# Passivity of iron

> A thin oxide layer stops the attack


## Objectives

- Explain why concentrated nitric acid causes the passivity of iron.
- State the two ways the book gives to remove the passive layer.

## 1. Properties of iron




## 2. The strong acid that stops

Predict: an iron nail goes into concentrated nitric acid. What happens?

- It reacts at first, then stops reacting. ✓
- It fizzes and gives hydrogen, like a dilute acid.
- It dissolves quickly.

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.

[ar-EG] المسمار بيتفاعل في الأول وبعدين بيقف: الـ concentrated nitric acid بيعمل passivity للـ iron. بتتكوّن طبقة oxide رفيعة على السطح وبتحمي الـ metal من إنه يكمّل تفاعل.
## 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. Concentrated $\ce{HNO3}$ dissolves iron quickly.


## 4. Follow the surface

Move the slider, stage by stage. Stage 1. Concentrated nitric acid meets clean iron. 2. A thin oxide layer forms on the surface. 3. The layer protects the iron: no further reaction. 4. Abrasion or dilute hydrochloric acid removes the layer.


## 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. Passivity is permanent.


## 6. Your turn

A passive iron nail is put into copper (II) sulphate solution. Nothing happens. After rubbing it with sandpaper, copper coats it. Why?

- Rubbing removed the oxide layer, so fresh iron could displace copper. ✓
- Rubbing made the iron passive.
- Iron is below copper, so it needed rubbing to react.
- Sandpaper added copper to the nail.

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.

[ar-EG] الحكّ بورق الصنفرة شال طبقة الـ oxide، فالـ iron الجديد قدر يطرد الـ copper من محلول الـ copper (II) sulphate. الـ iron فوق الـ copper في الـ activity series، لكن الطبقة اللي عملت الـ passivity كانت موقّفة التفاعل قبل الحكّ.
## 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. Give reason (Q7.10): concentrated nitric acid causes the passivity of iron. Key words: thin layer of oxide, protects the metal from further reaction. Can you explain why iron stops reacting with concentrated nitric acid, while dilute HCl dissolves it? 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.



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