I reacted 0.40 M CuCl2 with 0.50 g of granular Al. After waiting for the reaction to be complete I filtered out the copper. After allowing the filter paper to sit to dry overnight, I observed the copper and it was surrounded by a green color that I expected after the copper oxidized. What I didn't expect was a yellow ring about 2-3 mm in width at the outer edge of the entire filter paper. What would the yellow ring be made of or made from?
1 Answer
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The yellow ring at the outer edge of the filter paper is probably due to some ferric compound like $\ce{FeCl3}$ or $\ce{Fe(OH)_nCl_{3-n}}$. Ferric compounds are usually brown and intensely colored. Their origin is probably an impurity in the metal aluminium : it is rather difficult to remove the last proportion of iron in even pure aluminium samples. Usually pure aluminium contains traces of iron (and other elements like $\ce{Si}$ and $\ce{Mn}$). As iron $\ce{Fe}$ and aluminium $\ce{Al}$ react the same way with $\ce{CuCl2}$, this reaction produces first $\ce{Cu}$ plus $\ce{Fe^{2+}}$ ions. But in the presence of air, this $\ce{Fe^{2+}}$ion is easily oxidized into brown ferric ion $\ce{[Fe(OH)]^{2+}}$ by $\ce{O2}$ according to :$$\ce{4Fe^{2+} + O2 + 2H2O -> 4[Fe(OH)]^{2+}}$$ In non-acidic water this ion is quickly hydrolyzed and produces a precipitate $\ce{Fe(OH)3}$ or $\ce{[FeO(OH)]}$ in a reaction like $$\ce{[Fe(OH)]^{2+} + 4 H2O -> Fe(OH)3 + 2 H3O+}$$ $$\ce{[Fe(OH)]^{2+} + 3 H2O -> FeOOH + 2 H3O+}$$ But in presence of $\ce{Al^{3+}}$ in solution, the solution becomes acidic due to hydrolysis of $\ce{Al^{3+}}$ and the precipitation of $\ce{Fe(OH)3}$ does not occur
