Question

$CaOC{{l}_{2}}$ (Bleaching powder) has two types of chlorine with different oxidation numbers. The sum of both the oxidation numbers is _______.

Answer 1

$CaOC{{l}_{2}}$ is a neutrally charged compound made up of hypochlorite and chlorine anions and a calcium cation. Oxygen has more electronegativity compared to chlorine. The overall charge on species is equal to the sum of the oxidation numbers of atoms involved in the structure.

Complete step by step answer:
Let’s see the structure of bleaching powder to find the oxidation number of chlorine atoms.
$C{{l}^{-}}\text{ }C{{a}^{++}}{{\text{ }}^{-}}OCl$

Here, we can see that one hypochlorite anion and one chloride anion is bonded with a calcium atom which has +2 charge.
Let’s find out the oxidation number of chlorine anion first. We can say that for $CaOC{{l}_{2}}$ ,
Overall charge = Oxidation number of Calcium + charge on hypochlorite anion + oxidation number of chlorine anion.
Here, the overall charge on $CaOC{{l}_{2}}$ is zero. So,
0 = (+2) + (-1) + oxidation number of chlorine anion
Oxidation number of chlorine anion = -2+1
Oxidation number of chlorine anion = (-1)

Now, let’s find out the oxidation number of chlorine atoms in hypochlorite anion.
Overall charge = Oxidation number of oxygen + oxidation number of chlorine
Here, we can see that overall charge on $^{-}OCl$ has to be (-1). Here oxygen will have negative charge because it is more electronegative in comparison with chlorine atom.
So, we can write that (-1) = (-2) + oxidation number of chlorine
Oxidation number of chlorine = +2 – 1
Oxidation number of chlorine in hypochlorite anion = +1
Hence if we do the sum of both oxidation numbers of chlorine then we will get
+1 + (-1) = 0.
So, the sum of oxidation numbers of both chlorine atoms in $CaOC{{l}_{2}}$ is zero.

Note: Remember that when more electronegative atoms make compounds with chlorine, then chlorine has a positive oxidation number as its electronegativity will be lower. Note that calcium always has +2 oxidation state in salts, so if you take oxidation number of chlorine equal to (-1) and oxygen’s oxidation number as (-2), in $CaOC{{l}_{2}}$ , then calcium will not have +2 charge which is not possible.