   Chapter 12, Problem 15Q

Chapter
Section
Textbook Problem

# Table 12.2 illustrates how the average rate of a reaction decreases with time. Why does the average rate decrease with time? How does the instantaneous rate of a reaction depend on time? Why are initial rates used by convention?

Interpretation Introduction

Interpretation:

The reason why average rate of a reaction decreases with time has to be explained.  And the dependence of instantaneous rate on time and the reason for using initial rates also has to be explained.

Concept introduction:

The change on concentration of reactant or product per unit time is called as the rate of a reaction.  Let's consider the reaction where the reactant A is giving product P .

AP

The rate of the reaction can be expressed as given below.

Rate=[A]Δt

Where,

Δ[A] is the change in concentration of the reactant.

Δt is the change in time.

Explanation

• Rate is the change in concentration of the reactant or product per unit time.  At the starting of a reaction the concentration of products will be zero.  As the time increases or as the reaction proceeds, the concentration of the product increases and that of the reactant decreases.  When the concentration of product is zero, the rate of the forward reaction will be high.  As times goes, the product concentration increases and the reverse reaction also occurs.  Hence, the rate of the reaction decreases with time.
• Average rate is the change in the concentration of the reactant or product with unit change in time.  Average rate is calculated for long time intervals.  Instantaneous rate is the rate of a reaction at any instant.  It is the differential of concentration per a small change in time...

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