It's a little bit complicated because hypochlorite reacts with chloride and a source of protons to form chlorine, but if no chloride is added, hypochlorite itself can slowly form chloride via two reactions:
2 ClO- --> 2 Cl- + O2
3 ClO- --> 2 Cl- + ClO3-
The chloride then reacts with further hypochlorite and a source of protons to form chlorine gas:
ClO- + Cl- + 2 [H+] --> Cl2 + H2O
This reaction proceeds slowly with a bad source of protons, such as water, but much faster with stronger acids. Commonly hydrochloric acid is used, which is also a source of chloride:
ClO- + 2 HCl --> Cl2 + H2O + Cl-
With an external source of chloride, the reaction proceeds faster, and also gives a better yield because half of the chlorine atoms in the Cl2 actually come from the chloride source (in this case HCl), plus when hypochlorite has to form "it's own" chloride, some of the chlorine atoms end up in the chlorate ions (first equation). Adding chloride also means you get (almost) no oxygen with your chlorine gas (second equation). So if you use a different acid it's a good idea to add something like NaCl.
>bad stuff
chlorine is "bad stuff", sort of
>choramines
Are formed with ammonia and some nitrogenous organic compounds such as urea, but not on its own.
