Consider the following statements:
I) The value of rate constant is never negative
II) A zero order reaction is an elementary reaction
III). For elementary reactions, molecularity and order are equal
IV). Average and instantaneous rate of reaction are defined for micro and macroscopic time interval respectively The correct statement(s) is/are

Options

A. I, II, Ill
B. Only III
C. I, III
D. I, II, IV

Answer

To determine the correct statements among the given options, we will analyze each statement one by one.

### Step 1: Analyze Statement I
**Statement I:** The value of the rate constant is never negative.

- The rate constant (k) is a proportionality factor in the rate law of a chemical reaction. It is a measure of how fast the concentration of reactants changes over time. Since it represents a rate, it cannot be negative.

**Conclusion:** This statement is **true**.

### Step 2: Analyze Statement II
**Statement II:** A zero-order reaction is an elementary reaction.

- A zero-order reaction is characterized by a constant rate that is independent of the concentration of the reactants. While some zero-order reactions can be elementary (single-step reactions), not all zero-order reactions are elementary. Zero-order kinetics can also arise from complex mechanisms.

**Conclusion:** This statement is **false**.

### Step 3: Analyze Statement III
**Statement III:** For elementary reactions, molecularity and order are equal.

- Elementary reactions are single-step reactions where the molecularity (the number of molecules involved in the reaction) is equal to the reaction order. For example, a bimolecular elementary reaction has a molecularity of 2 and an order of 2.

**Conclusion:** This statement is **true**.

### Step 4: Analyze Statement IV
**Statement IV:** Average and instantaneous rate of reaction are defined for micro and macroscopic time intervals respectively.

- The average rate of reaction is calculated over a longer time interval (macroscopic), while the instantaneous rate is determined at a specific moment (microscopic). Therefore, the definitions in this statement are reversed.

**Conclusion:** This statement is **false**.

### Final Conclusion
The correct statements are:
- Statement I: True
- Statement II: False
- Statement III: True
- Statement IV: False

Thus, the correct statements are **I and III**.