaseous volumes:

  • This law was enunciated by Gay Lussac in 1808.
  • According to this law, when gases react to give products under the same condition of temperature and pressure, they react in the simple whole number ratio by volumes.
  • Example:

H2           +             Cl2           —–>     2HCl

1 vol.                     1 vol.                     2 vol.  (1:1:2)

One volume of hydrogen gas and 2 volumes of chlorine gas combine to give two volumes of Hydrogen chloride gas under the same condition of temperature and pressure.

Similarly,

N2           +             3H2         —–>     2NH3

1 vol.                     3 vol.                     2 vol.  (1:3:2)

One volume of nitrogen gas and 3 volumes of hydrogen gas combine to give 2 volumes of ammonia gas under the same condition of temperature and pressure.

Stoichiometry (Quantitative relationship between reactants and products)