To complete and balance the reaction between chlorine gas (Cl₂) and hydroxide ions (OH^-) to produce chloride ions (Cl^-) and hypochlorite ions (ClO^-), we need to take into account both the conservation of mass and the conservation of charge. This reaction occurs in aqueous solution, and it depends on the conditions such as temperature and the concentration of the hydroxide ions. There are actually two possible reactions. In a basic solution, we can have a balanced equation that reflects the production of both chloride and hypochlorite ions:

$$\text{Cl}_2 + 2\text{OH}^- \to \text{Cl}^- + \text{ClO}^- + \text{H}_2\text{O}$$

Explanation:

- The chlorine gas (Cl₂) reacts with hydroxide ions (OH^-) present in the aqueous solution.

- Two hydroxide ions are required to balance the equation, ensuring that the chlorine from Cl₂ is distributed between the produced chloride ions (Cl^-) and hypochlorite ions (ClO^-).

- The equation is balanced as written because there are two chlorine atoms on each side, the charges are balanced (2 negative charges on the reactant side from the hydroxide ions and 2 negative charges on the product side from Cl^- and ClO^- ions), and the additional product is water (H₂O), formed by the combination of the hydrogen from the hydroxide ions and an oxygen atom from one of the hydroxide ions.

This equation highlights how the presence of OH^- drives the disproportionation of Cl₂, splitting it into two different oxidation states in the products (Cl^- at -1 and ClO^- at +1). This is a common type of reaction for halogens in the presence of base.