11.2 Collision Theory And Transition State Theory

Objectives

1.Collision theory.

   Explain the requirements for effective collision:

      i. minimum energy

     ii. Correct orientation

2. Transition state theory

    i. Define activation energy of a reaction with reference to the energy profile      diagram.

   ii. Define activated complex.

  iii. State the characteristics of an activated complex.

Collision Theory

Collision Theory is the theory to explain the rate of chemical reactions. It is based on;

1-    molecule must collide to react

2-    molecules must possess a certain minimum kinetic energy (activation energy) to initiate the chemical reaction .

3-    molecule must collide in the right orientation in order that the collisions will result  in a reaction.

Only effective collisions cause formation of product that is collisions of molecules with Ea and at correct orientation.        

Rate directly proportional to the number of effective collisions / time

           

Activation Energy

The activation energy (Ea) is the min. kinetic energy that molecules must posses in order for a chemical reaction to occur. The Ea is shown on reaction profiles diagrams, illustrate the role of the Ea as an energy barrier that must be overcome by the reactants before they can form products.

Orientation factor/stearic factor : is how reacting molecules are oriented relative to each other.

·         Collision that cannot produce chemical change(ineffective collision)

·         Collision that lead a reaction(effective collision)

Transition State Theory

o The transition state theory describes what happens to the reactant molecules prior to their change into products.

o An intermediate stage lies between the reactants and the products is transition state.

o The transitional species with partial bonds is activated complex.

A Reaction Profile

Characteristics Of An Activated Complex

• Very unstable, It has a short half-life (exists at the instant the reacting system has the highest potential energy.

• Its potential energy is greater than reactants or products.

• In an activated complex, the bonds in the reactant molecules are in the process of breaking while the new bonds in the product molecules are starting to form.

• The activated complex and the reactants are in chemical equilibriumharacteristics

• It decomposes to form products or reactants.

Example

1. Define the term activation energy

2. Draw a potential energy diagram for an endothermic reaction. Indicate on the

drawing

    i. the potential energy of reactants

   ii. the potential energy of the products

  iii. the energies of activation for the forward and reverse reactions

  iv. the heat of reaction.

Solution

The activation energy (Ea)…..is the minimum energy is required

to initiate the chemical reaction and produce the product. It must be

supplied by collisions.