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

If we prepare a solution of $\ce{FeCl2}$ by dissolving solid $\ce{FeCl2}$ powder in water, supposing the bottle of $\ce{FeCl2}$ was left open for sufficient time, is it possible for $\ce{FeCl2}$ to have oxidised to $\ce{FeCl3}$? I can't find a suitable reaction for this excecpt one which includes $\ce{HCl}$ which was definitely not present when preparing the mixture.

• Yes, it is very common. You actually have to protect Fe(II) solution from air when you do experiments if oxidation is something you want to avoid. – Greg Jun 6 '17 at 4:03

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.

$\ce{Fe^2+ -> Fe^3+ + e- E^o = +0.771 V}$
$\ce{O2 + 2H2O + 4e- -> 4OH- E^o = +0.40 V }$