Animated Solution for Chemistry - s and p-Block Elements: The products obtained when chlorine gas reacts with cold and dilute aqueous NaOH are

Welcome, future chemists and doctors! Today, we are going to dive into one of the most classic, high-yield, and conceptually beautiful reactions in inorganic chemistry. We are talking about the dance of halogens with alkalis. Specifically, we are going to look at what happens when chlorine gas meets sodium hydroxide.

But here is the catch—chemistry is not just about mixing things together; it is about how you mix them. The conditions matter immensely. In this problem, we are dealing with cold and dilute aqueous $\text{NaOH}$. Let's unravel this mystery step by step!

Imagine you have a flask containing a cold, dilute solution of sodium hydroxide ($\text{NaOH}$). You start bubbling pale green chlorine gas (${\text{Cl}}_2$) into it. What happens? To understand this, we need to look at the nature of chlorine.

Chlorine is a halogen, sitting in Group 17 of the periodic table. In its elemental form, ${\text{Cl}}_2$, it has an oxidation state of exactly $0$. It is hungry for electrons, but it can also be persuaded to give them up under the right circumstances. When chlorine encounters the basic environment provided by the hydroxide ions (${\text{OH}}^{-}$), it undergoes a fascinating transformation known as a disproportionation reaction.

Disproportionation

A disproportionation reaction is a special type of redox reaction where a single substance acts as both the oxidizing agent and the reducing agent. It is like playing both sides of a chess board simultaneously! Some of the chlorine atoms will grab an electron (getting reduced), while others will lose an electron (getting oxidized).

Because our conditions are cold and dilute, the reaction is relatively mild. The energy provided is only enough to push the oxidation state of chlorine up by one step.

The balanced chemical equation for this beautiful process is:

Let's break down the products. We have sodium chloride ($\text{NaCl}$), sodium hypochlorite ($\text{NaClO}$), and water (${\text{H}}_2\text{O}$).

Now, let's look closely at the oxidation states of chlorine in our products.

In sodium chloride ($\text{NaCl}$), chlorine has gained an electron to become the chloride ion. Its oxidation state is $-1$.

In sodium hypochlorite ($\text{NaClO}$), chlorine is bonded to the highly electronegative oxygen atom. Here, it has effectively lost an electron, giving it an oxidation state of $+1$.

The question asks for the specific ions produced in the solution. When these ionic compounds dissolve in the aqueous medium, they dissociate: $\text{NaCl}$ gives us the chloride ion (${\text{Cl}}^{-}$). $\text{NaClO}$ gives us the hypochlorite ion (${\text{ClO}}^{-}$).

Therefore, the products obtained are ${\text{Cl}}^{-}$ and ${\text{ClO}}^{-}$. This perfectly matches option (c).

Did you know that this exact reaction is how industrial bleach is manufactured? Sodium hypochlorite ($\text{NaClO}$) is the active ingredient in household bleach. It is a powerful oxidizing agent, which is why it is so effective at killing bacteria and removing stains. So, the next time you see a bottle of bleach, you will know exactly the chemistry that went into making it!

Turning Up the Heat

Before we wrap up, let's do a quick thought experiment. What if we changed the conditions? What if, instead of cold and dilute, we used hot and concentrated sodium hydroxide?

When you turn up the heat and increase the concentration, you provide much more energy to the system. The disproportionation goes further! Instead of stopping at the $+1$ oxidation state, the chlorine gets oxidized all the way to the $+5$ state.

The reaction would look like this:

In this case, the products are the chloride ion (${\text{Cl}}^{-}$) and the chlorate ion (${\text{ClO}}_3^{-}$).

This is a classic trap set by examiners in competitive exams like JEE and NEET. They will test your attention to detail. Always read the conditions carefully! Cold and dilute gives hypohalite, while hot and concentrated gives halate. Keep this golden rule in your mind, and you will never get this wrong. Keep exploring, keep questioning, and let the magic of chemistry unfold!