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Chemical reactions take place when molecules interact with each other. At a microscopic level, the molecules collide with each other and these collisions result in chemical reactions. However, not all the collisions will result in a chemical reaction. The molecules must possess enough kinetic energy to overcome the threshold energy...

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Chemical reactions take place when molecules interact with each other. At a microscopic level, the molecules collide with each other and these collisions result in chemical reactions. However, not all the collisions will result in a chemical reaction. The molecules must possess enough kinetic energy to overcome the threshold energy barrier, which will enable them to overcome the repulsive forces. Apart from the kinetic energy, molecules are also required to possess a certain specific orientation for the collisions to be successful. The collisions of molecules that possess adequate kinetic energy and the appropriate orientation result in a chemical reaction. And all such collisions are termed as effective collisions (since they effectively cause the chemical reaction).

Increasing the temperature and/or the amount of molecules are some ways to increase the number of effective collisions.

Hope this helps. 

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