Animated Solution for Chemistry - s and p-Block Elements: Among the following oxoacids, the correct decreasing order of acid strength is

When faced with a lineup of oxoacids like ${\text{HClO}}_4$, ${\text{HClO}}_3$, ${\text{HClO}}_2$, and $\text{HOCl}$, determining their acidic strength might seem daunting at first glance. However, the secret lies not in the acids themselves, but in what they leave behind.

The golden rule of acidity is simple: Acidic Strength is directly proportional to the stability of its conjugate base.

Imagine an acid as a generous donor of ${\text{H}}^{+}$ ions. The easier it is for the acid to let go of that proton, the stronger the acid is. But an acid will only willingly donate its proton if the resulting species—the conjugate base—is stable and comfortable carrying the leftover negative charge.

Let's strip away the ${\text{H}}^{+}$ ions from our four contenders and examine the resulting conjugate bases:

1. ${\text{HClO}}_4\rightleftharpoons {\text{H}}^{+}+{\text{ClO}}_4^{-}$ (Perchlorate ion) 2. ${\text{HClO}}_3\rightleftharpoons {\text{H}}^{+}+{\text{ClO}}_3^{-}$ (Chlorate ion) 3. ${\text{HClO}}_2\rightleftharpoons {\text{H}}^{+}+{\text{ClO}}_2^{-}$ (Chlorite ion) 4. $\text{HOCl}\rightleftharpoons {\text{H}}^{+}+{\text{ClO}}^{-}$ (Hypochlorite ion)

Now, the million-dollar question: Which of these ions is the most stable? The answer lies in the magical phenomenon of resonance.

Resonance is nature's way of sharing the burden. When a negative charge is localized on a single atom, it creates instability. But if that charge can be spread out—delocalized—over multiple atoms, the overall energy of the system drops, leading to greater stability.

Let's look at our ions: - In ${\text{ClO}}_4^{-}$, the negative charge is beautifully delocalized across four highly electronegative oxygen atoms. This creates four equivalent resonance structures, making it incredibly stable. - In ${\text{ClO}}_3^{-}$, the charge is shared among three oxygen atoms. It's stable, but not quite as stable as perchlorate. - In ${\text{ClO}}_2^{-}$, the charge is shared between just two oxygen atoms. - Finally, in ${\text{ClO}}^{-}$, there is no resonance at all. The negative charge is entirely stuck on a single oxygen atom, making it the least stable of the bunch.

Therefore, the stability order of the conjugate bases is:

Since the stability of the conjugate base dictates the strength of the parent acid, we can confidently conclude the order of acidic strength:

If you're short on time during an exam, there's a brilliant shortcut. For oxoacids of the same central atom, the acidic strength increases with the oxidation state of the central atom.

Let's calculate the oxidation state of Chlorine in each: - ${\text{HClO}}_4$: Cl is +7 - ${\text{HClO}}_3$: Cl is +5 - ${\text{HClO}}_2$: Cl is +3 - $\text{HOCl}$: Cl is +1

As you can see, the higher the oxidation state, the stronger the acid. This trick works like a charm and is a massive time-saver!