Question

Write the overall order of a reaction if A is present in large excess.

Hint:

If one reactant is in excess → pseudo-order reaction. Rate depends only on the limiting reactant.

Answer 1

Step 1: Understanding the Rate Law.
The rate law for a reaction depends on the concentrations of the reactants and their respective orders in the reaction. If the rate law is given as:
\[ \text{Rate} = k[A]^m[B]^n \] where:
- \( k \) is the rate constant,
- \( [A] \) and \( [B] \) are the concentrations of reactants A and B, respectively,
- \( m \) and \( n \) are the orders with respect to each reactant.

Step 2: Effect of Large Excess of A.
If A is present in large excess, its concentration does not significantly change during the reaction. This means the concentration of \( A \) remains nearly constant, and it can be considered as a constant factor in the rate law.
Thus, the rate law can be simplified to:
\[ \text{Rate} = k' [B]^n \] where \( k' = k[A]^m \) is a new rate constant, considering \( A \) is in large excess.

Step 3: Conclusion.
When A is in large excess, the overall order of the reaction depends only on the concentration of \( B \). Therefore, the overall order of the reaction is \( n \), which is the order with respect to \( B \). The reaction becomes effectively dependent on the concentration of \( B \) alone.