We report the first simulation of a surface chemical reaction performed with the ab initio molecular dynamics approach. A set of trajectories with different initial conditions has been generated for single Cl2 molecules impinging on the Si(111)-2 x 1 surface with incident translational energy of 1 eV. We observe a high probability of dissociation, triggered by active sites on the pi-bonded chains, and accompanied by a large surface response and local rehybridization effects.