Redox reaction for sour solution

[Mathman23]

Hi All,

$\mathrm{Fe^{3+}(aq) + SCN^{-} (aq) \rightleftharpoons FeSCN^{2+}(aq)}$

I add some $\mathrm{NaHSO_{3}(s)}$ to the above reaction !

This makes the above chemical solution sour, consequently in a sour solution $\mathrm{HSO^{-}_{3}}$ will reduce Fe^{3+} to Fe^{2+}, meanwhile $HSO_{3}^{-}$ is itself oxidated to $SO^{2-}_{4}$.

a/ The color of the original resolution changes then adding $\mathrm{NaHSO_{3}(s)}$ to yellow.

b/ By adding $\mathrm{NaHSO_{3}(s)}$ to the orginal solution the equilibrium direction changes, such that it runs from right to left.

c/ I need help writing the reaction between $HSO^{-}_{3}$ and $SO^{-}_{4}$.

d/ Which influence does the adding of $HSO^{-}_{3}$ have on the concentration of $Fe^{3+}$

I hope that there someone out there who is willing to assist me since this is my last post !

Sincerely
Fred

 

[chem_tr]

Hello,

Let me first do the redox thing in atomic level, since it's easier than dealing with a bunch of atoms.

We know that iron(III) is reduced to iron(II), and in turn, sulfite (S⁴⁺) is oxidized to sulfate (S⁶⁺):

$$S^{4+} \longrightarrow S^{6+}+2e^-$$
$$2Fe^{3+}+2e^-\longrightarrow 2Fe^{2+}$$
--------------------------------
$$S^{4+}+2Fe^{3+} \longrightarrow S^{6+}+2Fe^{2+}$$

In the real redox, you'll need to put some other correcting factors (water and hydroxide, I think) to balance additional oxygens.

Now that we've got rid of the redox reaction, we may look other things. When you add some sodium bisulfate to the medium, a redox reaction occurs; and so the thiocyanate complex changes from $\displaystyle Fe(SCN)^{2+}$ to $\displaystyle Fe(SCN)^+$; this gives a totally different color. Iron(II) sulfate is also formed from the reaction, and the color may have also come from this one, so I don't think that the equilibrium shifts to the left, but I may be wrong; I don't insist on that.

 

[ravenprp]

Hi Fred,

Thank you for sharing your question with us. It seems like you are conducting an interesting experiment with the reaction between Fe^{3+} and SCN^{-}. Adding NaHSO_{3}(s) to the solution will indeed result in a change in the equilibrium direction, as you correctly pointed out. This is because NaHSO_{3}(s) acts as a reducing agent, converting Fe^{3+} to Fe^{2+} and itself being oxidized to SO^{2-}_{4}. This change in oxidation states can also cause a change in color, which you observed to be yellow.

To answer your questions:

a/ The color change to yellow is likely due to the formation of Fe^{2+}. Fe^{2+} is typically pale green or yellow in color, whereas Fe^{3+} is usually red or orange.

b/ Adding NaHSO_{3}(s) does indeed change the equilibrium direction, as mentioned above. This is because the addition of a reducing agent (NaHSO_{3}(s)) shifts the equilibrium towards the reactants (Fe^{3+} and SCN^{-}).

c/ The reaction between HSO^{-}_{3} and SO^{-}_{4} can be written as HSO^{-}_{3} + SO^{-}_{4} \rightarrow SO^{2-}_{4} + H^{+}. This is a redox reaction, with HSO^{-}_{3} being reduced to SO^{2-}_{4} and SO^{-}_{4} being oxidized to SO^{2-}_{4}.

d/ The addition of HSO^{-}_{3} will decrease the concentration of Fe^{3+} in the solution, as it is being reduced to Fe^{2+}. This can be seen by the change in color from red/orange to yellow.

I hope this helps to clarify your doubts. Don't hesitate to reach out if you have any other questions. Good luck with your experiment!