Thiosulphate acidic decomposition
The general answer to this question is no, one cannot simply predict the products of any reaction.  How do we know that one of the products is not $\ce{S2O6^{2-}}$ or perhaps even $\ce{S2O5Cl2^{2-}}$?

The best that can be done without any guidance from experiment is to compute the thermodynamically favored products.  One does this (at standard temperature and pressure) by writing down all plausible products and then computing the Gibbs free energy ($\Delta G$) for the reactions.  The one with the smallest $\Delta G$ is the thermodynamically preferred.

But be warned:  there are mechanistic and kinetic factors to take into account.  The thermodynamically favored reaction may, as a practical matter, not take place.  The standard example is diamond --> graphite, which is thermodynamically allowed, but don't hold your breath waiting for it to happen.