Chemistry - Why would a solution of FeCl2 appear brown/yellow sort of like FeCl3?

Solution 1:

The yellow/brown color you are seeing in the solution of $\ce{FeCl2}$ is due to formation of various types of hydrated iron oxide and not iron(III) chloride. They may include both the anhydrous($\ce{FeOOH}$) or the monohydrate($\ce{FeOOH.H2O}$) often referred to as ferric hydroxide($\ce{Fe(OH)3}$). They are generally yellow to brown in color. The reaction is given here:

$$\ce{4FeCl2 + 6H2O + O2 → 4FeO(OH) + 8HCl}$$

Reaction between iron chloride (II), water and oxygen(moisture) to form iron metahydroxide(Iron(III) oxide-hydroxide) and hydrogen chloride. The reaction takes place at reflux.

Moreover, if you further heat iron chloride in oxygen at 450-480℃, iron oxide will form.

References(all pdf links)


Solution 2:

$$ \begin{align} \ce{Fe^2+ &-> Fe^3+ + e-} &\quad E^\circ &= \pu{+0.771 V} \tag{R1}\\ \ce{O2 + 2 H2O + 4 e- &-> 4OH-} &\quad E^\circ &= \pu{+0.40 V} \tag{R2} \end{align} $$

Thus, oxidation of iron(II) to iron(III) ions is feasible with atmospheric oxygen as the overall cell potential for the reaction is $\pu{+1.171 V}$ and it does occur at a reasonably observable rate.