Consider the reactions:
- Between hydrogen and chlorine to form hydrogen chloride
- Between hydrogen and oxygen to form steam (gaseous water)
- Between hydrogen and nitrogen to form ammonia gas
Explain what Gay- Lussac would have observed carring out these experiemnet and how Avogadro would have interpreted these observations.
Consider the reaction between hydrogen and chlorine to form hydrogen chloride.
While carrying out this experiement, Gay- Lussac would have observed that, at constant temperature and pressure, n volume gaseous hydrogen reacting with n volume of gaseous chlorine to produce 2n volumes of gaseous HCl.
Therefore, the volumes of gaseous species in this reaction (reactant or product) are all in the simple whole number ratio of 1:1:2.
Avogadro, on the other hand, was concerned with the number of molecules in definite volumes of gaseous substances, and would have interpreted this observation of Gay-Lussac as follows:
At the same temperature and pressure, m number of molecules of gaseous hydrogen reacts with m number of molecules gaseous chlorine to produce 2m number of molecules of gaseous hydrogen chloride.
Thus, one molecule of gaseous hydrogen reacts with one molecule gaseous chlorine to produce two molecules of gaseous hydrogen chloride.
Or, one molecule of HCl would require 1/2 molecule of gaseous hydrogen and 1/2 molecule of gaseous chlorine.
So the smallest possible unit of gaseous hydrogen or chlorine is 1/2 molecule.
Since atoms are indivisible (this was the picture in those days), the smallest possible unit of any elementary substance should be its one atom.
Therefore, for elementary gaseous substances like hydrogen or chlorine, 1/2 molecule= 1 atom
`rArr ` 1 molecule = 2 atoms
In other words, elementary gases undergoing chemical combination are all diatomic.
Same set of logic as well as conclusions are applicable to other systems of gaseous reactions too.