Textbook Problem

Define reaction rate. Distinguish between the initial rate, average rate, and instantaneous rate of a chemical reaction. Which of these rates is usually fastest? The initial rate is the rate used by convention. Give a possible explanation as to why.

Interpretation Introduction

Interpretation: The definition of reaction rate is to be stated. A distinction between the terms initial rate, average rate and the instantaneous rate of a chemical reaction and the fastest rate among these is to be determined. An explanation regarding the usage of the initial rate as the rate by convention is to be stated.

Concept introduction: The change observed in the concentration of a reactant or a product per unit time is known as the rate of the particular reaction.

Answer to Problem 1RQ

Answer

The change observed in the concentration of a reactant or a product per unit time is known as the rate of the particular reaction.

Explanation of Solution

To determine: The definition of reaction rate.

The reaction rate or the rate of a reaction is defined as the change observed in the concentration of a reactant or a product per unit time.

For the reaction,

$\text{A}\to \text{P}$

$\begin{array}{c}\text{Rate}\text{of}\text{Reaction }=\text{Rate}\text{of}\text{disappearance}\text{of}\text{A}\\ =\frac{\text{Decrease}\text{in}\text{concentration}\text{of}\text{A}}{\text{Time}\text{Taken}}\\ =\frac{-\Delta \left[\text{A}\right]}{\Delta \text{T}}\end{array}$

$\begin{array}{c}\text{Rate}\text{of}\text{Reaction }=\text{Rate}\text{of}\text{appearance}\text{of}\text{P}\\ =\frac{\text{Increase}\text{in}\text{concentration}\text{of}\text{P}}{\text{Time}\text{Taken}}\\ =\frac{\text{ +}\Delta \left[\text{P}\right]}{\Delta \text{T}}\end{array}$

To determine: A distinction between the terms initial rate, average rate and the instantaneous rate of a chemical reaction and the fastest rate among the given rates.

Solution: The initial rate of a reaction is faster than the average rate and the instantaneous rate of a given chemical reaction.

The rate of a reaction observed at any instant of time is known as the ‘instantaneous rate of a reaction’. It can also be defined as thechange of concentration per unit time.

The rate of a reaction that is measured at comparatively longer intervals of time is known as the ‘average rate of the reaction’. The rate of a reaction keeps on changing throughout the reaction process. Hence, the rate of reaction generally observed is the average rate of the reaction during a specific time interval.

The instantaneous rate of a reaction that is measured initially or at $\text{t}=0$ is known as the ‘initial rate of the reaction’. It can be determined experimentally as well as graphically.

The initial rate of a reaction is faster than the average rate and the instantaneous rate of a given chemical reaction.

To determine: An explanation regarding the usage of the initial rate as the rate by convention.

Solution: The rate of a reaction at $\text{t}=0$ depends only on the forward reaction as the formation of products has not been initiated. Therefore, the initial rate of a reaction is the rate used by convention.

The instantaneous rate of a reaction that is measured initially or at $\text{t}=0$ is known as the initial rate of the reaction. At this time interval the formation of products has not started yet.

Hence, the rate at this instant depends only upon the forward reaction. The reverse reaction does not occur as there is no formation of products. Therefore, the initial rate of a reaction is the rate used by convention.

Conclusion

The initial rate of a reaction is faster than the average rate and the instantaneous rate of a given chemical reaction.

The rate of a reaction at $\text{t}=0$ depends only on the forward reaction as the formation of products has not been initiated. Therefore, the initial rate of a reaction is the rate used by convention.