Question

4.)Sketch a potential energy diagram for the endothermic, elementary reaction

A + B à C + D

and on it denote the activation energies, heat of reaction.Also indicate the reactants, products transition state.

5.)Explain collision theory?

6.)Explain why all reactions have an activation energy, using your knowledge of collision theory

Answer 1

5)
Postulates of collision theory:

1) Chemical reactions can occur only upon collisions between reactant molecules.

2) However all the collisions do not lead to the formation of products. Otherwise all the reactions could occur within a fraction of second since there are millions of collisions possible.

3) Only those collisions taking place between molecules in proper orientation and possessing certain minimum amount of energy can lead to the formation of products.

The minimum amount of energy required by the colliding molecules to yield the products is called Threshold Energy (ET).

The reactant molecules must cross this energy barrier to be converted to products.

4) The energy possessed by the normal reactant molecules under STP conditions is called 'Average Energy' .

The difference between threshold energy and average energy of the reacting molecules is called Activation energy (Ea).

Ea = Threshold energy (ET) - Average energy (ER)

5) Some molecules will get energy equal to or greater than the threshold energy during collisions and are called as activated molecules. These are denoted by asterisk(*).

6) The collisions taking place between activated molecules are called activated collisions (or) effective collisions (or) fruitful collisions. The products are formed only during the activated collisions.

A + A ---------> A* + A                 (normal collision; no product is formed)

A* + A* ---------> products         (effective collision)

Where

A = normal molecule

A* = Activated molecule

Activated molecules constitute a small fraction of total molecules. Therefore the effective collisions constitute a small fraction of total collisions. Hence all the chemical reactions do not occur in a fraction of a second.

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6)

Eplanation:
1)for every reaction it has its own freeenergy (DG= -Ve) when reactents are aproaching for collide that their free energy ( kinetic energy) must over come the activation energy otherwise the reaction does not takes.

2)Even if the species are orientated properly, you still won't get a reaction unless the particles collide with a certain minimum energy called the activation energy of the reaction