Question

The most important oxides of iron are magnetite, \(\mathrm{Fe}_{3} \mathrm{O}_{4}\), and hematite, \(\mathrm{Fe}_{2} \mathrm{O}_{3}\). (a) What are the oxidation states of iron in these compounds? (b) One of these iron oxides is ferrimagnetic, and the other is antiferromagnetic. Which iron oxide is likely to show which type of magnetism? Explain. Transition-Metal Complexes (Section 23.2)

Short answer

(a) The oxidation states of iron in magnetite (\(\mathrm{Fe}_{3} \mathrm{O}_{4}\)) are +2 and +3 and only +3 in hematite (\(\mathrm{Fe}_{2} \mathrm{O}_{3}\)). (b) Magnetite is likely to show ferrimagnetic properties, and hematite is likely to show antiferromagnetic properties due to their respective oxidation states and magnetic moments.

Step-by-step solution

Determine the oxidation states of iron in magnetite and hematite

To find the average oxidation states of iron in the given iron oxides, we can use the formula: Average oxidation state = (Total charge of metal ions) / (Number of metal ions) For magnetite, \(\mathrm{Fe}_{3} \mathrm{O}_{4}\): Oxygen has an oxidation state of -2. So, the total negative charge on the oxygen atoms = 4 × (-2) = -8. We know that the total positive charge on the iron atoms must balance the total negative charge, so it must be +8. Number of iron ions in the formula = 3. Therefore, the average oxidation state of iron in magnetite = 8/3. For hematite, \(\mathrm{Fe}_{2} \mathrm{O}_{3}\): Oxygen has an oxidation state of -2. So, the total negative charge on the oxygen atoms = 3 × (-2) = -6. We know that the total positive charge on the iron atoms must balance the total negative charge, so it must be +6. Number of iron ions in the formula = 2. Therefore, the average oxidation state of iron in hematite = 6/2 = 3.

Determine the magnetism of each iron oxide based on their oxidation states

To predict the type of magnetism each iron oxide may exhibit, we should consider the magnetic characteristics of their respective oxidation states. Iron has two common oxidation states: +2 (in magnetite) and +3 (in hematite). Iron (II) ions have one unpaired electron and can display ferromagnetic or ferrimagnetic behavior (attraction towards a magnetic field), while Iron (III) ions have similar electron configurations to antiferromagnetic materials (not attracted to a magnetic field). Since magnetite (\(\mathrm{Fe}_{3} \mathrm{O}_{4}\)) contains both iron (II) and (III) ions, it will likely show ferrimagnetic properties as the iron (II) ions attract the magnetic field and the iron (III) ions partially counterbalance this attraction. Hematite (\(\mathrm{Fe}_{2} \mathrm{O}_{3}\)), having exclusively iron (III) ions, is expected to show antiferromagnetic properties due to its aligned magnetic moments that cancel each other out.

Final Answer

(a) The oxidation states of iron in magnetite (\(\mathrm{Fe}_{3} \mathrm{O}_{4}\)) are +2 and +3 and only +3 in hematite (\(\mathrm{Fe}_{2} \mathrm{O}_{3}\)). (b) Magnetite is likely to show ferrimagnetic properties, and hematite is likely to show antiferromagnetic properties due to their respective oxidation states and magnetic moments.

Key concepts

Oxidation States

The oxidation state is a concept used to describe the degree of oxidation of an atom in a chemical compound. Essentially, it helps in understanding how electrons are distributed among the different atoms in a molecule. For transition metal oxides, determining the oxidation states is crucial as they influence the chemical and physical properties of the material.
In the case of iron oxides, we have magnetite (\( \mathrm{Fe}_{3} \mathrm{O}_{4} \)) and hematite (\( \mathrm{Fe}_{2} \mathrm{O}_{3} \)). To find the oxidation states:

• Oxide ions (\( \mathrm{O}^{2-} \)) have a known oxidation state of -2.
• In magnetite, the total of oxidation states must be zero, distributed among four oxygen and three iron atoms. Calculation shows an average oxidation state for iron to be \( +\frac{8}{3} \)..
• For hematite, it's simpler: two iron atoms balance the charge from three oxygen atoms, giving each iron an oxidation state of \( +3 \).

Understanding these oxidation states is foundational in predicting the behavior of these compounds in various chemical reactions, including redox reactions.

Magnetism

Magnetism in transition metal compounds is fascinating and is tied closely to the arrangement of electrons around metal atoms. Iron oxides provide an exciting example of this relationship. Magnetism comes in several types, with ferrimagnetism and antiferromagnetism being relevant to our discussion.
Iron can show different magnetic properties depending on the oxidation state and electron arrangement:

• Ferrimagnetism: Occurs when unpaired electrons lead to partial alignment of magnetic moments, creating a net magnetic moment. Magnetite (\( \mathrm{Fe}_{3} \mathrm{O}_{4} \)) is ferrimagnetic due to the presence of both iron (II) and iron (III) ions, resulting in more aligned spins.
• Antiferromagnetism: Results when magnetic moments align opposite to each other in equal magnitude, canceling each other out. Hematite (\( \mathrm{Fe}_{2} \mathrm{O}_{3} \)) fits this description, with iron (III) ions leading to this anti-alignment.

These magnetic properties depend on electron-electron interactions, which modify how these materials act in magnetic fields, influencing their realms of application in technology and materials science.

Iron Compounds

Iron compounds, primarily magnetite and hematite, are vital in various fields, from geology to technology. These compounds showcase the diverse properties of iron, a metal that can exist in multiple oxidation states and show distinct magnetic behaviors.
Here's a closer look at the key iron oxides:

• Magnetite (\( \mathrm{Fe}_{3} \mathrm{O}_{4} \)): A compound with both +2 and +3 iron ions, giving it unique electronic properties. It's ferrimagnetic, making it useful in magnetic storage and electronic devices.
• Hematite (\( \mathrm{Fe}_{2} \mathrm{O}_{3} \)): Composed of iron (III) ions, hematite exhibits antiferromagnetism at low temperatures, and it's widespread in geological formations. Its distinctive appearance and properties make it valuable in pigment production and as a catalyst in industrial processes.

These iron oxides highlight how variations in oxidation states and electron configurations can change physical properties, leading to a wide range of applications and a better understanding of mineralogical processes.