Question

Give the unit of rate of reaction.
A. Molar
B. Second
C. $\dfrac{\operatorname{Sec}ond}{Molar}$
D.$\dfrac{Molar}{\operatorname{Second}}$

Answer 1

Hint: Rate of a reaction is defined as the amount of reactant disappeared or the amount of product generated when a given interval of time is taken.

Complete answer:
In order to understand the question, we have to learn about the rate of reaction. The rate of a chemical reaction is the speed or velocity at which a reaction takes place.
The rate of chemical reaction may be expressed in any of the following ways:
1. The rate of decrease in concentration of any one of the reactants.
2. The rate of increase in concentration of any one of the products at a given temperature.
For example, for a hypothetical reaction,
     \[\operatorname{Reactant} \to \operatorname{Product}\]
Let us assume that the volume of the system remains constant. If ${{[R]}_{1}}$ and ${{[P]}_{1}}$ are the concentrations of reactant and product respectively at time ${{t}_{1}}$ and ${{[R]}_{2}}$ and ${{[P]}_{2}}$ are their concentrations at time ${{t}_{2}}$ then, we can say that the rate of disappearance of the reactant should be equal to the rate of appearance of the product, so that the laws of chemical kinetics are followed properly. Now, we can say that ‘rate’ means the ratio of change. In case of chemical reactions, we specify or analyse the change with respect to the time. So, the rate of a chemical reaction can be said as the disappearance of the reactant or the appearance of a product in a specific range of time. Now, the concentration of the reactant or the product has the unit ‘Mole’, whereas we assign units of time as seconds. As, $rate=\dfrac{concentration}{time}$, so we can assign the unit of reaction as $\dfrac{Molar}{\operatorname{Second}}$.

This gives option D as the correct answer for this question.

Note:
It is to be noted that when the reactants are disappearing, then the rate of the reaction is always accompanied by a negative sign, but for products it is accompanied by a positive sign for convenience.