Question

For a reaction A + B -> C, if [B] is much larger than [A] so that [B] essentially remains constant over the course of the reaction, the kinetics will be _____.

a) First order with respect to A
b) Zero order with respect to A
c) Second order with respect to A
d) Unpredictable

Answer 1

In a reaction A + B -> C, where [B] remains constant, the kinetics will be first order with respect to A because the rate of the reaction only depends on the concentration of the limiting reactant A.

Step-by-step explanation:

For the reaction A + B -> C, when concentration of B ([B]) remains essentially constant, it acts as a surplus which means its change does not affect the rate of reaction significantly. Therefore, the kinetics of this reaction will primarily depend on the concentration of A ([A]). As [B] is in excess and appears to be constant, the reaction rate can be described by the concentration of A alone. The rate of the reaction becomes directly proportional to [A] and the kinetics is said to be first order with respect to A.

The rate law for this scenario can be simplified to rate = k [A], where k is the rate constant. This simplification assumes that the concentration of B is constant and, therefore, is included in the rate constant k, making the reaction appear first-order in A, even though the true mechanism when both reactants are variables could be second-order or another order.