Why chromium resists the effect of atmosphere
Active metal, protected surface
1. Economic importance of the first transition series
2. Shiny for years
Chromium is a chemically active metal. So why do chrome-plated taps stay shiny for years?
Pick the right answer
Predict: why doesn't chromium keep reacting with air?
- A. A layer forms on its surface and protects it
- B. Chromium is an unreactive metal, like gold
- C. Chromium does not meet oxygen in air
Explanation
Chromium is chemically active, so it does react with oxygen of air. The oxide that forms makes a non-porous layer on the surface, and this layer prevents the continued interaction with oxygen.
3. Active, so it reacts first
Remember: an active metal reacts readily, for example with oxygen.
Chromium does react with the oxygen of air. A metal oxide forms on its surface.
4Cr + 3O2 → 2Cr2O3
So the protection is not a lack of activity. It comes from this oxide.
4. Watch the layer form
Move the slider to follow the surface, stage by stage.
Stage 1
The molecular volume of the oxide is larger than that of the metal atoms. So it forms a non-porous layer. This layer prevents the continued interaction with oxygen of air.
5. Not every oxide protects
True or false?
Any oxide layer protects a metal.
Explanation
Protection depends on the kind of layer: a non-porous layer, like chromium's oxide, keeps oxygen out; a porous layer, like rust, lets air and moisture through.
This is why chromium is useful in metal plating.
6. Give the reason
True or false?
Chromium resists the effect of air because it is an inactive metal.
Explanation
Book p.6: chromium is chemically active, but the molecular volume of its oxide is larger than that of the metal atoms, so the oxide forms a non-porous layer that prevents the continued interaction with oxygen of air.
Pick the right answer
Aluminum is above iron in the activity series. Yet aluminum window frames do not corrode away. Which is the best explanation?
- A. A non-porous oxide layer forms on its surface and blocks oxygen
- B. Aluminum is less active than iron
- C. Any oxide on a metal stops further corrosion
Explanation
Aluminum is more active than iron, yet its window frames do not corrode away. As with chromium, an active metal is protected when its oxide forms a non-porous layer that blocks oxygen.
7. Build the chain
Put in order
Put the steps of the explanation in order.
- Chromium, an active metal, reacts with oxygen of air
- A metal oxide forms on its surface
- The oxide's molecular volume is larger than that of the metal atoms
- A non-porous layer covers the surface
- Oxygen of air cannot continue reacting with the metal
Fill in the blanks
The molecular volume of the oxide is _____ than that of the metal atoms, so it forms a _____ layer.
Explanation
Larger molecular volume → a continuous, non-porous layer.
8. Summary
Back to the taps: chromium reacts, but its oxide seals it.
- Chromium is a chemically active metal.
- Its oxide's molecular volume is larger than that of the metal atoms: a non-porous layer forms.
- Rust on iron is porous, so it does not protect the iron.
| Chromium | Iron | |
|---|---|---|
| Chemically active metal | yes | yes |
| Layer on the surface | metal oxide | rust |
| Kind of layer | non-porous | porous |
| Oxygen of air | cannot continue reacting with the metal | keeps reaching the metal |
Can you explain why rust does not protect iron, but chromium oxide protects chromium?