Question

When two reactants A and B are mixed to give products C and D, the reaction quotient at the initial stages of the reaction:
A. Is zero
B. Decreases with time
C. Is independent of time
D. Increases with time

Answer 1

Hint: $Q(\text{Reaction Rate)}=\dfrac{{{[C]}^{c}}{{[D]}^{d}}}{{{[A]}^{a}}{{[B]}^{b}}}$,
for the chemical reaction:
$aA+bB\to cC+dD$

Step-by-Step Solution:
From the hint given above, we know that the reaction rate of the given chemical reaction:

$aA+bB\to cC+dD$

is given by:
$Q(\text{Reaction Rate)}=\dfrac{{{[C]}^{c}}{{[D]}^{d}}}{{{[A]}^{a}}{{[B]}^{b}}}$
Now at t = 0, the concentration of the products C and D is zero, therefore at the very beginning of the reaction, its reaction rate (Q) = 0.
As the reaction proceeds towards completion, the concentration of the products C and D increases with time as that of the reactants A and B decreases, with the later reacting to produce the former.
This results in a consistent increase in the rate of reaction with respect to time as the reaction proceeds to completion.
Therefore, as a result our analysis, the answer to this question is d) Increases with time.

Note: Be very careful of the difference between Equilibrium Constant and the Reaction Quotient, as the former is only concerned with reversible reactions whereas the latter is applicable for all reactions.