Animated Solution for Chemistry - Redox Reactions: Oxidation number of $\text{Cl}$ in ${\text{CaOCl}}_2$ (bleaching powder) is

When you first encounter the chemical formula for bleaching powder, ${\text{CaOCl}}_2$, it looks deceptively simple. It is incredibly tempting to treat it like a standard compound and calculate an average oxidation state for the two chlorine atoms. If you were to do that, you might assume calcium is $+2$, oxygen is $-2$, and therefore the two chlorine atoms must average out to an oxidation state of $0$.

However, chemistry is rarely that straightforward, and this is a classic trap set by examiners. The average oxidation state doesn't tell the whole story of what is happening at the molecular level.

To truly understand the oxidation states, we need to look at the actual ionic structure of bleaching powder. It is not a simple salt; it is a mixed salt.

When bleaching powder dissociates, it doesn't just break apart into random atoms. It forms three distinct ions: one calcium cation (${\text{Ca}}^{2+}$), one hypochlorite anion (${\text{OCl}}^{-}$), and one chloride anion (${\text{Cl}}^{-}$). Because the two chlorine atoms reside in completely different ionic environments, they experience different electron distributions and, consequently, have different oxidation states.

Let's break down the two anions separately to find the oxidation state of each chlorine atom.

First, consider the hypochlorite ion, ${\text{OCl}}^{-}$. In this polyatomic ion, oxygen is bonded to chlorine. Since oxygen is significantly more electronegative than chlorine, it hogs the shared electrons, taking its standard oxidation state of $-2$. The entire ion has a net charge of $-1$. We can set up a simple algebraic equation where $x$ is the oxidation state of chlorine:

Solving for $x$, we get $x=+1$. So, the chlorine atom in the hypochlorite ion has an oxidation state of $+1$.

Next, let's look at the chloride ion, ${\text{Cl}}^{-}$. This one is much simpler. It is a monatomic ion with a charge of $-1$. By definition, the oxidation state of a monatomic ion is equal to its charge. Therefore, the oxidation state of this chlorine atom is $-1$.

By analyzing the true ionic structure of bleaching powder, we've uncovered a fascinating chemical reality. Within a single formula unit of ${\text{CaOCl}}_2$, chlorine exists in two completely different oxidation states simultaneously: $+1$ and $-1$.

This dual nature is what makes bleaching powder such a unique and frequently tested compound in competitive exams. Always remember to look beyond the empirical formula and consider the actual structural components of mixed salts!