Part C: Concept Checks

Transcript

0:00Introduction

Hello everyone! Let me start with a question from the lab. In an acid and base titration, we add a few drops of an indicator, and its colour change tells us when to stop. But in a potassium permanganate titration, we add no indicator at all. So how does the flask tell us when to stop? Keep that question in mind, because Example one will answer it. This is Part C of the unit problem bank on practical chemistry: short concept checks, where every answer is a reason. Questions like these come up often in JEE, so let's begin.

0:37Check 1: why fuse the compound with sodium?

Check number one is about Lassaigne's test. The question on screen asks why a compound must be fused with sodium before we test its nitrogen. Remember, our lab tests are ionic tests. So why can we not test the nitrogen directly? Yes, exactly, because in the compound, nitrogen, the N on screen, is held by covalent bonds. It is not present as an ion, so it gives no ionic test. Okay? So we fuse the compound with sodium metal. On screen, N a is sodium and C is carbon. Sodium plus carbon plus nitrogen gives N a C N, sodium cyanide, and the arrow means gives. Now the nitrogen sits in an ionic cyanide. Right? The sodium fusion extract is boiled with freshly prepared ferrous sulphate solution, F e S O four, with the four written low, and then acidified with dilute sulphuric acid, H two S O four. The cyanide is detected as Prussian blue. So fusion turns covalent nitrogen into ionic cyanide, and only then can the test see it. Clear? Now a small trap. Hydrazine contains nitrogen, but no carbon at all. Will Lassaigne's test show its nitrogen? It will not, and that is the trap. Look at the fusion equation again: sodium needs carbon as well as nitrogen to make the cyanide. With no carbon, no sodium cyanide forms, so the test fails. Got it?

2:09Check 2: phenolphthalein or methyl orange?

Check number two is about indicators. The graph shows p H against the volume of zero point one molar sodium hydroxide added, in millilitres; capital M means molar. The solid curve, labelled H C l, is for twenty five millilitres of hydrochloric acid, a strong acid. The dashed curve, labelled C H three COOH, is for acetic acid, a weak acid. The dotted line marks the equivalence point, and the black dot, labelled p H equals p K a, we will not need today. With me? Now the question on screen asks why we use phenolphthalein, not methyl orange, when oxalic acid is titrated against sodium hydroxide. Oxalic acid is a weak acid, and N a O H, sodium hydroxide, is a strong base. So it is the same case as the acetic acid on the dashed curve. Okay? So where does the equivalence point of a weak acid with a strong base lie? In the acidic region, or in the alkaline region? It lies in the alkaline region, that is correct. Look at the dashed curve: its steep part, near the equivalence line, sits high up on the graph. Right? The upper shaded band is phenolphthalein, with a range of p H eight point three to ten point zero. It changes colour within the steep part of the curve, and that is exactly what an indicator must do. Clear? Methyl orange, the lower band, from three point one to four point four, lies outside this steep part. The dashed curve crosses it early, long before the equivalence point, so its colour change would come too soon. That is why we choose phenolphthalein. Got it?

3:49Check 3: which acid for permanganate titrations?

Check number three is about permanganate titrations, where the flask must be acidic. The question asks why we use sulphuric acid, and neither hydrochloric nor nitric acid. Many students say that any strong acid will do. Is that right? No, it is not, and that is the trap. Permanganate oxidises hydrochloric acid, H C l on screen, to chlorine. So some permanganate is used up on the acid itself. Okay? Nitric acid, HNO three, is itself an oxidising agent, so it cannot be used either. Right? But dilute sulphuric acid, H two S O four, is neither oxidised nor oxidising. It simply makes the solution acidic. That is why we use it. With me?

4:36Example 1: why KMnO4 titrations need no indicator

Now the two examples. Example one answers our opening question: why is no indicator added in permanganate titrations? Here is a hint: think about the colours of the ions. Pause the video and try it first. Done? Okay, let's check. The permanganate ion, M n O four minus, with the minus written high as its charge, is intensely purple. Clear? Now the purple drops fall into the sample in the flask. So what happens to the purple colour of permanganate there? Yes, exactly, it disappears, because the permanganate is reduced to manganese two plus, M n two plus on screen. And that ion is almost colourless. Right? So while the sample lasts, every drop loses its colour. But once the reaction is complete, the very first drop in excess has nothing left to react with, so it gives a visible pink colour. Okay? So permanganate is a self-indicator, its own indicator. The end point is from colourless to a permanent pale pink, and the arrow means changes to. And that answers our opening question: the flask tells us itself.

5:48Example 2: common-ion effect in group II and III

Got it? Now Example two, a step harder, from salt analysis. The Roman numerals on screen are group numbers. In group two, we use hydrochloric acid, H C l, and in group three, ammonium chloride, N H four C l. What single principle explains both? Pause the video and try it first. Done? Okay, let's check, starting from the rule. Any salt precipitates only when its ionic product, capital Q, is greater than its solubility product, K s p, with s p written low. Clear? In group two, hydrogen sulphide is passed in dilute hydrochloric acid. It ionises: H two S gives two H plus ions and one sulphide ion, S two minus, and the double half arrows mean an equilibrium. Now the acid adds plenty of H plus. What does that do to this equilibrium? Yes, exactly, it pushes it back, because H plus is the common ion. So the sulphide concentration falls; the square brackets mean concentration. At such a small sulphide level, the ionic product exceeds K s p only for the least soluble sulphides, those of group two, such as copper sulphide and lead sulphide, C u S and P b S on screen. With me? Now group three. Ammonium hydroxide, N H four O H, ionises into ammonium ions, N H four plus, and hydroxide ions, O H minus. We add ammonium chloride first. Now careful: which ion of ammonium chloride is the common ion here, the ammonium or the chloride? It is the ammonium ion, N H four plus, not the chloride, because ammonium appears in this equilibrium. So the equilibrium is pushed back, and the hydroxide level stays small. Only the least soluble hydroxides, those of group three, precipitate, such as iron three hydroxide and aluminium hydroxide; the O H in round brackets is taken three times. Okay? So one single principle explains both: the common-ion effect. The H plus lowers the sulphide level, the ammonium ion lowers the hydroxide level, and only the least soluble sulphides or hydroxides exceed their K s p. Got it?

8:03Quick revision: sodium fusion, indicators, KMnO4, common ion

So, quick revision. Sodium fusion turns covalent nitrogen into ionic cyanide, which we detect as Prussian blue. For a weak acid against a strong base, use phenolphthalein. Permanganate titrations use dilute sulphuric acid and no indicator, because permanganate is its own indicator. And the reagents of groups two and three work through the common-ion effect. Now go and try the practice problems on this page. All the best!