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Activation Energy

What is activation energy?

Activation energy in chemistry defines a certain amount of energy acquired by the atoms or molecule to be active before the chemical transformation or activated complex formation. This means before the chemical reaction proceeds, the reactant molecule or molecules must be in an energy-rich species or activated state.

Activated complex theory

The activated complex theory is also known as the theory of absolute reaction rate proposed by Arrhenius. According to Arrhenius when the reactants are converted into products, the reactant has passed through the crucial configuration or activation to higher energy levels. This is called the transition state. In the transition state, the reactants form an activated complex whose concentration can be calculated from the concentration of the reactant.

Activated complex example

The idea of the activated complex in the transition state better understands by the example of the formation of a molecule like hydrogen iodide. Suppose a hydrogen atom approaches halogen molecules to form an activated complex. When these two are far apart, the energy of the system is equal to the sum of individual energies. As the hydrogen atom comes close to the iodine molecule, the orbitals of the hydrogen and iodine become overlap. Therefore the iodine-iodine chemical bonding begins to stretch. The energy of the system now trends to activated or increases by the formation of the complex.

Activation energy diagram

In learning chemistry, the meaning of activation energy or activated complex is made clear with the schematic diagram or reaction rate coordinate vs energy graph given in the below picture.

Activation energy formula graph in chemistry for atoms or molecule to active before activated complex formation

The average energy of reactant and product in the above diagram is represented by ER and EP respectively. A minimum energy level is required for the chemical kinetics reaction denoted by Ex to which the reactant molecule must be reached for any chemical changes. Therefore, the excess or additional energy (Ex – ER) in the above diagram which the reactant required for the chemical transformation or activated complex formation is the activation energy E1 of the atoms or molecule.

After the chemical transformation, the product has an average free energy EB. Therefore an amount of energy like (Ex – EB) = E2 is calculated from the Van’t Hoff equation for any chemical transformation. If E2 > E1, the reaction is exothermic but when E2 < E1, the chemical transformation is endothermic.

Arrhenius equation for activation energy

Activation energy is usually represented by Ea and find from the Arrhenius mathematical formula, k = Ae-Ea/RT, where A = constant. It is obvious that the activation energy Ea in the Arrhenius equation must have the units of energy.

In enzyme catalysis reaction in biology, the chemical catalyst can be lowering the activation energy which permits a larger amount of reaction in a given time by the formation of the intermediate activated complex.