Question
Why is sulphuric acid a strong acid?
✓
Answer
Sulphuric acid ionises in an aqueous solution in two steps as follows:
Step 1:
\9\begin{aligned}
& H _2 SO _{4( aq )}+ H _2 O _{( l )} \longrightarrow \\
& H _3 O ^{+}{ }_{( aq )}+ HSO _{4 \text { (aq) }}^{-} K_{ a }>10 \\
&
\end{aligned}$
Step 2 :
$\begin{aligned}
& HSO _{4( aq )}^{-}+ H _2 O _{( l )} \longrightarrow \\
& H _3 O _{\text {(aq) }}^{+}+ SO _4^{2-}\left(\text { (aq) } K_{ a }=1.2 \times 10^{-2}\right. \\
&
\end{aligned}$
The larger value of $K _{ a }\left( K _{ a }>10\right)$ shows more dissociation of the acid into $H _3 O ^{+}$and $H _2 SO _4$. Thus $H _2 SO _4$ is a strong acid.
Step 1:
\9\begin{aligned}
& H _2 SO _{4( aq )}+ H _2 O _{( l )} \longrightarrow \\
& H _3 O ^{+}{ }_{( aq )}+ HSO _{4 \text { (aq) }}^{-} K_{ a }>10 \\
&
\end{aligned}$
Step 2 :
$\begin{aligned}
& HSO _{4( aq )}^{-}+ H _2 O _{( l )} \longrightarrow \\
& H _3 O _{\text {(aq) }}^{+}+ SO _4^{2-}\left(\text { (aq) } K_{ a }=1.2 \times 10^{-2}\right. \\
&
\end{aligned}$
The larger value of $K _{ a }\left( K _{ a }>10\right)$ shows more dissociation of the acid into $H _3 O ^{+}$and $H _2 SO _4$. Thus $H _2 SO _4$ is a strong acid.
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