The rate of elementary chemical reaction is directly proportional to:

Options

A. Active masses of reactants
B. Equilibrium constant
C. Active masses of products
D. Pressure of reaction

Answer

To solve the question regarding the rate of an elementary chemical reaction, we will analyze the relationship between the rate of reaction and the various factors mentioned in the options provided.

### Step-by-Step Solution:

1. **Understanding Elementary Reactions**:
- An elementary reaction is a single step reaction where reactants are converted to products in one step. The rate of such reactions can be expressed in terms of the concentrations of the reactants.

2. **Defining Rate of Reaction**:
- For an elementary reaction involving reactants A and B, the rate can be expressed as:
\[ \text{Rate} = k [A]^m [B]^n \]
- Here, \(k\) is the rate constant, and \(m\) and \(n\) are the orders of the reaction with respect to reactants A and B, respectively.

3. **Molecularity and Order**:
- In elementary reactions, the order of the reaction is equal to its molecularity. For a reaction like \(A + B \rightarrow P\), the molecularity is 2 (since two reactant molecules are involved), and thus the rate expression simplifies to:
\[ \text{Rate} = k [A]^1 [B]^1 = k [A][B] \]

4. **Active Mass**:
- The term "active mass" refers to the concentration of the reactants in the reaction. In this case, it corresponds to the molar concentrations of A and B.

5. **Analyzing the Options**:
- **Option 1: Active mass of reactant**: This is correct, as the rate of the reaction is directly proportional to the concentrations (active masses) of the reactants.
- **Option 2: Equilibrium constant**: This is incorrect; the equilibrium constant relates to the concentrations of products and reactants at equilibrium, not the rate of reaction.
- **Option 3: Active mass of product**: This is incorrect; the rate expression does not include the concentrations of products.
- **Option 4: Pressure of reaction**: This is also incorrect; while pressure can affect reaction rates, it is not included in the rate expression for elementary reactions.

6. **Conclusion**:
- Therefore, the rate of an elementary chemical reaction is directly proportional to the active mass of the reactants. Hence, the correct answer is **Option 1: Active mass of reactant**.