Chlorine on reaction with hot and concentrated sodium hydroxide gives

The reaction of chlorine gas with sodium hydroxide is a classic example of a disproportionation reaction (also known as a self-redox reaction). In this type of reaction, the exact same element (chlorine) is simultaneously oxidized to a higher oxidation state and reduced to a lower oxidation state.

2. Chemical Equation for Hot and Concentrated NaOH

When chlorine gas (Cl2) reacts with hot and concentrated sodium hydroxide (NaOH), it undergoes a deep oxidation-reduction process to form sodium chloride, sodium chlorate, and water.

The balanced molecular equation is: 3 Cl2 + 6 NaOH -> 5 NaCl + NaClO3 + 3 H2O

The ionic form of the products focuses on the anions released in solution:

• NaCl dissociates into Na+ and Cl- (Chloride ion)
• NaClO3 dissociates into Na+ and ClO3- (Chlorate ion)

3. Oxidation State Changes

Let us look at what happens to the oxidation states of the chlorine atoms:

• In elemental chlorine (Cl2), the oxidation state of chlorine is 0.
• In the chloride ion (Cl-), the oxidation state of chlorine is -1 (Reduction).
• In the chlorate ion (ClO3-), the oxidation state of chlorine is +5 (Oxidation).

4. Comparison with Cold and Dilute Conditions

It is helpful to contrast this with the other common exam condition to avoid confusion:

• With Cold and Dilute NaOH: The reaction is milder and yields chloride and hypochlorite ions: Cl2 + 2 NaOH -> NaCl + NaClO + H2O Anions formed: Cl- and ClO-
• With Hot and Concentrated NaOH: The higher temperature drives the oxidation state of chlorine further up to +5: Anions formed: Cl- and ClO3-