Question

The term $\dfrac{{ - dx}}{{dt}}$ in the expression refers to:
A) The concentration of reactants
B) Increase in concentration of reactants
C) Decrease in concentration of reactants
D) The average rate of reaction

Answer 1

Hint: For any reaction in chemical kinetics we can write the expression of the rate, in that there is always a decrease in the concentration of reactants while an increase in concentration of products. This decrease of reactant concentration is because of the fact that reactants are changing into products that’s why their amount is decreasing and product amount is increasing.

Complete step-by-step answer:
Let’s understand the concept by taking an example of reaction, in this reaction we have A and B which are converting into product C. So the rate is the velocity by which the reaction is taking place. $A + B\xrightarrow{k}\,C$ Here, in this reaction (k) is the rate constant if the reaction takes place in the presence of a catalyst then the rate will be higher for faster reaction.

Now see if we want to write the rate expression we can include the concentration of A and B because these are the reactants. Also, we can write the rate as decrease in concentration of reactants.
$Rate = \,k\,\left[ A \right]\,\left[ B \right]$
$Rate = \,\dfrac{{ - d\left[ A \right]}}{{dt}}$ And also $Rate = \,\dfrac{{ - d\left[ B \right]}}{{dt}}$
So, here $\dfrac{{ - d\left[ A \right]}}{{dt}}$ and $\,\dfrac{{ - d\left[ B \right]}}{{dt}}$ gives the information as the decrease in the concentration of reactants. Similarly in the above question, we have given $\dfrac{{ - dx}}{{dt}}$ which will also give the decrease in the concentration of reactants.

Hence the correct answer is option ‘C’.

Note: When you are solving the equation rate the negative sign is put before the rate of reactants just to make their concentration position. There is a decrease in the concentration of reactant and increase in the concentration of products. The rate expression is written on behalf of reactants and products by putting stoichiometric coefficients.