CHEMICAL KINETICS

ACTIVATION ENERGY

We know that nitrogen and oxygen combine to form nitrogen oxides.


Here is an interesting question. Air contains both oxygen and nitrogen, but why they do not combine  to  form nitrogen oxides by themselves.

To know the answer we should know what is activation energy?

" It is the additional energy required by the colliding molecules to attain threshold energy in addition to its potential energy."

Let's take a look at this energy level diagram.The potential energy of the reactant molecules i.e., in this case nitrogen and oxygen is indicated in the left side. The peak portion is called threshold energy. The molecules must reach the peak to undergo effective collision.

Now the molecules attained threshold energy by gaining additional energy which is called as activation energy.Thus the portion indicated between the < < marks gives you the activation energy.

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Now you can see that nitrogen and oxygen have reacted to form nitric oxide.Also the P.E difference between the reactants and products is taken as enthalpy change.Here it is positive thus an endothermic process.

 So what's the answer for  the question asked before? yes nitrogen and oxygen in air needs activation energy to form nitric oxide.Unless they get it they wont react with each other.