Microscopic/Macroscopic Views of
Chemical Reactions

For a chemical reaction to take place atoms and molecules must interact with one another. Atom-atom, atom-molecule or molecule-molecule collisions must take place. Thinking of the reaction at this atomic level is the "microscopic" view of the reaction. On the other hand, when we do experiments with chemical reactions in the laboratory we see the effects of a very large collection of atomic and molecular collisions. This latter perspective is the "macroscopic" view.

  • Microscopic view of reactions: single atomic/molecular collisions
  • Macroscopic view of reactions: large collections of atomic/molecular collisions

Consider the reaction between hydrogen (H2) and oxygen (O2). From the microscopic view we see the H2 and O2 molecules collide and exchange atoms. (Place mouse cursor over the image below to see the animation.)

This reaction is, of course, the formation of water. While the animation above is a simplified version of the microscopic view, it nevertheless depicts the essence of a chemical reaction as an exchange or rearrangement of atoms with old bonds breaking and new bonds forming during the reaction process.

The macroscopic view of the above reaction is different only in that we are now concerned with a large collection of hydrogen molecules colliding with oxygen molecules. The number of H2 and O2 molecules necessary to be seen experimentally is huge - billions of trillions of individual collisions of these molecules. Later we will learn that the chemist has a particular name for this large number - it is called the "mole".

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C101 Class Notes
Prof. N. De Leon