Understanding Pseudo First Order Rate Constant

A pseudo first order reaction is a reaction that is actually of a higher order (like second order) but is made to behave like a first order reaction by keeping one reactant in large excess. The rate constant for such a reaction is called the pseudo first order rate constant.

Step 1: Analyze the Options

Let's evaluate each statement one by one.

Step 2: Evaluate Statement 1

Statement: "Its value will not change if volume is changed."

The pseudo first order rate constant, $k_{\text{obs}}$, is a constant for a given temperature and concentration conditions. Changing the volume changes the concentrations of the reactants. However, since one reactant is in large excess, its concentration remains effectively constant even after a volume change. Therefore, $k_{\text{obs}}$ should not change. This statement is correct.

Step 3: Evaluate Statement 2

Statement: "It is dependent on the concentration of the reactant."

By definition, a rate constant is independent of the concentration of reactants. The pseudo first order rate constant is an effective rate constant, but it is still a constant for a given set of conditions (temperature and initial concentration of the excess reactant). It does not change as the reaction progresses and the concentration of the limiting reactant decreases. This statement is not correct.

Step 4: Evaluate Statement 3

Statement: "Its value is dependent on the temperature."

Like all rate constants, the value of the pseudo first order rate constant is governed by the Arrhenius equation and increases with an increase in temperature. This statement is correct.

Step 5: Evaluate Statement 4

Statement: "Its value is independent of the reactant present in small amount."

The pseudo first order rate constant, $k_{\text{obs}}$, is defined as $k_{\text{obs}}=k[\text{B}{]}^0$, where [B]₀ is the initial concentration of the reactant in large excess. It is independent of the concentration of the reactant present in small amounts. This statement is correct.

Final Answer

The statement that is not correct is: "It is dependent on the concentration of the reactant."

Related Topics & Formulae

Key Theory

For a true second-order reaction: $\text{Rate}=k\left[\text{A}\right]\left[\text{B}\right]$

If reactant B is in large excess ($\left[\text{B}\right]\approx [\text{B}{]}^0$), the rate law becomes:

$\text{Rate}=(k[\text{B}{]}^0)[\text{A}]=k_{\text{obs}}[\text{A}]$

Here, $k_{\text{obs}}$ is the pseudo first order rate constant.

Related Concepts

• Order of Reaction
• Integrated Rate Laws
• Method of Isolation (for determining order)
• Arrhenius Equation