Welcome to the Reactions of Chlorine!
Hi there! Today, we are going to explore how chlorine behaves when it meets water and sodium hydroxide. Chlorine is a very "busy" element; it loves to react! You might already know it as the smell of a swimming pool or the active ingredient in bleach. In this chapter, we will see why it is so good at cleaning and how it can actually act as both an oxidising agent and a reducing agent at the exact same time! Don't worry if that sounds complicated—we'll break it down step-by-step.
1. Prerequisite: What is Disproportionation?
Before we look at the specific reactions, we need to understand one special word: disproportionation. Imagine a person standing in the middle of a see-saw. If they suddenly split into two people, where one goes up and the other goes down, that's a bit like a disproportionation reaction.
In Chemistry, disproportionation is a reaction where the same element is simultaneously oxidised (its oxidation number goes up) and reduced (its oxidation number goes down).
2. Chlorine and Cold Sodium Hydroxide (\(NaOH\))
When we add chlorine gas to cold, dilute aqueous sodium hydroxide, it reacts to form two different salts and water. This is the reaction used to make household bleach!
The Equation:
\(Cl_{2}(g) + 2NaOH(aq) \rightarrow NaCl(aq) + NaClO(aq) + H_{2}O(l)\)
Step-by-Step Breakdown of Oxidation Numbers:
Let's track the "movement" of the chlorine atoms:
1. In \(Cl_{2}\) (the reactant), the oxidation number of chlorine is 0.
2. In \(NaCl\) (sodium chloride), the chlorine has been reduced to -1.
3. In \(NaClO\) (sodium chlorate(I)), the chlorine has been oxidised to +1.
Key Takeaway: Because the chlorine went from 0 to both -1 and +1, this is a disproportionation reaction.
Did you know? \(NaClO\) is called sodium chlorate(I). The Roman numeral (I) tells you the oxidation state of the chlorine!
3. Chlorine and Hot Sodium Hydroxide (\(NaOH\))
If we turn up the heat and use hot, concentrated sodium hydroxide, the chlorine gets even more "excited" and pushes its oxidation state even higher.
The Equation:
\(3Cl_{2}(g) + 6NaOH(aq) \rightarrow 5NaCl(aq) + NaClO_{3}(aq) + 3H_{2}O(l)\)
Step-by-Step Breakdown of Oxidation Numbers:
1. In \(Cl_{2}\), the oxidation number is 0.
2. In \(NaCl\), the chlorine is reduced to -1.
3. In \(NaClO_{3}\) (sodium chlorate(V)), the chlorine is oxidised all the way to +5.
Memory Aid: Think of it like this—when it's cold, the chlorine stays low (+1). When it's hot, the chlorine jumps high (+5)!
Quick Review Box:
Cold \(NaOH\): Forms \(NaClO\) (Chlorine is +1)
Hot \(NaOH\): Forms \(NaClO_{3}\) (Chlorine is +5)
4. Chlorine in Water Purification
Chlorine is added to our drinking water and swimming pools to kill dangerous bacteria. This is another example of a disproportionation reaction.
The Reaction with Water:
When chlorine is added to water, the following reversible reaction occurs:
\(Cl_{2}(g) + H_{2}O(l) \rightleftharpoons HCl(aq) + HOCl(aq)\)
Why does this kill bacteria?
The reaction produces \(HOCl\) (chloric(I) acid). This acid is the "active ingredient." It can further dissociate (break apart) in water:
\(HOCl(aq) \rightleftharpoons H^{+}(aq) + ClO^{-}(aq)\)
Both \(HOCl\) and the \(ClO^{-}\) (chlorate(I) ion) are powerful oxidising agents. They act like chemical "grenades" that attack and destroy the cell walls and enzymes of bacteria, making the water safe to drink.
Oxidation Numbers in this reaction:
1. \(Cl_{2}\) starts at 0.
2. In \(HCl\), chlorine is -1 (reduced).
3. In \(HOCl\), chlorine is +1 (oxidised).
Common Mistake to Avoid: Students often forget that this reaction is reversible (shown by the \(\rightleftharpoons\) symbol). In water treatment, we use just enough chlorine to kill bacteria without making the water harmful to us!
Summary Checklist
Before you finish, make sure you can:
• Define disproportionation clearly.
• Write the equation for chlorine + cold \(NaOH\) and identify the +1 oxidation state.
• Write the equation for chlorine + hot \(NaOH\) and identify the +5 oxidation state.
• Explain how \(HOCl\) and \(ClO^{-}\) are formed in water and why they are useful for purification.
Keep practicing these equations! Writing them out three times each is a great way to make them stick in your memory. You've got this!