Overview

Transcript

0:00Introduction

Hello everyone! Let us start with a small puzzle. In the lab, you are handed a white salt, and its anion, the negative ion, could be any one of eight. And the first reagent you reach for is sulphuric acid. Can one acid really sort eight anions? It can, if you use it at two strengths. Today we map this whole chapter, and salt analysis questions come up often in JEE, so let's begin.

0:27One acid, two strengths, three classes

So, here is the map of this chapter, a figure from the course. Three boxes on top, and each arrow points down to the anions that box detects, each written with its charge. The first box is dilute sulphuric acid, H two S O four; the small two and four written low are subscripts, counting the atoms. We add it to the salt in the cold, then warm it gently, and three anions show up here. Okay? The second box is concentrated sulphuric acid, warmed with the salt, and four more anions show up here. Right? And the third box is for the anion that gives no gas with either acid, and that is sulphate. Three plus four plus one makes all eight. Now tell me, at the bench, which strength of acid do you try first? If you said the dilute acid, very good. You always start with the dilute acid, and the salt is warmed with concentrated acid only if the dilute acid gave nothing. With me?

1:27Dilute acid: carbonate, sulphide, nitrite

Now the first box in detail. These three are anions of weak or unstable acids, and a dilute strong acid sets those acids free. So start with carbonate. When a carbonate meets dilute acid, which gas comes off? Yes, exactly, the gas is carbon dioxide, the C O two on screen. Carbonate is written C O three with a two minus up high, a superscript that shows the charge. We see brisk effervescence of a colourless, odourless gas that turns lime water milky. Clear? Next, sulphide, the S with a two minus. It gives hydrogen sulphide, H two S, which smells of rotten eggs and turns lead acetate paper black. Got it? And nitrite, N O two with a single minus. It gives brown fumes, and the gas turns starch iodide paper blue. Right?

2:20Concentrated acid: the halides and nitrate

Now the second box, with warm concentrated acid. First, chloride, the C l with a minus sign up high. It gives hydrogen chloride, H C l, colourless and pungent, and with ammonia, N H three, it gives white fumes. Okay? Bromide, B r with a minus, gives reddish brown vapour of bromine, B r two. And iodide, the I with a minus, gives violet vapour of iodine, the I two on screen. So why does chloride give a colourless gas, while bromide and iodide give coloured halogens? It is all about reducing power. It rises from H C l, to H B r, to H I, and the less than signs on screen show that order. So concentrated sulphuric acid does not oxidise H C l at all, but it oxidises H B r partly to bromine, and H I almost completely to iodine. With me? Last in this box, nitrate, N O three with a minus. It gives brown fumes of N O two, and the fumes grow deeper when copper turnings are added. Clear?

3:24The brown fumes trap

Now let me show you a classic trap. You add acid to a salt, and brown fumes come out. Brown fumes, so the salt is a nitrate, right? No, and that is exactly the trap, because first you must ask which acid gave the fumes. Brown fumes with dilute acid mean nitrite, from the first box, and not nitrate. Okay? And even with concentrated acid, brown can mean two anions: nitrate, or bromide, whose bromine vapour, B r two, is reddish brown. Right? So add copper turnings. With a nitrate, the brown fumes become denser and the solution turns blue, and that settles the question. Got it?

4:06Sulphate: no gas, so test in solution

Now the third box, sulphate, S O four with a two minus. It gives no gas with dilute or with concentrated acid, so we test it in solution. Okay? To the solution, acidified with dilute hydrochloric acid, H C l, add barium chloride, B a C l two, and a white precipitate of barium sulphate, B a S O four, forms. But how can we be sure that this white solid is barium sulphate? We check that it does not dissolve in acid. Barium sulphate is insoluble in dilute hydrochloric acid and in dilute nitric acid, HNO three. Clear? And there is a second test. Lead acetate, in acetic acid, gives white lead sulphate, P b S O four, and this precipitate dissolves in hot ammonium acetate. With me?

4:56Example 1: brisk effervescence turns lime water milky

Now example one, from the next concept in this chapter, on anions detected with dilute acid. There is a white salt that gives brisk effervescence with dilute sulphuric acid in the cold. The gas has no smell, and it turns lime water milky. Which anion is present? Pause the video and try it first. Done? Okay, let's check. There is a gas with dilute acid, so we are in the first box: carbonate, sulphide or nitrite. Clear? The gas has no smell, so it is not hydrogen sulphide, and sulphide is out. That leaves carbonate or nitrite. Which clue decides between them? The lime water decides it, so if you said that, very good. Of the gases in the first box, the one that turns lime water milky is carbon dioxide, so the anion is carbonate. Right? And here is why it turns milky. Lime water is calcium hydroxide, written C a, then O H in brackets with a small two, for two O H groups. With carbon dioxide it gives calcium carbonate, C a C O three, plus water, and the arrow means gives. Calcium carbonate is insoluble, and that is the milkiness.

6:12Example 2: colourless gas with conc H2SO4, then MnO2

Got it? Now a harder one, example two, from the concept on anions detected with concentrated acid. There is a salt that is unaffected by dilute acid. But with concentrated acid it gives a colourless gas, which turns moist blue litmus red and gives white fumes with ammonia. On adding manganese dioxide, M n O two, a greenish yellow gas appears. Identify the two gases. Here is a hint: follow the map, box by box. Pause the video and try it first. Done? Okay, let's check. Nothing happens with dilute acid, so the first box is out: no carbonate, sulphide or nitrite. Clear? So there is no gas with dilute acid. Then it must be sulphate, the anion that gives no gas, right? No, and that is the trap. Sulphate gives no gas even with concentrated acid, but this salt does give one. So we are in the second box. Okay? In the second box, bromide, iodide and nitrate all give coloured vapours. This gas is colourless, it turns blue litmus red, so it is acidic, and it gives white fumes with ammonia. So the gas is hydrogen chloride, H C l. Got it? Now look at what the manganese dioxide does. It oxidises H C l to chlorine, C l two, which is the greenish yellow gas. Right? So the two gases are hydrogen chloride, and then chlorine, and the salt contains chloride. See how the map did all the work? With me?

7:50Quick revision: dilute acid, conc acid and sulphate groups

So, quick revision. Dilute sulphuric acid finds carbonate, sulphide and nitrite. Warm concentrated acid finds chloride, bromide, iodide and nitrate, but only if the dilute acid gave nothing. And sulphate gives no gas with either acid, so it is tested in solution, with barium chloride. Now go through the concepts that follow, and try their practice problems. All the best!