Passivity of iron
A thin oxide layer stops the attack
1. Properties of iron
2. The strong acid that stops
Dilute acids dissolve iron. So a concentrated acid should dissolve it even faster. Or should it?
Pick the right answer
Predict: an iron nail goes into concentrated nitric acid. What happens?
- A. It dissolves quickly.
- B. It reacts at first, then stops reacting.
- C. It fizzes and gives hydrogen, like a dilute acid.
Explanation
The nail reacts at first, then stops: concentrated nitric acid causes passivity of iron. A thin oxide layer forms on the surface and protects the metal from further reaction.
3. A thin layer of oxide
Passivity of iron
Caused by concentrated nitric acid: a thin layer of oxide forms, which protects the metal from further reaction.
Remember chromium: its oxide layer stops further reaction with air. Here, an oxide layer stops the acid.
4. Follow the surface
Move the slider, stage by stage.
Stage 1
1. Concentrated nitric acid meets clean iron.
5. Not permanent
This layer can be removed by abrasion or by using dilute hydrochloric acid.
Abrasion
Rubbing a surface hard, for example with sandpaper.
With the layer gone, fresh iron is exposed and can react again.
- Passive iron: thin oxide layer → Fresh iron exposed (abrasion or dilute HCl)
- Fresh iron exposed → Iron reacts again
6. Your turn
Pick the right answer
A passive iron nail is put into copper (II) sulphate solution. Nothing happens. After rubbing it with sandpaper, copper coats it. Why?
- A. Rubbing removed the oxide layer, so fresh iron could displace copper.
- B. Sandpaper added copper to the nail.
- C. Iron is below copper, so it needed rubbing to react.
- D. Rubbing made the iron passive.
Explanation
Rubbing with sandpaper removed the oxide layer, so fresh iron could displace copper from the copper (II) sulphate solution. Iron is above copper in the activity series, but the passive layer stopped the reaction before rubbing.
True or false?
Once iron is made passive, it can never react again.
Explanation
The statement is false: the protective oxide layer can be removed by abrasion or by using dilute hydrochloric acid, and then the iron can react again.
Fill in the blanks
The passive layer is removed by _____ or by dilute _____ acid.
Explanation
Abrasion (rubbing) or dilute hydrochloric acid.
7. Summary
- Concentrated nitric acid makes iron passive: a thin layer of oxide forms.
- The layer protects the metal from further reaction.
- It is removed by abrasion or by dilute hydrochloric acid.
- Same idea as chromium: a surface oxide layer stops further reaction.
Can you explain why iron stops reacting with concentrated nitric acid, while dilute HCl dissolves it?
Reveal
Concentrated nitric acid forms a thin layer of oxide on the iron, and the layer protects the metal from further reaction. Dilute HCl removes that layer, so fresh iron stays exposed and reacts.