Question

Which of the following about \(\mathrm{F}_{3} \mathrm{O}_{4}\) is truc? (1) The iron in it is trivalcnt. (2) The iron in it is divalent. (3) The iron in it bas fractional valcncy of \(8 / 3\). (4) It has both divalent and trivalcnt iron.

Short answer

Options (3) and (4) are true.

Step-by-step solution

Understand the Compound \(\text{Fe}_3\text{O}_4\)

\(\text{Fe}_3\text{O}_4\) is a mixed oxide consisting of iron in two different oxidation states. The compound is a combination of \(\text{FeO}\) and \(\text{Fe}_2\text{O}_3\), where iron exists in +2 and +3 oxidation states respectively.

Analyze \(\text{FeO}\) and \(\text{Fe}_2\text{O}_3\)

In \(\text{FeO}\), iron is in +2 oxidation state (divalent), and in \(\text{Fe}_2\text{O}_3\), iron is in +3 oxidation state (trivalent). The combination results in \(\text{Fe}_3\text{O}_4\) having both iron states.

Calculate Average Oxidation State

To confirm the oxidation states, calculate the average oxidation state of iron in \(\text{Fe}_3\text{O}_4\). Let \(x\) be the oxidation state of iron: \text{Total oxidation states} = \(3x + 4(-2) = 0\) \(3x - 8 = 0\) \(x = \frac{8}{3}\). This gives an average oxidation state, which implies the presence of both +2 and +3 states.

Verify Options

From the analysis: Option 1 is incorrect as not all iron is trivalent. Option 2 is incorrect as not all iron is divalent. Option 3 is true as the fractional valency average is \(\frac{8}{3}\). Option 4 is true as it has both divalent (Fe\(^{2+}\)) and trivalent (Fe\(^{3+}\)) iron.

Key concepts

Mixed Oxides

A mixed oxide is a compound that contains more than one type of metal ion in different oxidation states. In the case of \(\text{Fe}_3\text{O}_4\), it is known as magnetite and is a mixture of iron(II) oxide (\(\text{FeO}\)) and iron(III) oxide (\(\text{Fe}_2\text{O}_3\)). This means \(\text{Fe}_3\text{O}_4\) has iron present in two different oxidation states. In simpler terms, when iron forms \(\text{Fe}_3\text{O}_4\), some iron atoms are in the +2 state, and others are in the +3 state. This combination is what makes \(\text{Fe}_3\text{O}_4\) a mixed oxide.

• Iron(II) oxide (\(\text{FeO}\)) contains iron in the +2 oxidation state.
• Iron(III) oxide (\(\text{Fe}_2\text{O}_3\)) contains iron in the +3 oxidation state.

Oxidation States

Oxidation states, or oxidation numbers, are a conceptual charge assigned to atoms in a compound to help keep track of electron transfers during chemical reactions. For \(\text{Fe}_3\text{O}_4\), we are dealing with two oxidation states: +2 and +3.
To find the average oxidation state in \(\text{Fe}_3\text{O}_4\), you use the formula for the total oxidation state.
Considering oxygen is always -2, the sum of oxidation states in the compound must be zero since it is neutral.
For \(\text{Fe}_3\text{O}_4\):
Let \ x \ be the average oxidation state of Fe.
\[ 3x + 4(-2) = 0 \ \]
Simplifying:
\[ 3x - 8 = 0 \ \]
\[ 3x = 8 \ \]
\[ x = \frac{8}{3} \ \]
So, the average oxidation state of Fe in \(\text{Fe}_3\text{O}_4\) is \(\frac{8}{3}\).

Fractional Valency

Fractional valency occurs when an element in a compound has more than one oxidation state, as seen in mixed oxides like \(\text{Fe}_3\text{O}_4\). The average oxidation state of iron in this compound is \(\frac{8}{3}\). This value is not a whole number, it shows that iron is present in fractional amounts of \(\text{Fe}^{2+}\) and \(\text{Fe}^{3+}\) ions.
This does not mean that any iron atom has an oxidation state of \(\frac{8}{3}\). Rather, it indicates the existence of a combination of iron atoms in +2 and +3 states such that the average appears fractional. This can be visualized as:

• 2 Fe atoms in +3 state (from \(\text{Fe}_2\text{O}_3\))
• 1 Fe atom in +2 state (from \(\text{FeO}\))

Divalent Iron

Divalent iron refers to iron in the +2 oxidation state, denoted as \(\text{Fe}^{2+}\). In \(\text{Fe}_3\text{O}_4\), this is present as part of its mixed oxide nature. Specifically, \(\text{FeO}\) component of the compound is where iron exists in this +2 state. In general, \(\text{Fe}^{2+}\) is also known as ferrous iron. The presence of divalent iron in \(\text{Fe}_3\text{O}_4\) means that not all the iron atoms are in the same state, contributing to the overall mixed character of the oxide.

• Ferrous iron is reactive and often forms ions easily in aqueous solutions.
• It is commonly encountered in the geological, biological, and industrial contexts.

Trivalent Iron

Trivalent iron refers to iron in the +3 oxidation state, denoted as \(\text{Fe}^{3+}\). In \(\text{Fe}_3\text{O}_4\), the iron that comes from the \(\text{Fe}_2\text{O}_3\) component is in this state. Generally, \(\text{Fe}^{3+}\) is known as ferric iron.
The presence of trivalent iron in conjunction with divalent iron in \(\text{Fe}_3\text{O}_4\) demonstrates how this compound has a mix of two different oxidation states, which is reflective in its fractional valency.

• Ferric iron often forms stable compounds and is widely found in minerals.
• It plays significant roles in processes like oxygen transport in biology and industrial activities like steel production.