Arrhenius equation

Aug 26, 2022, 16:45 IST

About Arrhenius equation

Before moving to Arrhenius equation explanation lets understand activation energy and its use in Arrhenius equation

Activation energy

Activation energy = Threshold energy – Average kinetic energy of the reacting molecules.

(a) Zero activation energy = Fraction of effective collision (f) will be very large = Very fast reaction (Instantaneous reaction).

(b)  Low activation energies = Fraction of effective collision (f) will be large = Fast reactions.

(c)  High activation energies = Fraction of effective collision (f) will be small = Slow reaction.

The activation energy   depends upon the nature of chemical bonds undergoing rupture and is independent of enthalpies of reactants and products.

According to the concept of activation energy, the reactants do not change directly into the products. The reactant first absorb energy equal to activation energy and form activated complex. At this state, the molecules must have energy at least equal to the threshold energy. This means that the reaction involves some energy barrier which must be overcome before products are formed. The energy barrier is known as activation energy barrier.

(2) Transition state theory 

(i) According to transition state theory the activated complex is supposed to be in equilibrium with the reactant molecules.

(ii) Once the transition state is formed it can either return to the initial reactants or proceeds to form the products.

(iii) Assuming that once formed the transition state proceeds to products we can say that rate is proportional to concentration of transition state.

Mathematically, Rate  Transition state

Rate= Constant × Transition state

Arrhenius equation Definition  

(Temperature dependence of rate constant):

Arrhenius suggested an equation which describes rate constant as a function of temperature .can be explain as k=Ae to the power -Ea/RT

where A = frequency factor

Ea =Energy of activation; R= the gas constant. 

T =Temperature in Kelvin.

At two temperatures T1 and T2 their rate constant are given by k1 and k2. 

Find below example and detail explanation of Arrhenius equation

For more details about the chemistry formulas check out the formula sheet