Question

When two reaction A and B are mixed to give products C and D , the reaction quotient Q 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

Reaction quotient is a measure of the relative amount of products and reactants present in a reaction at a given time .
The formula is $Q = \dfrac{{\left[ {\operatorname{Products} } \right]}}{{\left[ {\operatorname{Reaction} } \right]}}$

Complete step by step answer:
Reaction quotient is a thermodynamic quantity that provides a measurement of the relative quantities of products and reactants present in a reaction .
In equilibrium , the value of the reaction quotient remains the same and is equal to the equilibrium constant .
For a general chemical reaction ,
$aA + bB \rightleftharpoons cC + dD$
The reaction quotient can be written as the ratio of the concentrations of the products and the reactants raised to the power of the stoichiometric coefficients .
$Q = \dfrac{{{{\left[ C \right]}^c}{{\left[ D \right]}^d}}}{{{{\left[ A \right]}^a}{{\left[ B \right]}^b}}}$
For the given reaction in question
$A + B \rightleftharpoons C + D$
The reaction quotient is
$\therefore {\text{ Q = }}\dfrac{{\left[ C \right]\left[ D \right]}}{{\left[ A \right]\left[ B \right]}}$
At the initial stages of the reaction , formation of the products C and D takes place, that is the concentration of C and D will increase and the concentrations of A and B will decrease .
Therefore , the Q will increase with respect to time as the numerator increases with respect to time causing the value of Q to increase .
Hence option (D) is the correct answer.

Note:
The reaction quotient (Q) also helps in figuring out the direction of the reaction that is in which direction the reaction will proceed . If $Q > K$ , then it will favour the reactants and if $Q < K$ , then the reaction favours the products .