The red-brown precipitate shown in the picture is produced in the reaction between an iron salt and a dilute alkali solution. When the precipitate is isolated and dried and then heated in an ignition tube, water vapor was found to be present along with another iron compound, X. What is a possible identity of X? (A) Fe(OH)3, (B) FeCl3, (C) FeSO4, (D) FeO, (E) Fe2O3.
This question is asking us to find the identity of X, the red-brown precipitate shown in the picture. Producing X is a two-step process. First, there’s a reaction between an iron salt and a dilute alkali solution. Next, the product of that reaction is isolated, dried, and heated to produce water vapor alongside our mystery compound. If we can characterize what is going on during those two steps, we can find a possible identity for X.
First, let’s describe the initial reaction. We know that one of the reactants is an iron salt. In other words, it’s an iron atom and an unknown anion combined to make an ionic compound. The other reactant is a dilute alkali solution. Alkali solutions contain hydroxide ions alongside cations. However, in this case, the cation is unknown. In a double-displacement reaction like this, the two unknown ions will combine to form some sort of salt or ionic solution. We’re more concerned about the other combination of ions. The combination of an iron ion and a hydroxide ion will make iron hydroxide.
However, since we don’t know if the ion we started with is an iron(II) ion or an iron(III) ion, two possible products emerge: iron(II) hydroxide or iron(III) hydroxide. For now, let’s consider what would happen if we heated each of these two products. The hydroxide ions in each of these two compounds have hydrogen atoms and oxygen atoms that when heated separate out as water molecules, leaving behind an iron oxide. In the first reaction here, heated iron(II) hydroxide produces iron(II) oxide plus water. In the second reaction, heating two moles of iron(III) hydroxide produces one mole of iron(III) oxide and three moles of water.
We can also call this heating process a thermal decomposition, thermal meaning it uses heat and decomposition meaning that the one reactant breaks down into multiple products. The product of this thermal decomposition is the red-brown product X that we’re trying to identify. Each of the two possibilities we’ve arrived at is one of the answer choices to this question. So how do we know which one is the unknown compound?
The final clue is in the color, red brown. Iron(II) oxide is a black powder, while iron(III) oxide has the red-brown color described in the question. So we can select answer choice (E), Fe2O3, as the correct answer to this problem.
When we combine an iron salt and a dilute alkali solution, some form of iron hydroxide will form. When we heat those forms of iron hydroxide, some form of iron oxide will form. The iron oxide that matches the red-brown image in the problem is iron(III) oxide. So, what is a possible identity of the unknown iron compound X? That’s choice (E) Fe2O3.