Question

For a hypothetical elementary reaction (both zero and first order rate constants), initially only two moles of 'A' are present. Then the total number of moles of 'A', and at the end of the reaction are:

a) 2, 0

b) 2, 1

c) 2, 2

d) 0, 0

Answer 1

The total number of moles of 'A' at the end of a reaction, for both zero-order and first-order reactions, effectively becomes 0, though the paths to reach this conclusion differ. Zero-order reactions reduce the concentration of 'A' linearly over time, whereas first-order reactions decrease the concentration exponentially.

Step-by-step explanation:

For a hypothetical elementary reaction with both zero-order and first-order rate constants, initially having only two moles of 'A' present, the total number of moles of 'A' at the end of the reaction can be determined by the characteristics of the reaction order.

Zero-order reactions proceed at a constant rate, independent of the concentration of the reactant. The integrated rate equation for the zero-order reaction is [A] = -kt + [A]0, where [A]0 is the initial concentration, k is the rate constant, and t is time. Ultimately, the concentration of 'A' will decrease linearly with time until all 'A' has been consumed, leaving 0 moles at the end.

In contrast, the rate of a first-order reaction is directly proportional to the concentration of the reactant. The rate law in this case is rate = k[A], and as the reaction progresses, the amount of 'A' decreases exponentially, but theoretically never reaches zero. However, for practical purposes, after a sufficient amount of time has passed, the concentration of 'A' can be so small that it is effectively zero.