Question

How would you balance $PC{l_5}\, + \,{H_2}O\, \to \,{H_3}P{O_{4\,}}\, + \,HCl$?

Answer 1

Hint:We will balance this chemical reaction by Oxidation number method. After balancing the oxidation numbers of the atoms at both sides, we need to balance the remaining atoms O and H by hit and trial method. Follow the procedure of balancing the reaction stepwise in order to arrive at the correct result.

Complete answer:
-The balancing is done in following steps:
$PC{l_5}\, + \,{H_2}O\, \to \,{H_3}P{O_{4\,}}\, + \,HCl$
-Write the oxidation number of each atom in the equation
${P^{ + 5}}C{l^{ - 1}}_5\, + \,{H^{ + 1}}_2{O^{ - 2}}\, \to \,{H^{ + 1}}_3{P^{ + 5}}{O^{ - 2}}_4\, + {H^{ + 1}}C{l^{ - 1}}$
-Identify the atoms which undergo change in oxidation number. ${P^{ + 5}}C{l^{ - 1}}_5\, + \,{H^{ + 1}}_2{O^{ - 2}}\, \to \,{H^{ + 1}}_3{P^{ + 5}}{O^{ - 2}}_4\, + {H^{ + 1}}C{l^{ - 1}}$
-We can clearly see that the oxidation number of each element is the same. Hence, this is not a redox reaction. It is simply a hydrolysis reaction.
-Calculate the increase and decrease in oxidation number with respect to reactant atoms.
-No increase or decrease in oxidation number in the given reaction. No need to do this step.
-Equate the increase and decrease in oxidation number.
-No need to do this step.
-Balance Cl atoms in the given reaction as P atoms are already balanced.
$PC{l_5}\, + \,{H_2}O\, \to \,{H_3}P{O_4}\, + 5HCl$
-Balance H atoms in the given reaction.
$PC{l_5}\, + 4{H_2}O\, \to {H_3}P{O_4}\, + 5HCl$
-Now, we will see that the O atoms are already balanced in this reaction.
 $PC{l_5}\, + 4{H_2}O\, \to {H_3}P{O_4}\, + 5HCl$
This is the required balanced equation.

Note:The balancing of a chemical reaction is done to equalize the atoms of different elements or compounds which are involved in it. It is done to fulfil the requirement of the law of conservation of mass. We need to identify carefully the oxidation number of the reactants and products. Also identify carefully the elements undergoing increase and decrease in oxidation number.