Chlorine, Bleaching Powder and Hydrogen Chloride

Transcript

0:00Introduction

Hello everyone! Let us start with a small puzzle. Drop a dry, coloured cloth into a jar of perfectly dry chlorine. Wait, and nothing happens to the colour. Now moisten the cloth with a little water, and the colour fades away, for good! So why does chlorine need water to bleach? This topic comes up often in JEE, so let's find out.

0:25Preparing chlorine, Cl₂

First, how do we make chlorine, C l two? Add hydrochloric acid to manganese dioxide: M n O two plus four H C l gives M n C l two, plus C l two, plus two H two O, water. The small numbers below count atoms, and the arrow means gives. Okay? Potassium permanganate, K M n O four, works too. Two K M n O four plus sixteen H C l gives two K C l, two M n C l two, eight water, and five C l two. Right? In the Deacon process, air oxidises H C l: four H C l plus O two gives two C l two plus two water. Above the arrow are the conditions: the catalyst C u C l two, and seven hundred and twenty three kelvin, with a capital K, okay? And the electrolysis of brine gives chlorine at the anode, the positive electrode. Got it?

1:14Chlorine with alkali and slaked lime

Now pass chlorine into cold, dilute sodium hydroxide: two N a O H plus C l two gives N a C l, plus N a O C l, sodium hypochlorite, plus water. With me? Make the alkali hot and concentrated, and the product changes: six N a O H plus three C l two gives five N a C l, plus N a C l O three, sodium chlorate, plus three water. So cold gives hypochlorite, hot gives chlorate. Okay? Next, dry slaked lime: C a, then O H in brackets, two; the two outside the brackets doubles the O H group. Two of these plus two C l two give calcium hypochlorite, that is C a, then O C l in brackets, two, plus calcium chloride, C a C l two, plus two water. Right? That mixture is bleaching powder, with roughly this long formula; the wavy equals sign means approximately, and the centre dots join its parts. We usually write C a O C l two. Clear?

2:08Chlorine with ammonia, and as an oxidising agent

With ammonia, it depends on which is in excess: excess N H three gives ammonium chloride, N H four C l, and nitrogen; excess chlorine gives N C l three and hydrogen chloride. Chlorine also oxidises. In acidified ferrous sulphate, iron is F e two plus, the raised two plus being its charge. So what does chlorine turn it into? Yes, exactly, into iron three plus! Two F e S O four plus H two S O four plus C l two gives ferric sulphate, F e two, then S O four in brackets, three, where iron is plus three, along with two H C l molecules. Got it? The same way, sulphite becomes sulphate: N a two S O three plus C l two plus water gives N a two S O four plus two H C l, okay? And S O two plus two water plus C l two gives H two S O four, sulphuric acid, plus two H C l again. Right? With excess chlorine, even iodine is oxidised: I two plus six water plus five C l two gives two HIO three, iodic acid, plus ten H C l molecules. Clear?

3:13Why chlorine bleaches only when moist

Now, back to our puzzle: dry chlorine does nothing to a dry cloth. So what is the water doing? First, chlorine reacts with the water: C l two plus H two O gives H C l plus HOC l, hypochlorous acid. With me? Then HOC l gives H C l and O in square brackets, which means nascent oxygen, freshly formed. It oxidises the dye, permanently. So, puzzle solved! No water means no HOC l, no nascent oxygen, and no bleaching. Right?

3:47Hydrogen chloride, HCl

Now hydrogen chloride. Heat sodium chloride with sulphuric acid at four hundred and twenty kelvin, written above the arrow, to get N a HSO four and hydrogen chloride gas. The gas fumes in moist air, as it dissolves in water vapour to form tiny droplets. Okay? H C l is an acid, so does pure, anhydrous H C l, with no water at all, conduct electricity? No, it does not, because anhydrous H C l is covalent; there are no ions to carry the current. Got it? But in water, it ionises completely: H C l plus H two O gives H three O plus, the hydronium ion, plus C l minus. Now there are ions, so it conducts well. With me?

4:32Aqua regia, HBr and HI

Now mix three parts H C l with one part nitric acid, HNO three; the colon shows the ratio. This is aqua regia, and it dissolves even gold and platinum, though neither acid alone dissolves gold. Clear? Gold, A u, plus four H plus, plus nitrate, N O three minus, plus four chloride ions, gives A u C l four minus, plus N O, plus two water. Nitric acid oxidises the gold, and the chloride ions hold it in this complex ion, pulling the reaction forward. Right? Hydrogen bromide and hydrogen iodide, H B r and H I, come from bromides and iodides with phosphoric acid, H three P O four, or by hydrolysing P B r three and P I three. But for H C l we used sulphuric acid, so why not here? Because sulphuric acid oxidises them, the H B r and H I, so we use phosphoric acid instead. Okay?

5:26Example 1: oxidation numbers of Cl in bleaching powder

Now example one. What is the oxidation number of chlorine in bleaching powder, C a O C l two? Pause the video and try it first. Done? Okay, let's check. Call the oxidation number of chlorine x. Calcium is plus two, oxygen minus two, and the two chlorines give two times x. So two times x equals zero, and the answer is zero, right? Oh no, careful, that is the trap! Bleaching powder is a mixed salt, C a, then O C l in brackets, then C l, so its two chlorine atoms are not the same. Clear? One chlorine is in hypochlorite, O C l minus, so it is plus one. The other is chloride, C l minus, so it is minus one. So the answer is plus one and minus one; zero was only their average. Got it?

6:19Example 2: available chlorine in bleaching powder

Now example two, a numerical one. What is the maximum percentage of available chlorine in bleaching powder, taken as C a O C l two? Pause the video and try it first. Done? Okay, let's check. With sulphuric acid, bleaching powder gives off its chlorine: C a O C l two plus H two S O four gives C a S O four, plus C l two, plus water. So one mole gives one mole of C l two. With me? Now capital M, the molar mass: forty point one, plus sixteen point zero, plus seventy one point zero for two chlorines, gives about one hundred and twenty seven grams per mole. And it releases seventy one point zero grams of C l two. Okay? So the percentage is seventy one point zero over one hundred and twenty seven, times one hundred, which gives fifty five point nine percent. But notice the word maximum. Does a real bottle give that much? No, it does not; commercial samples have only about thirty five percent. Fifty five point nine is the limit for pure C a O C l two. Right?

7:27Quick revision: Cl2 preparation, alkali reactions, HCl

So, quick revision. Chlorine comes from M n O two or K M n O four with H C l, or by the Deacon process. Cold alkali gives hypochlorite, hot alkali gives chlorate, and dry slaked lime gives bleaching powder. Bleaching needs moisture, and dry H C l is covalent. Now go and try the practice problems on this page. All the best!