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Partial Equation Method:
In all such cases as stated at limitation of the hit and trial method, balancing can be done more easily by supposing the complex reaction to take place in steps, writing equations for these individual steps and finally adding them up. This is known as the method of partial equations. Balancing such equations by this method will illustrate its usefulness.
The reaction between zinc and dilute nitric acid to give zinc nitrate, nitrous oxide and water is supposed to be completed in the following steps:
a) First dil. HNO3 reacts with zinc to yield zinc nitrate and nascent hydrogen.
b) The nascent hydrogen obtained in the first step reduces nitric acid to nitrous oxide and water. Products like nascent hydrogen which do not appear in the final products are called Intermediate products.
Writing simple equation for the two steps and adding them is such a way that the intermediate products (nascent hydrogen) cancel, then we will get the required solution. To equalize nascent hydrogen atoms, multiply the first equation by 4. On adding, the nascent hydrogen atoms cancel each other, so that we have:
Zn + 2HNO3 = Zn (NO3)2 + 2H] ×4
2HNO3 + 8H = N2O+ 5H2O
4Zn + 10HNO3 = 4Zn (NO3)2+N2O+5H2O
Action of chlorine on cold caustic soda solution is supposed to be completed in the given steps:
a) Chlorine reacts with water to yield hydrochloric acid and hypochlorous acid.
b) Hydrochloric acid produces in reacts with NaOH to give NaCl and water.
c) Hypochlorous acid produced in reacts with NaOH to yield sodium hypochlorite and water.
Writing simple equation for these steps and adding them in such a way that the intermediate products (HCl and HClO in this ease) cancel, we shall have the desired equation.
Cl2 + H2O = HCl + HClO
HCl + NaOH = NaCl + H2O
HClO + NaOH = NaClO + NaClO + H2O
Cl2 + 2NaOH = NaCl + NaClO + H2O