Chemical Kinetics
Table of Contents
Can activation energy ever be negative?
Well, in some cases, activation energy can actually attain a numerically negative value. It is true for typically barrier less reactions where the activation energy is calculated from reactant to transition state. The advancing reaction relies only on the capture of the molecules in a potential well.
In simple words, there are certain reactions in which the reaction rate is inversely proportional to the temperature. Thus, increasing the temperature reduces the probability of molecules colliding as the reaction cross section decreases. It happens because increasing the temperature increases the momentum of molecules due to which the molecules are carried away from the potential collision zone.
For more interesting facts about activation energy and its supply click here.
Factors which affect the reaction rate
This topic sounds familiar. Isn’t it? Well, for a moment you might think so until you actually discover what lies ahead. You must have studied the basic factors that affect the rate of reaction of chemical processes.
There are quite a few other factors that play an important role in altering the reaction rate. These factors are-
- Particle size- A smaller particle size of reactants provides a greater surface area for the collisions to take place on. Therefore, we know that the greater the surface area the faster is the rate of reaction.
- Isotope effect- The reaction rate for different isotope molecules may be different. This results due to different isotopologues having different configurations and stability. (Reference)
- Solvent effect- Presence of a suitable solvent might rapidly increase the reaction rate where a lack of one might decrease it. Also, higher the viscosity lower is the rate of diffusion of the particles which implies lower reaction rate.
- Physical state- When reactants are in different phases, the reaction is limited to a tie-up between the reactants because the reaction takes place only at their area of contact. On the other hand, when the reactants are in same phase, let’s say in an aqueous solution, thermal motion helps bring the reactants into contact. (Source)
- Mixing and stirring- Mixing the reactants together increases their ability to interact thus increasing the rate of the chemical reaction.
- Pressure- Increasing the pressure in a gaseous chemical reaction increases the number of collisions between the reactants, thus, increasing the rate of reaction. (Reference)
- Radiation or light- Reactants that are affected by this factor sometimes produce free radicals and these free radicals react almost immediately with other molecules due to their excited state forming products.
Law of mass action vs rate law
The question is why the predictions of law of mass action may not agree with that of the rate law? Which law is correct in determining the reaction rate?
- Well, it’s just that law of mass action is valid only for ideal cases. For a reaction, the power to which active masses of each reactant is raised is fixed because it is always equal to its stoichiometric coefficient. However, the reaction rate differs with the change in physical conditions.
- While, in the rate law, the power to which active masses of each reactant is raised may or may not be equal to its stoichiometric coefficient. Therefore, the rate equation comprises of actual factors that change in every experimental condition.
Thus, both the laws are correct. Although, rate law that is applied considering only the reactants of a reaction is considered quite experimental in nature and hence is generally more appropriate and applicable while the law of mass action due to its limited approach becomes kind of theoretical in nature.
To know other key differences in the approach of these two laws, click here.
Why do we need to calculate and control the rate of reaction?
The study of reaction rates is very crucial in kinetics. If a reaction occurs very fast it can cause a great damage in much less time. We need to control it according to our own necessity. Let’s take an example of rusting process of iron. This process is pretty fast but if we don’t want this reaction to take place, we either try to delay or control it. Thus, we either paint the iron or cover it with a material that slows down the process.
The following video will help you get a better understanding of this concept.