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Aims: We study the chemistry of the Type IIb supernova ejecta that led to theCas A supernova remnant to assess the chemical type and quantity of dust thatforms and evolves in the remnant phase. We later model a dense oxygen-richejecta knot that is crossed by the reverse shock in Cas A to study theevolution of the clump gas phase and the possibility to reform dust clusters inthe post-reverse shock gas.  Methods: A chemical network including all processes efficient at high gastemperatures and densities is considered. The formation of key bimolecularspecies (CO, SiO) and dust clusters is described. Stiff, coupled, ordinary,differential equations are solved for the conditions pertaining to both the SNejecta and the post-reverse shock gas.  Results: We find that the ejecta of Type IIb SNe are unable to form largeamounts of molecules and dust clusters as opposed to their Type II-Pcounterparts because of their diffuse ejecta. The gas density needs to beincreased by several orders of magnitude to allow the formation of dustclusters. We show that the chemical composition of the dust clusters changesdrastically and gains in chemical complexity with increasing gas density.Hence, the ejecta of the Cas A supernova progenitor must have been in the formof dense clumps to account for the dust chemical composition and massesinferred from infrared observations of Cas A. We show that the ejecta moleculesin a clump that is processed by the reverse shock reform in the post-reverseshock gas with lower abundances than those of the initial ejecta clump, exceptSiO. These molecules include CO, SiS and O2. Dust clusters are destroyed by thereverse shock and do not reform in the post-reverse shock gas, even for thehighest gas density. These results indicate that the synthesis of dust grainsfrom the gas phase in the dense knots of Cas A and in other supernova remnantsis unlikely.

Molecules and dust in Cas A: I - Synthesis in the supernova phase and processing by the reverse shock in the clumpy remnant