Which of the following is a first order reaction:

Options

A. `NH_5 NO_2 (s) to N_2+ 2H_2O`
B. `2HI to H_2 + I_2`
C. `2NO_2 to 2NO + O_2`
D. `2NO + O_2 to 2NO_2`

Answer

To determine which of the given options is a first order reaction, we need to understand the concept of reaction order in chemical kinetics. The order of a reaction is defined as the sum of the powers of the concentration terms in the rate law expression. A first order reaction is characterized by the fact that the rate of reaction is directly proportional to the concentration of one reactant.

### Step-by-Step Solution:

1. **Understand Reaction Order:**
- The order of a reaction indicates how the rate of the reaction depends on the concentration of the reactants. A first order reaction means that the rate is proportional to the concentration of one reactant.

2. **Analyze Each Option:**
- We need to check each option provided to see how many reactant molecules are involved in the rate-determining step of the reaction.

3. **Evaluate Option A:**
- If option A has only one reactant molecule, it indicates that the rate of the reaction is dependent on the concentration of this single reactant. Therefore, this is a first order reaction.

4. **Evaluate Option B:**
- If option B has two reactant molecules, the rate of reaction would depend on the concentration of both reactants. This indicates a higher order than first order, likely second order.

5. **Evaluate Option C:**
- Similar to option B, if option C also has two reactant molecules, it is not a first order reaction.

6. **Evaluate Option D:**
- If option D has two or more reactant molecules, it also indicates that the reaction is not first order.

7. **Conclusion:**
- After evaluating all options, we find that only option A corresponds to a first order reaction since it involves only one reactant molecule.

### Final Answer:
The correct option that represents a first order reaction is **Option A**.

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