Difference Between Reaction Rate and Specific Rate Constant

The key difference between reaction rate and specific rate constant is that reaction rate gives an indication of the speed at which the reactions are converted to products while specific rate constant is proportionality constant.

When one or more reactants are converted to products, they may go through different modifications and energy changes. The chemical bonds in the reactants are broken, and new bonds are formed to generate products which are totally different from the reactants. These types of chemical modifications are known as chemical reactions. There are numerous variables controlling the reactions. By studying thermodynamics and kinetics, we can draw a lot of conclusions about a reaction and how we can control them. Thermodynamics is the study of transformations of energy. It is only concerned with the energetic and the position of the equilibrium in a reaction. It has nothing to say about how fast the equilibrium is reached. This question is the domain of kinetics.

CONTENTS

1. Overview and Key Difference
2. What is Reaction Rate 
3. What is Specific Rate Constant
4. Side by Side Comparison – Reaction Rate vs Specific Rate Constant in Tabular Form
5. Summary

What is Reaction Rate?

Reaction rate is simply the indication of the speed of the reaction. It can be regarded as a parameter which determines how fast or how slow the reaction is. Naturally, some reactions are very slow, so we cannot even see the reaction taking place unless we observe it for a very long time. For example, rock weathering by chemical processes is a really slow reaction which takes place over the years. In contrast, the reaction between a piece of potassium and water is very rapid and produces a large amount of heat; this is considered a vigorous reaction.

Consider the following reaction where reactants A and B are going to the products C and D.

a A + b B    →   c C + d D

The rate for the reaction can be given in terms of either of two reactants or products.

Rate =  -(1/a) d[A]/dt = -(1/b) d[B]/dt = -(1/c) d[C]/dt = -(1/d) d[D]/dt

a,b,c and d are stoichiometric coefficients of the reactants and products. For the reactants, the rate equation is written with a minus sign because the products are depleting as the reaction proceeds. However, as the products are increasing, they are given positive signs.

Figure 01: Reaction Rate Increases with Increasing Temperature

Chemical kinetics is the study of the reaction rates and there are many factors affecting the speed of the reaction. These factors include concentrations of the reactants, catalysts, temperature, solvent effects, pH, sometimes the product concentrations, etc. These factors can be optimized to have the maximal reaction rate or can be adjusted to manipulate the required reaction rates.

What is Specific Rate Constant?

If we write the rate equation in relation to the reactant A in the above-given reaction, it is as follows.

R= -K [A]a [B]b

In this reaction, k is the rate constant. This is known as specific rate constant when the concentration of each reactant is unity; i.e. one mole/dm3. It is a proportionality constant which depends on the temperature. Rate and specific rate constant of a reaction can be found by experiments.

What is the Difference Between Reaction Rate and Specific Rate Constant?

Reaction rate gives an indication of the speed at which the reactions are converted to products while specific rate constant is proportionality constant. So, this is the key difference between reaction rate and specific rate constant. More importantly, specific rate constant is a part of the reaction rate. Specific rate constant only cannot give a valid statement of the reaction speed.

Summary – Reaction Rate vs Specific Rate Constant

The key difference between reaction rate and specific rate constant is that reaction rate gives an indication of the speed at which the reactions are converted to products while specific rate constant is proportionality constant.

Image Courtesy:

1. “Reaction Rate Increase with Increasing Temperature” By Brazosport College – Own work (CC BY-SA 3.0) via Commons Wikimedia

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