Single Replacement: Kl(s) + Cl2(g)

Double Replacement: CuCl2(aq) + Na2S(aq)

2\,KI\, +\, Cl_2\, =\, 2\,KCl + I_2

This reaction works because chlorine is a stronger oxidizing agent than iodine.

CuCl_2(aq)\, + \, Na_2S(aq)\, = \, CuS + 2\, NaCl

The driving force in this case is the formation of CuS which is quite insoluble in water.