Stoichiometry is the calculation of the quantities of reactants and products involved in the chemical reaction. It is the study of the relationship between the number of mole of the reactants and products of a chemical reaction. A stoichiometric equation is a short scientific representation of a chemical reaction.
Rules for writing stoichiometric equations
i.In order to write the stoichiometric equation correctly, we must know the reacting substances, all the products formed and their chemical formula.
ii.The formulae of the reactant must be written on the left side of arrow with a positive sign between them.
iii.The formulae of the products formed are written on the right side of the arrow mark. If there is more than one product, a positive sign is placed between them. The equation thus obtained is called skeleton equation. For example, the Chemical reaction between Barium chloride and sodium sulphate producing BaSO4 and NaCl is represented by the equation as
BaCl2 + Na2SO4 ® BaSO4 + NaCl
This skeleton equation itself is a balanced one. But in many cases the skeleton equation is not a balanced one.
For example, the decomposition of Lead Nitrate giving Lead oxide, NO2 and oxygen. The skeletal equation for this reaction is
Pb(NO3)2 ® PbO + NO2 + O2
iv.In the skeleton equation, the numbers and kinds of particles present on both sides of the arrow are not equal.
During balancing the equation, the formulae of substances should not be altered, but the number of molecules with it only be suitably changed.
vi. Important conditions such as temperature, pressure, catalyst etc., may be noted above (or) below the arrow of the equation.
vii.An upward arrow (-) is placed on the right side of the formula of a gaseous product and a downward arrow (¯) on the right side of the formulae of a precipitated product.
viii.All the reactants and products should be written as molecules including the elements like hydrogen, oxygen, nitrogen, fluorine chlorine, bromine and iodine as H2, O2, N2, F2, Cl2, Br2 and I2