What Happens When Bleach and Vinegar Are Mixed
Chlorine bleach contains sodium hypochlorite or NaOCl. Because bleach is sodium hypochlorite in water, the sodium hypochlorite in bleach actually exists as hypochlorous acid:
NaOCl + H2O ↔ HOCl + Na+ + OH-
Hypochlorous acid is a strong oxidizer. This is what makes it so good at bleaching and disinfection. If you mix bleach with an acid, chlorine gas will be produced. For example, if you mix bleach with toilet bowl cleaner, which contains hydrochloric acid:
HOCl + HCl ↔ H2O + Cl2
Chlorine gas attacks mucous membranes, such as your eyes, throat, and lungs and can kill you, so causing that reaction isn't in your best interest. If you mix bleach with another acid, such as the acetic acid found in vinegar, you get essentially the same result:
2HOCl + 2HAc ↔ Cl2 + 2H2O + 2Ac- (Ac : CH3COO)
There is an equilibrium between the chlorine species that is influenced by pH. When the pH is lowered, as by adding toilet bowl cleaner or vinegar, the ratio of chlorine gas in increased. When the pH is raised, the ratio of hypochlorite ion is increased. Hypochlorite ion is a less efficient oxidizer than hypochlorous acid, so some people will intentionally lower the pH of bleach to increase the oxidizing power of the chemical, even though chlorine gas is produced as a result.
What You Should Do Instead
Don't poison yourself! Rather than increasing the activity of the bleach by adding vinegar to it, just buy fresh bleach! Chlorine bleach has a shelf life. This is particularly true if your bleach has been hanging around for several months. It's far safer for you to buy fresh bleach than to risk releasing a chemical weapon on yourself by mixing bleach with another chemical. You can use bleach and vinegar for cleaning, just make sure you rinse before switching products.