Question

For a first-order irreversible reaction carried out in a batch reactor, the rate of reaction is proportional to:

• A. Square of concentration
• B. Concentration of reactant
• C. Inverse of concentration
• D. Product concentration

Hint:

Remember the units of the rate constant \( k \) to identify the order:
- Zero order: \( \text{mol}/(\text{L} \cdot \text{s}) \)
- First order: \( \text{s}^{-1} \)
- Second order: \( \text{L}/(\text{mol} \cdot \text{s}) \)
The unit for a reaction of order \( n \) is \( (\text{conc})^{1-n} \text{time}^{-1} \).

Answer 1

The Correct Option is B

Step 1: Clarifying what “order of reaction” means. 
In reaction kinetics, the order of a reaction tells us how the reaction rate depends on the concentration of reactants.
A reaction is called first order when the rate changes directly with the concentration of a single reactant.

Step 2: Writing the general rate expression.
For an irreversible reaction of the form \( A \rightarrow \text{Products} \), the rate equation can be written as:

\[ -r_A = k C_A^n \]

Here:
\( -r_A \) represents the rate of disappearance of reactant \( A \).
\( k \) is the rate constant.
\( C_A \) is the concentration of \( A \).
\( n \) denotes the order of the reaction.

Step 3: Applying first-order conditions.
For a first-order reaction, the value of \( n \) is 1.
Substituting this into the rate equation gives:

\[ -r_A = k C_A \]

This relationship shows a direct proportionality between reaction rate and reactant concentration.
When the concentration of \( A \) increases, the reaction rate increases by the same factor.

Step 4: Final conclusion.
In a first-order irreversible reaction, the rate of reaction is directly proportional to the concentration of the reactant.