Answer:-

The requirements for molecules to react and form products are as follows:

i. Activation Energy: For a reaction to occur, the colliding reactant molecules must possess the minimum kinetic energy, known as activation energy. The reaction will only happen if the colliding molecules have kinetic energies equal to or greater than the activation energy.

ii. Orientation of Reactant Molecules:

a. In reactions involving complex molecules or ions, reactant molecules must collide with the proper orientation. The molecules need to be oriented relative to each other in such a way that the reacting groups approach closely.

b. For example, consider the reaction A + C - B → A - B + C. If A collides with C approaching toward A, it would not lead to a reaction. The reactant molecules would collide and separate due to the improper orientation of C - B.

c. The reaction is successful as a result of the proper orientation of C - B. Only a fraction of such collisions leads to the conversion of reactants to products.

iii. Potential Energy Barrier:

Consider the reaction A + C - B → A - B + C.

a. During the collision, a new bond A - B develops, and at the same time, the bond C - B breaks. An activated complex is formed in which all three atoms are weakly connected together: A + C - B → A-----B-----C → A - B + C.

b. To attain the configuration A-----B-----C, atoms need to gain energy, which comes from the kinetic energy of colliding molecules. The energy barrier between reactants and products is illustrated in the diagram below:

Potential Energy Barrier

c. To form an activated complex, the reactant molecules need to overcome the energy barrier by climbing up. The height of the barrier is called activation energy (E_a). Therefore, reactant molecules transform into products only if they possess energy equal to or greater than this activation energy. Consequently, only a few collisions lead to the formation of products.