Reaction Rate vs Specific 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 kind of chemical modification is known as chemical reactions. There are numerous variables controlling the reactions. Mainly, 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.
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 really slow reaction which takes place over the years. In contrast, piece of potassium reaction with water is very rapid, and thus producing a large amount of heat, and it is considered as 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.
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.
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 the specific rate constant of a reaction can be found by experiments.
What is the difference between Reaction Rate and Specific Rate Constant?
• The reaction rate gives an indication of the speed at which the reactions are converted to products. Specific rate constant is proportionality constant.
• Specific rate constant is a part of reaction rate.
• Specific rate constant only cannot give a valid statement of the reaction speed.