Which of the following is correct about the adsorption of `N_(2)` over lron ?

Options

A. It is always physically adsorbed
B. Extent of adsorption over iron decreases with the increase in temperature first and then increases
C. It is always chemically adsorbed
D. `N_(2)` is never adsorbed over iron

Answer

To determine the correct statement about the adsorption of \( N_2 \) over iron, we can analyze the process of adsorption in detail. Here’s a step-by-step breakdown of the solution:

### Step 1: Understand Adsorption
Adsorption is the process where molecules from a gas or liquid adhere to the surface of a solid. This can occur through two main types: physisorption (physical adsorption) and chemisorption (chemical adsorption).

**Hint:** Remember the difference between physisorption and chemisorption: physisorption involves weak van der Waals forces, while chemisorption involves stronger chemical bonds.

### Step 2: Analyze the Behavior of \( N_2 \) on Iron
- At room temperature, \( N_2 \) is known to undergo chemisorption on iron. This means that at this temperature, \( N_2 \) molecules are chemically bonded to the surface of iron.

**Hint:** Chemisorption typically occurs at higher temperatures, but some gases can still chemisorb at room temperature depending on their nature and the surface.

### Step 3: Consider Low Temperature Effects
- At lower temperatures, specifically around 83 K, \( N_2 \) is physically adsorbed onto iron. This indicates that at this temperature, the adsorption is due to weaker interactions rather than chemical bonding.

**Hint:** Lower temperatures often favor physisorption due to reduced kinetic energy of the gas molecules.

### Step 4: Examine High-Temperature Effects
- When the temperature is raised to 773 K, \( N_2 \) is chemisorbed as atomic nitrogen. This indicates that at higher temperatures, the kinetic energy allows for stronger interactions, leading to chemical bonding.

**Hint:** Higher temperatures can promote chemisorption as they provide the energy needed to overcome activation barriers for chemical reactions.

### Step 5: Evaluate the Statements
Now, let's evaluate the statements provided in the question:
1. **"It is always physically absorbed."** - This is incorrect because \( N_2 \) is chemisorbed at room temperature.
2. **"Extent of adsorption over iron decreases with increase in temperature."** - This is also incorrect as the extent of adsorption can initially decrease but then can increase at higher temperatures due to chemisorption.
3. **"It is always chemically absorbed."** - This is correct as \( N_2 \) can be chemisorbed at room temperature and at higher temperatures.
4. **"N2 is never absorbed over iron."** - This is incorrect since \( N_2 \) does adsorb on iron.

### Conclusion
The correct statement regarding the adsorption of \( N_2 \) over iron is that it is always chemically absorbed.

**Final Answer:** The correct option is that \( N_2 \) is always chemically absorbed.