Chlorine production from the addition of hydrochloric acid to sodium hypochlorite solution
Bleach is an aqueous solution of sodium hypochlorite (NaClO) (Wikipedia).
When hydrochloric acid (HCl) is added to a solution of NaClO, ClO- is firstly protonated to form hypochlorous acid (HClO) :
$$ \ce{ClO- + H+ <=>> HClO} \qquad \qquad K_1=1/K_a $$Then, HClO is converted to chlorine (Cl2) according to the following reaction (e.g. Wikipedia):
$$ \ce{HClO + H+ + Cl- <=>> Cl2 + H2O} \qquad \qquad K_2 $$Those two reactions are illustrated with the below concentration versus pH diagram created with PHREEQC, using a specific database ; and explain why mixing bleach with hydrochloric acid can liberate (toxic) chlorine gas.
Distribution at 25 °C of Cl2, HClO and ClO- according to pH through HCl adding to NaClO solution
Similar diagrams are exposed in Deborde and von Gunten (2008) and Hamano and Ikeda (1995) (Note that the Cl2/HClO equivalence pH is dependent on the K2 taken value) :
