Question

When one of the reactants is present in such great excess that its concentration may be considered constant or nearly so, the reaction is said to be of___

• A. Second order
• B. Pseudo-order
• C. Zero order
• D. Third order

Answer 1

The Correct Option is B

The question deals with the concept of reaction orders in physical chemistry, specifically focusing on cases where one reactant is in excess. Let's break down the solution:

In chemical kinetics, the order of a reaction is determined by how the rate of reaction depends on the concentration of the reactants. However, in some reactions, one reactant is present in such a large excess that its concentration does not significantly change during the reaction. This leads to the concept of pseudo-order reactions.

Pseudo-Order Reaction: When one reactant is present in a large excess, its concentration remains approximately constant throughout the reaction. Consequently, the reaction can be simplified to appear as if it is of a lower order than it actually is.

For example, consider a reaction that is normally second-order:

\(A + B \rightarrow \text{Products}\)

Under normal conditions, the rate equation is:

\(\text{Rate} = k[A][B]\) 

If reactant \(B\) is in excess, its concentration remains nearly constant, and the rate equation simplifies to:

\(\text{Rate} = k' [A]\) where \(k' = k[B]\) (constant)

This effectively makes the reaction appear first-order with respect to reactant \(A\). This is an example of a pseudo-order reaction, specifically pseudo-first order in this case.

Now, let's evaluate the given options:

• Second Order: This would imply dependence on the concentration of two reactants, which doesn't simplify.
• Pseudo-Order: Correct. It appears to be of a lower order due to the excess concentration of one reactant.
• Zero Order: The rate is independent of the concentration of reactants, which is not the concept described here.
• Third Order: Implies dependence on the concentration of three reactants, which doesn't match the scenario.

Conclusion: When one of the reactants is in great excess such that its concentration remains nearly constant, the reaction is described as a pseudo-order reaction. Thus, the correct answer is pseudo-order.