Reaction Rate vs Rate Constant
When one or more reactants are converting to products, they may go through different modifications and energy changes. The chemical bonds in the reactants are breaking, and new bonds are forming to generate products, which are totally different from the reactants. This chemical modification is 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 to control them. Thermodynamics is the study of transformations of energy. It is only concerned with the energy and position of the equilibrium in a reaction. It has nothing to say about how fast the equilibrium is reached. That is in the domain of kinetics.
Reaction rate is simply the indication of the speed of the reaction. So 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 slow reaction, which takes place over years. In contrast, reaction of a piece of potassium with water is very rapid, producing a large amount of heat; thus, it is considered as a vigorous reaction.
Consider the following reaction, where reactants A and B are transforming in to 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 the two reactants or products.
Rate = -(1/a) (dA/dt) = -(1/b) (dB/dt) = (1/c) (dC/dt) = (1/d) (dD/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.
Chemical kinetics is the study of the reaction rates, and there are many factors affecting the speed of the reaction. These factors are 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.
If we write the rate equation in relation to the reactant A for the reaction given above, it is as follows.
R= -K [A]a [B]b
In this reaction, k is the rate constant. It is a proportionality constant, which depends on the temperature. Rate and the rate constant of a reaction can be found by experiments.
What is the difference between Reaction Rate and Rate Constant?
• Rate constant depends on the temperature, whereas the rate depends on many other variables, as well.
• Rate constant is a proportionality constant, which is a part of the reaction rate.
• Both reaction rate and the rate constant are associated with giving an indication of the reaction speed. However, only the rate constant cannot give a valid statement of the reaction speed.