What is Arrhenius Theory?
This theory states that there is temperature dependence of reaction rates. It states that acids are actually substances which will dissociate in water to give atoms or molecules which are electrically charged and are ions. One of the ion is a hydrogen ion (H +) and there is a base ion which ionise in water to give hydroxide. (Ion is an atom or molecule with net electric charge). This theory was introduced by Swedish scientist Svante Arrhenius in the year 1887.
It was however Jacobus Henricus who in the year 1884 noted the equation for the temperature dependence of equilibrium constants. It was later developed by Arrhenius. The equation determines rate of chemical reaction and for calculation of energy activation. Arrhenius formulated and gave a physical justification for this theory. Nowadays we use this equation and Arrhenius theory to measure and record the temperature variation of coefficients and many other thermally induced reactions.
The Arrhenius reaction can be expressed as:
k= Ae -Ea/RT where -Ea/RT is to the power to.
Here k is the rate constant, T is the temperature (kelvins), A is the pre-exponential factor, Ea is the activation energy for the reaction and R is the universal gas constant.
If we take an exponential factor A the units corresponding to it are same and identical to those with the constant rate and will also be different on the reaction order. Here if the reaction is of first order it will have the same units and for that reason it is called the frequency of the reaction. Simply we can say that k is the number of collisions that are result for a reaction pert second. A is the number of collisions happening per second and there is proper orientation happening. Any collision resulting in a reaction will increase the temperature or decrease the activation energy through the use of catalysts and thus increase the rate of the reaction.
When we consider the kinetic energy and their small temperature, we can assume that we can approximate the activation energy as being independent of the temperature. If we consider many practical conditions then the weak temperature is dependent on the pre-exponential factor. However if we consider the barrier less reactions this is an exception. with this equation we can also estimate the rate of reaction that increases by a factor for every temperature increase seen (10 degree C).
What are the limitations of the Arrhenius activation energy? Here both the activation energy and rate constant k are determined and they represent macroscopic parameters which are not related to threshold energies and the success of some individual collisions at the molecular level. Here if we consider a collision between two molecules X and Y (the collision is basic so the reaction is elementary). The angle of collision, the translational energy, the internal energy which is the vibration basically will all determine the chance of the collision which will produce a product XY molecule. If we measure the collision parameters then we see many individual collisions had happened and the reaction rates are at molecular level and hence we conduct experiments under collisional conditions and this is often known as molecular reaction dynamics.
Let’s take another example or situation where the Arrhenius equation parameters will fall short in heterogenous catalysts. Here all the molecules will not collide directly and hence we cannot apply the simple cross section theory. There can be many deviations to the Arrhenius law in the glass forming process in glass forming matter. The law predicts that the structural units that is atoms and ions and molecules are slowing down at a slower rate through the glass transition that is observed. Here the structural units slow down at a faster way and rate than that is predicted by the Arrhenius law. This particular observation is more reasonable where we consider that units must always overcome the energy barrier. This is means of thermal activation energy. The thermal energy here must be high to allow for translational motion of the units which then allows the material viscosity hence the overall structure of the material.
Theoretical interpretation of the Arrhenius concept of activation energy equation :
The Swedish scientist Arrhenius did argue that for the reactants to transform into products they must first get a mximunum amount of energy called the activation energy Ea. At a temperate that is static lets say T the fraction of molecules that will have kinetic energy that is greater than Ea and can be calculated from mechanics. Here the concept of activation energy thus explains the exponential energy of the relationship and in some way of the other it is available in all the kinetic theories. One approach to this theory is that molecules are reacting if they collide with a kinetic energy along the centre that will exceed the Ea.
Sharp you knowledge by solving similar questions, refer: Acids, Bases And Salts from Class 10 Chemistry – Find All the Questions & Answers