Rule no. 5 the law of micro-reversibility

This is a consequence of the second law of thermodynamics. The development of a reaction must in some way follow the same mechanical pathway as the reverse reaction. 

This principle, which must be respected at the scale of an elementary step, is not valid when considering the overall reaction, as shown in the example of calcium carbonate decomposition  

In this decomposition reaction, it is evident that the hme formed will be on the surface of the carbonate grains (between the carbonate and the gas), whereas in the reverse reaction the carbonate would be on the surface of the lime (between the lime and the gas). In the first case, carbon dioxide has to go through the lime, whereas in the second case, it has to go through the carbonate. These paths are not the reverse 

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