The interaction of chlorosilanes and the silicon surface is an important part of reaction processes, such as hydrochlorination and chemical vapor deposition, involved in the production of high purity silicon. We used planewave based density functional theory (DFT) to investigate periodic slabs of the Si(100)-c(4 x 2) surface for adsorption of H-2, HCl, SiCl2, dichlorosilane (SiH2Cl2 or DCS), trichlorosilane (SiHCl3 or TCS) and silicon tetrachloride (SiCl4 or STC). The effects of surface coverage, molecule orientation, adsorption site and multiple molecule adsorption were studied. All the molecules could undergo dissociative chemisorption in the right orientation and site placement, with HCl and SiCl2 possessing the strongest binding energies. The H-2 molecule preferred lower coverage, the HCl molecule was not much affected by coverage while the SiCl2 molecule strongly preferred higher coverage and the STC molecule was affected negatively by both too high or low coverage. The elementary steps leading to transfer of surface crystal silicon atoms to gas phase molecules as part of the chlorination or hydrochlorination process were then looked at through reaction pathway analysis. The formation of SiCl2 from a surface dimer Si atom was found to prefer an intradimer route with a reaction barrier of 3.62 eV (83.48 kcal/mol), going down to 3.10 eV (71.49 kcal/mol) after removal of the first surface Si atom. The subsequent formation of TCS from this SiCl2 was found to have reaction barrier of 1.07 eV (24.68 kcal/mol). STC could also be formed from this SiCl(2 )molecule with a reaction barrier of 2.86 eV (65.95 kcal/mol).