Effect of air and water vapour
Red-hot iron gives magnetic iron oxide
1. Properties of iron
2. Red-hot iron in oxygen
Damp air rusts iron slowly. Now heat iron until it glows red, in dry oxygen.
Pick the right answer
Predict: which oxide forms on red-hot iron in dry oxygen?
- A. Fe2O3, like hematite
- B. Fe3O4, magnetic iron oxide
- C. None: iron only rusts in damp air
Explanation
Red-hot iron reacts with dry air or oxygen to give magnetic iron oxide, Fe3O4: 3Fe + 2O2 → Fe3O4.
3. Effect of air
gathered3Fe(s) + 2O2(g) →[Δ]\\ Fe3O4(s)gathered
Red hot iron reacts with dry air or oxygen producing magnetic iron oxide.
Magnetic iron oxide
Fe3O4: a black oxide that is attracted by a magnet. In nature it is the ore magnetite.
Why 3 and 2?
- Fe3O4 holds 3 Fe atoms, so write 3Fe.
- Fe3O4 holds 4 O atoms. That is 2 O2 molecules.
- Iron gains oxygen, so iron is oxidized.
4. Effect of water vapour
Red hot iron (500 °C) reacts with water vapour producing magnetic iron oxide and hydrogen gas.
gatheredFe + H2O(v) →[500\,^C]\\ Fe3O4 + H2gathered
Water vapour gives its oxygen to iron. So water vapour is reduced to hydrogen.
5. Balance the water vapour equation
Balance the equation
Balance the equation of red hot iron with water vapour.
Fe + H2O →[500\,^C] Fe3O4 + H2
Balanced equation
3Fe + 4H2O →[500\,^C] Fe3O4 + 4H2
Explanation
Fe3O4 fixes 3 Fe and 4 O. Four H2O hold 8 H atoms: four H2.
Need a hint?
- Start like the air equation: how many Fe atoms are in Fe3O4?
- How many O atoms are in Fe3O4? Each H2O brings one.
- Count the H atoms on the left. Each H2 takes two.
6. Air or water vapour?
Pick the right answer
Experiment A: red hot iron in dry air. Experiment B: red hot iron (500 °C) in water vapour. Which statement is correct?
- A. Both give Fe3O4; only B also gives hydrogen gas.
- B. A gives Fe2O3; B gives Fe3O4.
- C. B gives Fe(OH)2 and hydrogen gas.
- D. Both give hydrogen gas.
Explanation
Both experiments give magnetic iron oxide, Fe3O4. In B, the water vapour also releases hydrogen gas: 3Fe + 4H2O → Fe3O4 + 4H2. In A, iron reacts with the oxygen of the air, so no hydrogen forms.
Fill in the blanks
In 3Fe + 4H2O → Fe3O4 + 4H2, the substance that is reduced is _____.
Explanation
Water vapour loses its oxygen to iron, so it is reduced to hydrogen.
7. Summary
- Red hot iron + dry air or oxygen → Fe3O4, magnetic iron oxide.
- Red hot iron (500 °C) + water vapour → Fe3O4 + hydrogen gas.
- Iron is oxidized in both. Water vapour is reduced to hydrogen.
Can you explain why water vapour, not iron, is the substance reduced?
Reveal
Water vapour gives its oxygen to iron, so water vapour is reduced to hydrogen. Iron gains that oxygen, so iron is oxidized.