Consider the following statement :
1 The rate of reaction is always proportinal to the concentration of reactants.
2 The order of an elementary chemical reaction step can be determined by examing its stoichimoetry.
3 The first order reactions follows an exponential time course.
Of these statements :

Options

A. 1, 2 and 3 are correct
B. 1 and 2 are correct
C. 2 and 3 are correct
D. 1 and 3 are correct

Explanation

To analyze the given statements regarding chemical kinetics, we will evaluate each statement one by one.

### Step 1: Evaluate Statement 1
**Statement:** The rate of reaction is always proportional to the concentration of reactants.

**Analysis:** This statement is **incorrect**. In the case of zero-order reactions, the rate of reaction is constant and does not depend on the concentration of reactants. For example, in a zero-order reaction, the rate can be expressed as:
\[ \text{Rate} = k \]
where \( k \) is the rate constant. Thus, the rate is not proportional to the concentration of reactants.

### Step 2: Evaluate Statement 2
**Statement:** The order of an elementary chemical reaction step can be determined by examining its stoichiometry.

**Analysis:** This statement is **correct**. For elementary reactions, the rate law can be directly derived from the stoichiometry of the reaction. For instance, if we consider an elementary reaction:
\[ 2A + B \rightarrow C \]
The rate law can be expressed as:
\[ \text{Rate} = k[A]^2[B]^1 \]
Here, the order of the reaction with respect to \( A \) is 2, and with respect to \( B \) is 1, which corresponds to their stoichiometric coefficients.

### Step 3: Evaluate Statement 3
**Statement:** The first-order reactions follow an exponential time course.

**Analysis:** This statement is **correct**. For first-order reactions, the concentration of the reactant decreases exponentially over time. The relationship can be expressed as:
\[ [A] = [A_0] e^{-kt} \]
where \( [A] \) is the concentration at time \( t \), \( [A_0] \) is the initial concentration, \( k \) is the rate constant, and \( t \) is time. This shows that the concentration decreases exponentially with time.

### Conclusion
Based on the evaluations:
- Statement 1 is incorrect.
- Statement 2 is correct.
- Statement 3 is correct.

Thus, the correct answer is that statements 2 and 3 are true.