The Principle of Group Separation

Transcript

0:00Introduction

Hello everyone! Here is a lab puzzle. Copper sulphide and zinc sulphide both hardly dissolve in water. Yet bubble hydrogen sulphide through copper and zinc ions in dilute hydrochloric acid, and only copper sulphide comes down. Same gas, same flask. So how does the gas choose? Keep that question in mind, because by the end of this video we will settle it with real numbers. The idea behind it runs the whole group table, and it comes up often in JEE practical chemistry. So, let's begin.

0:33When does a salt precipitate?

So, first the rule behind every group. The ions of a salt are sitting in solution. When does the salt come out as a solid, a precipitate? It precipitates when the ionic product, capital Q, from the ion concentrations present, is greater than the solubility product, K s p, where s p is a subscript. While Q stays below K s p, the ions stay dissolved. With me? Now the clever part. Each group reagent supplies an anion, like sulphide, hydroxide or carbonate, and the common-ion effect controls how much. So only the cations of that group exceed their K s p. That is the whole principle. Clear?

1:17The group reagents, one after another

Here is the whole scheme, from the course figure: the reagents on the left, the cations they catch on the right. We add them in this fixed order; the Roman numerals are group numbers. After each one we filter, and the filtrate goes on to the next group. Okay? Look at lead, P b two plus; the raised two plus is its charge. It appears in group one as white lead chloride, and again in group two as black lead sulphide. Why two groups? Because lead chloride, P b C l two, with a subscript two, is only sparingly soluble, not truly insoluble. So group one catches only part of the lead, and hydrogen sulphide brings the rest down in group two. Got it? And group zero, the ammonium ion, N H four plus, is tested on the original salt: warmed with sodium hydroxide, it gives ammonia gas, which turns moist red litmus blue. We test it first, because the later group reagents add ammonium salts themselves. Right?

2:20Groups II and IV: one gas, two sulphide levels

Now the key gas, hydrogen sulphide. In water it ionises: one H two S gives two H plus ions and one sulphide ion, S two minus; the double half arrows mean an equilibrium. Okay? In group two, the gas goes into dilute hydrochloric acid, H C l on screen. The acid pours in H plus, a product of this equilibrium. So what happens to the equilibrium? Yes, exactly, it is pushed back, because H plus is the common ion. So the sulphide concentration becomes very small; the square brackets mean concentration. Right? Such a small sulphide level exceeds only the very low K s p of copper sulphide and lead sulphide, C u S and P b S on screen. So only they precipitate in group two. Got it? Now flip it. In group four, the same gas goes into ammonium hydroxide, N H four O H, an alkaline solution. What happens to the ionisation now? With no extra H plus to hold it back, H two S ionises freely, and the sulphide level is large. So zinc sulphide and nickel sulphide, with higher K s p values, now precipitate. That is why group two needs the acid first. With me?

3:43Group III: keep the hydroxide low

Next, group three. First we boil off the leftover hydrogen sulphide; once ammonia goes in, it would throw down the group four sulphides too early. Then a drop of concentrated nitric acid, HNO three, oxidises iron two plus to iron three plus; conc means concentrated. Okay? The reagents are ammonium chloride first, then ammonium hydroxide, which ionises into ammonium ions and hydroxide ions, O H minus. The ammonium chloride supplies the common ion, so this ionisation is pushed back. Right? So the hydroxide level stays small, and only the very insoluble iron three hydroxide and aluminium hydroxide precipitate; the O H in round brackets is taken three times. Zinc, nickel and magnesium stay in solution. Clear? Now a trap. Ammonium sulphate also gives plenty of ammonium ions. So could it replace ammonium chloride here? It cannot, and that is the trap: ammonium sulphate, N H four in brackets, taken twice, with S O four, brings in sulphate. It would precipitate barium, and partly calcium, as sulphates in group three, the wrong group. Got it?

4:59Groups V and VI: carbonate, then phosphate

Group five uses ammonium carbonate, with ammonium chloride and ammonium hydroxide, and barium and calcium carbonates precipitate. But magnesium is not caught here. Why not? It is the common-ion effect again: the ammonium salts keep the carbonate ion level, C O three with a two minus charge, too low for magnesium carbonate. Okay? So magnesium waits for group six: disodium hydrogen phosphate, N a two HPO four, with ammonium hydroxide, which precipitates magnesium ammonium phosphate. The table is complete. With me?

5:36Example 1: group reagents for Cu2+, Al3+, Zn2+ and Ca2+

Alright, example one. Name the group reagent and the precipitate for copper two plus, aluminium three plus, zinc two plus and calcium two plus. Pause the video and try it first. Done? Okay, let's check. Copper is group two: hydrogen sulphide in dilute hydrochloric acid, giving copper sulphide. Aluminium is group three: ammonium chloride plus ammonium hydroxide, giving aluminium hydroxide. Right? Now zinc. Many students put it in group two because it forms a sulphide, and that is the trap. Its higher K s p needs group four: hydrogen sulphide in ammonium hydroxide, giving zinc sulphide. And calcium is group five: ammonium carbonate, giving calcium carbonate. Clear?

6:27Example 2: which sulphide precipitates, CuS or ZnS

Now our puzzle, in numbers. The solution is zero point three molar in H plus, saturated with hydrogen sulphide at zero point one molar; capital M means moles per litre. K a one times K a two is one point zero times ten to the power minus twenty two. Find the sulphide concentration, then say which sulphide precipitates when copper and zinc ions are each zero point zero one molar. Pause the video and try it first. Done? Okay, let's check. For H two S giving two H plus and S two minus, the constant is K a one times K a two, with a one and a two as subscripts. So the sulphide concentration is K a one K a two times the H two S concentration, over the H plus concentration squared. Okay? Many students divide by zero point three here. Is that right? Not quite, because H plus is squared: two H plus ions form. So one point zero times ten to the power minus twenty two, times zero point one, over zero point three squared, which is zero point zero nine, gives one point one times ten to the power minus twenty two moles per litre. Clear? Now part two. Each metal ion is zero point zero one molar, so Q is zero point zero one times that sulphide level, which is one point one times ten to the power minus twenty four for both. Which K s p does it cross? The K s p of copper sulphide is six point three times ten to the power minus thirty six. Our Q is far bigger; the double greater-than sign means much greater than. So copper sulphide precipitates. Right? The K s p of zinc sulphide is one point six times ten to the power minus twenty four, and our Q is just below it; this sign means less than. So zinc stays dissolved. And that solves our puzzle: in acid, the same gas catches copper but not zinc.

8:39Quick revision: Q vs Ksp and the common-ion effect

So, quick revision. First, a salt precipitates only when its ionic product exceeds its solubility product. Second, the common-ion effect sets each reagent's anion level. Third, acid keeps sulphide low in group two, and ammonium chloride keeps hydroxide low in group three. Now try the practice problems on this page. All the best!