Sodium Hydroxide and Chlorine Gas Reaction | NaOH + Cl₂

Sodium hydroxide reacts with chlorine gas in different ways according to the concentration and temperature of the solution and it gives different products. NaOH and Cl₂ reaction is an oxidation - reduction reaction and those reactions are more special because they are disproportionation reactions.

Reaction Types of NaOH + Cl₂

• Cold dilute NaOH and Cl₂ reaction
• Hot concentrated NaOH and Cl₂ reaction

Cold Dilute NaOH and Cl₂ reaction | NaOH + Cl₂ = NaCl + NaOCl + H₂O

Cold dilute sodium hydroxide and chlorine gas reaction is disproportionation. Chlorine gas is oxidized and reduced to OCl^- ion and Cl^- ion respectively. As products sodium chloride, sodium chlorate(i) and water are given.

• Hypochlorite ion | chlorate ion(i) - OCl^-
• Sodium chloride - NaCl
• Sodium chlorate(i) - NaOCl

Aqueous NaCl, NaOCl solution is called as the Milton solution. Hypochlorite ion exists in bleaching powder.

Chemical Balanced Equation

In hypochlorite ion, chlorine atom is in the +1 oxidation state. In NaCl, chlorine is at -1 oxidation state.

Hot concentrated NaOH and Cl₂ reaction | NaOH + Cl₂ = NaCl + NaClO₃ + H₂O

Hot dilute sodium hydroxide and chlorine gas reaction is also a disproportionation reaction.Chlorine gas is oxidized and reduced to ClO₃^- ion and Cl^- ion respectively. As products sodium chloride, sodium chlorate(iii) and water are given.

• Chlorite ion - ClO₃^-
• Sodium chlorate(iii) - NaClO₃

Chemical Balanced Equation

In chlorite ion, chlorine atom is in the +5 oxidation state.

Hypochlorite | Chlorate(i) ion | OCl^-

OCl^- ion is an unstable ion it exists only in cold states. When temperature increases, it disproportionate to chloride ion and chlorate ion. Negative charge is on the oxygen atom.

As chlorine gas, other halogens except fluorine also react with sodium hydroxide and give similar products as NaOH + Cl₂ reaction.

Balancing the cold dilute NaOH and Cl₂ reaction

We know this reaction is a disproportionation reaction. When we see the products, it's clear that Cl₂ is oxidized to OCl^- and reduced to Cl^-. Oxidation numbers of cglorine in OCl^- ion is +1 and Cl^- ion is -1.

Now check the differences of each oxidation numbers in oxidation and reduction. It is one in both. Then exchange the differences.

Then balance other elements. Check Na atoms in both sides. There are two Na in the right side. So we put two in front of NaOH. Then check hydrogen balance. Left side has two hydrogen atoms so far now. Then only one H₂O molecule in the right side is enough to balance the hydrogen equation.

This type of redox reaction balancing example is reaction of chlorine gas with ammonia.

What is a disproportionation reaction?

A reaction in which a substance is simultaneously oxidized and reduced, giving two different products.

Examples

• NaOH + Cl₂ reaction
• Hydrogen peroxide decomposition to water and O₂

Production

Now we need to produce NaOCl and NaClO₃. So we need NaOH and Cl₂

Electrolysis of Brine (NaCl)

Electrolysis of brine solution will produce NaOH, Cl₂ gas and H₂ gas.

NaOCl

We can collect Cl₂ gas can cool the gas before send it to the produced NaOH to occur the reaction. NaOH solution also should be cold.

NaClO₃

Produced Cl₂ is sent to the hot NaOH.

In both occasions, NaCl is produced as a product. We can separate it by a separton method from NaOCl or NaClO₃.

Mechanism of Cl₂ + dilute cold NaOH reaction

When chlorine gas dissolve in water, it produce hypochlorous acid (HOCl) and hydrochloric acid (HCl). You know acids react with bases and produce a salt and water as products. So when NaOH is in the presence of HOCl and HCl solution, following reactions are occurred.

• HOCl and NaOH react and produce NaOCl and water.
• HCl and NaOH react and produce NaCl and water as products.

Safety of NaOH + Cl₂ reaction

This safery and harmful facts are both valid for reaction of Cl₂ with hot NaOH and cold NaOH.

• NaOH is corrosive and may severely irritate skin, eyes, and mucous membranes. Also corrosive to the metals.
• Cl₂ is a yellow - green toxic gas. When you dealing with Cl₂, be very careful to get protected from it.

What will happen, when Cl₂ gas is sent to the water?

Cl₂ hydrolysis in the water. HCl and HOCl are given as products. HCl is a strong acid and HOCl is a weak acid. HOCl is a very good disinfectant and more efficient than bleaching powder.

Can I use this chlorine and NaOH reaction to neutralize Cl₂ gas?

Yes. You can use NaOH and this is very efficient way to do the neutralization. But you have to consider about the cost of this neutralization method because NaOH is so expensive.

What is chlorinated water or chllrination of water?

The process of adding chlorine or chlorine compounds such as sodium hypochlorite (NaOCl) to water. This is done in swimming pools. This is done to kill certain bacteria and other microbes in tap water as chlorine is toxic to those microbes. That's why we feel smell of chlorine gas in the swiming pools.

Chlorine reacts with hot and conentrated NaOH, But why products are different?

When temperature is high, the ability to oxidation is high. Therefore produced NaOCl is oxidized to NaClO₃< from +1 to +5 (oxidation umber of chlorine).

How this reaction is different from sodium hydroxide and HCl reaction?

NaOH and HCl reacts as base and acid respectively and produce salt (NaCl) and water as products. That reaction is not a redox reaction.

disproportionation reaction of water with sodium hydroxide

There is no disproportionation reaction oof water and sodium hydroxide. Only sodium hydroxide dissolve very well in water and dissociate to sodium ion and hydroxyl ions completely.

ocl^- ion oxidation state?

We know electronegativity of oxygen is higher than chlorine. So negative charge should be on oxygen atom whole chlorine atom is oxidized due to hugher electronegativity of oxygen. There is a single bond between oxygen and chlorine. Therefore oxidation number of chlorine is +1.

What will be the pH value after cold dilute NaOH reacts with chlorine gas if reactants are completely finished?

We know NaOH is a strong base. So it shows higher pH value which is greater than 7 at 25⁰C.

How pH is calculated for a strong base?

When all NaOH reacts with all chlorine, there is no remaining NaOH and Cl₂. Produced NaCl and NaOCl are in the solution and we should consider, whether their ability to hydrolysis of water to make water acidic or basic.

Equipment to react chlorine with caustic soda?

Chlorine gas reacts with iron and produce ferric chloride. So we cannot use iron as the reactant container equipment.

Also, caustic soda slowly reacts with glass and form sodium silicate. So glass container is also not suitable for the reaction equipment.

Stability of NaCl

Sodium ion and chloride ion are stable in the water and do not involve in hydrolysis of water. So NaCl is a neutral salt.

Stability of NaOCl in water

Hypochlorite ion is not stable in water and involve in hydrolysis of water to prouce HOCl and OH^- ion. So aqueous solution will basic due to release of hydroxyl ion.

Due to formation of OH^- ions, aqueous solution gets basic and show pH values of greate than 7. But not much as aqueous NaOH solution.

reaction of chlorine with sodium hydroxide acid or base?

Reaction of chlorine with sodium hydroxide can be considered as an acid-base reaction. How do I say that?

First, chlorine dissolve in water and produce hydrochloric acid (HCl) and hypochlorous acid (HOCl). These two acids react with NaOH separately to give NaCl and NaOCl respectively.

Chlorine and NaOH reaction have been included in following syllabuses.

1. Cambridge International AS and A Level Chemistry
2. General Certificate of Education (Advanced Level) GRADES 12-13

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