A comparison between unimolecularand gaseous reactions. The thermal decomposition of gaseous acetaldehyde

We have recently observed that the thermal decomposition of acetone in the gaseous state is a homogeneous, unimolecular reaction. As in the decom­position of nitrogen pentoxide, the number of molecules which react in unit time is very many times greater than the number which could receive the necessary energy by collision with other molecules. With the object of seeing whether this fundamental characteristic of unimolecular reactions would be confirmed in yet another instance, we have examined the thermal decomposi­tion of gaseous acetaldehyde, which at 500° C. decomposes smoothly into methane and carbon monoxide. The chemical similarity between this and the decomposition of acetone made it seem probable that acetaldehyde might also decompose in a unimolecular manner. But although the new reaction proved to be homogeneous, it was bimolecular. We therefore have the opportunity of making a comparison between the molecular statistics of two chemically similar reactions taking place in the same region of temperature, one of which definitely depends on molecular collisions, while the other appears to be independent of them. The contrast between the two supports strongly our previous theoretical conclusions, because, while in the unimolecular reaction the number of molecules reacting bears no relation whatever to the number which could be activated by collision, in the bimolecular reaction there is almost perfect correlation between the number of collisions suffered by activated molecules and the observed rate of reaction.