When $\ce{HCl}$ reacts with finely powdered iron, it forms ferrous chloride and not ferric chloride. Why?

Answer. Its reaction with iron produces $\ce{H2}$.

$\ce{Fe + 2HCl -> FeCl2 + H2}$

Liberation of hydrogen prevents the formation of ferric chloride.

Now my question is why couldn't it be: $\ce{2Fe + 6HCl -> 2FeCl3 + 3H2}$ ?


1 Answer 1


Acids can oxidize iron since the redox potentials $E$ for $\mathrm{pH} = 0$ show that $\ce{H+}$ can oxidize $\ce{Fe}$ to $\ce{Fe^2+}$; but non-oxidizing acids cannot further oxidize $\ce{Fe^2+}$ to $\ce{Fe^3+}$:

$$\begin{alignat}{2} \ce{2H+ + 2e- \;&<=> H2}\quad &&E^\circ = +0.000\ \mathrm{V}\\ \ce{[Fe(H2O)6]^2+ + 2e- \;&<=> Fe}\quad &&E^\circ = -0.440\ \mathrm{V}\\ \ce{[Fe(H2O)6]^3+ + e- \;&<=> [Fe(H2O)6]^2+}\quad &&E^\circ = +0.771\ \mathrm{V}\\ \end{alignat}$$

However, oxygen can oxidize $\ce{Fe^2+}$ to $\ce{Fe^3+}$:

$$\begin{alignat}{2} \ce{O2 + 4H+ + 4e- \;&<=> 2H2O}\quad &&E^\circ = +1.229\ \mathrm{V}\\ \ce{[Fe(H2O)6]^3+ + e- \;&<=> [Fe(H2O)6]^2+}\quad &&E^\circ = +0.771\ \mathrm{V} \end{alignat}$$

Therefore, iron is oxidized to $\ce{Fe^3+}$ in dilute hydrochloric acid when in contact with air.


Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge that you have read and understand our privacy policy and code of conduct.

Not the answer you're looking for? Browse other questions tagged or ask your own question.