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WHY IS fecl2 formed instead of fecl3 when iron reacts with dilute hydrochloric acid?

When iron (Fe) reacts with dilute hydrochloric acid (HCl), the following reaction occurs:

Fe + 2HCl → FeCl2 + H2

Iron displaces hydrogen from hydrochloric acid and forms iron(II) chloride (FeCl2) along with the liberation of hydrogen gas (H2). This happens because iron is more reactive than hydrogen, so it can displace hydrogen from the acid.

However, if excess iron is present in the reaction, then iron(III) chloride (FeCl3) can also be formed. The reaction would be:

2Fe + 6HCl → 2FeCl3 + 3H2

In this case, iron displaces hydrogen again, but because an excess of iron is present, it can displace all the hydrogen from the hydrochloric acid and form iron(III) chloride instead of iron(II) chloride.

Therefore, whether FeCl2 or FeCl3 is formed depends on the amount of iron available in the reaction.

When iron reacts with dilute hydrochloric acid (HCl), it can form two different products: ferrous chloride (FeCl2) and ferric chloride (FeCl3). The formation of these products depends on various factors such as the concentration of the acid, the temperature, and the presence of any oxidizing agents.

In the case of dilute hydrochloric acid, which is a relatively weak acid, the formation of FeCl2 is favored over FeCl3. This is because iron(II) ions (Fe2+) are more stable than iron(III) ions (Fe3+) in an acidic medium.

To understand why FeCl2 is formed, we need to consider the reaction mechanism. When iron reacts with HCl, it undergoes an oxidation-reduction reaction, where iron is oxidized and hydrogen ions from the acid are reduced. The overall reaction can be summarized as follows:

Fe(s) + 2HCl(aq) → FeCl2(aq) + H2(g)

During this reaction, iron loses two electrons and is oxidized from its elemental state (Fe) to the +2 oxidation state (Fe2+). The hydrogen ions (H+) from the acid accept these electrons and are reduced to form hydrogen gas (H2). The chloride ions (Cl-) from the acid combine with the iron(II) ions to form ferrous chloride (FeCl2).

It's important to note that under more concentrated hydrochloric acid or in the presence of oxidizing agents, such as nitric acid or oxygen, the reaction can proceed further to form ferric chloride (FeCl3). In this case, iron is oxidized to the +3 oxidation state (Fe3+), and three chloride ions (Cl-) combine with one iron(III) ion to form ferric chloride.

In summary, when iron reacts with dilute hydrochloric acid, the formation of FeCl2 is favored due to the relatively weak acidity and absence of strong oxidizing agents.

The free energy is such that the reaction is spontaneous for iron to be oxidized to iron(II) with HCl but not to iron(III). In fact, if any iron(III) were formed the thermodynamics are such that it would be reduced to iron(II) immediately with the excess iron therre initially. Iron(II) solutions must be protected from the atmosphere or oxygen will oxidize iron(II) to iron(III). Usually that is done by adding a small piece of iron metal to the iron(II) solution. So when oxygen oxidizes a ferrous ion to ferric ion the iron metal reacts with it to be oxidized to ferrous and the ferric is reduce to ferrous and the integrity of the ferrous solution is not compromised. I used to tell my students to observe the ferrous solution and see that small piece of iron in the bottom. That always made a good test question, "Why is the piece of iron metal in ferrous solution?"