# Magnetic iron oxide Fe₃O₄

> A mixed oxide: two salts prove it


## Objectives

- Explain why $\ce{Fe3O4}$ is a mixed oxide, using its reaction with hot concentrated acids.
- Describe how $\ce{Fe3O4}$ is prepared and how it is oxidized in air, with conditions.

## 1. Iron oxides




## 2. One oxide, two salts?

Predict: how many different iron salts form?

- Two: iron (II) and iron (III) sulphates ✓
- One: iron (III) sulphate only
- None: it does not react with acids

Two salts form, iron (II) sulphate and iron (III) sulphate. $\ce{Fe3O4}$ reacts with hot concentrated acids to give both iron (II) and iron (III) salts, which shows it is a mixed oxide.

[ar-EG] بيتكوّن 2 salts: الـ iron (II) sulphate والـ iron (III) sulphate. الـ $\ce{Fe3O4}$ بيتفاعل مع الـ hot concentrated acids ويدّي iron (II) salts و iron (III) salts مع بعض، وده بيبيّن إنه mixed oxide.
## 3. A mixed oxide

Mixed oxide $\ce{Fe3O4}$ can be considered as mixed iron (II) and iron (III) oxides: FeO + $\ce{Fe2O3}$. FeO + Fe₂O₃ add up to Fe₃O₄ It is known naturally as magnetite. It is a strong magnet.


## 4. Preparation

By the action of hot air or steam on red hot iron. By reduction of iron (III) oxide with carbon monoxide: You met route 1 in 'Effect of air and water vapour'.


## 5. The thermometer ladder

Reduce the same oxide, $\ce{Fe2O3}$, at different temperatures. Move up the thermometer. Rung Reducing $\ce{Fe2O3}$ with CO always gives iron.


## 6. With hot concentrated acids: two salts

It reacts with hot concentrated acids to give iron (II) and iron (III) salts, proving that it is a mixed oxide. $\ce{Fe3O4}$ is a single oxide that gives one salt.


## 7. Worked: add the two parts

FeO part Fe₂O₃ part Add them. FeO + $\ce{Fe2O3}$ = $\ce{Fe3O4}$, and 1 + 3 = 4 $\ce{H2SO4}$. Sum = the book's equation


## 8. Heated in air

It is oxidized to iron (III) oxide when it is heated in air. Its iron (II) part is oxidized to iron (III).


## 9. Your turn

Now with hot concentrated hydrochloric acid. Balance it. Which result proves that $\ce{Fe3O4}$ is a mixed oxide? Magnetic iron oxide is reduced by hydrogen at 400–700 °C. What forms?


## 10. Summary

$\ce{Fe3O4}$: black, a strong magnet, a mixed iron (II) and iron (III) oxide (magnetite). Made by hot air or steam on red hot iron, or $\ce{Fe2O3}$ + CO at 230–300 °C. Hot conc. acids give iron (II) + iron (III) salts: proof it is mixed. Heated in air, it is oxidized to $\ce{Fe2O3}$. MCQ (Q1.3): black iron oxide with hot conc. acids gives iron (II) and iron (III) salts together. Can you explain why Fe₃O₄ gives two salts, but Fe₂O₃ only one? $\ce{Fe3O4}$ is a mixed oxide, FeO + $\ce{Fe2O3}$: its FeO part gives the iron (II) salt and its $\ce{Fe2O3}$ part gives the iron (III) salt. $\ce{Fe2O3}$ is iron (III) oxide, so it gives one salt: an iron (III) salt.



Chapter: Transition Elements (https://yaani.ai/en/learn/chemistry/chem-s3-en.ch1)
Source: https://yaani.ai/en/lesson/chem-s3-en.lesson-fe.oxide-fe3o4