Iron and Aluminium

Transcript

0:00Introduction

Hello everyone! Let me start with a little magic from the salt analysis lab. Take a white precipitate, dissolve it in dilute hydrochloric acid, add blue litmus, and then add ammonium hydroxide drop by drop. At the end, the liquid is colourless, and yet a blue mass floats in it. So where did the blue dye go? By the end of this lesson, you will explain it in one line. Today we meet iron and aluminium, the two group three cations in the JEE syllabus. Their tests are a common topic in JEE practical chemistry. So, let's begin.

0:36Group III: two hydroxides

Let us first recall the group three reagent; the heading writes three in Roman numerals. It is ammonium chloride, N H four C l, and then ammonium hydroxide, N H four O H; the small four below the H is a subscript. Okay? If iron three plus is present, written F e with a raised three plus, its charge, we get a reddish-brown precipitate of ferric hydroxide. Its O H group sits in round brackets, taken three times. Right? And aluminium three plus gives a white gelatinous, that is jelly-like, precipitate of aluminium hydroxide. Clear? Now let me ask you this. Both precipitates sit in one test tube, and we warm them with excess sodium hydroxide, N a O H on screen. What happens to each one? Yes, exactly, they behave differently, and that is the key. The white aluminium hydroxide dissolves, but the reddish-brown ferric hydroxide stays, and is filtered off. Look, that is how the two are separated. With me?

1:39Iron(III): Prussian blue and blood red

Now we confirm iron; in the heading, the Roman three in round brackets is its oxidation state. We dissolve the reddish-brown precipitate in dilute hydrochloric acid, H C l, add potassium ferrocyanide, and get Prussian blue. Okay? Here is the equation, and the arrow means gives. Ferrocyanide is iron with six cyanide groups, C N, inside square brackets, and the raised four minus is the charge of the whole ion. Four iron three plus ions and three ferrocyanide ions give Prussian blue: four iron atoms in front, and three ferrocyanide units. Right? For the second test, we add potassium thiocyanate, and we get a blood-red colour. Clear? One iron three plus ion joins one thiocyanate ion, SCN with a raised minus, to form this complex ion in square brackets, with a charge of two plus. That complex gives the blood-red colour. Got it?

2:37Aluminium: it dissolves, then comes back

Let us turn to aluminium. Aluminium hydroxide and sodium hydroxide give sodium aluminate: N a, then aluminium with four O H groups in square brackets. Aluminium hydroxide dissolves in dilute acid and also in excess alkali, so we call it amphoteric. Okay? Now the aluminium is hidden in the solution as aluminate. Let me ask you this: how can we bring the white precipitate back? Here is the trick: we boil the solution with ammonium chloride, and the white aluminium hydroxide is reprecipitated, which means it comes down again. Got it? The reason is simple. The ammonium ion, N H four plus, removes hydroxide ions, O H minus, as ammonia and water. So the p H falls, and aluminium hydroxide comes back. With me?

3:28The lake test

Now the special test for aluminium, the lake test, which is our opening magic. We dissolve the white precipitate in dilute hydrochloric acid, add blue litmus, and then ammonium hydroxide, drop by drop. Okay? And here is what we see: a blue mass floating in a colourless liquid. So tell me now, where did the blue dye go? Yes, exactly, the dye went onto the aluminium hydroxide itself. Freshly precipitated aluminium hydroxide adsorbs the blue litmus completely; adsorbs means the dye sticks to its surface. So the mass is blue, the liquid is colourless, and our mystery is solved. Clear?

4:09Iron(II) for comparison: ferro or ferri?

Now, for comparison, iron two plus, the ferrous ion; the Roman two in the heading is its oxidation state. With sodium hydroxide, it gives a dirty green precipitate of ferrous hydroxide, and air slowly oxidises it to brown ferric hydroxide. Okay? Here many students slip. Both iron ions can give a deep blue colour, but with different reagents. For iron two plus, is it potassium ferrocyanide, or potassium ferricyanide? That is correct if you said potassium ferricyanide: K three, then iron with six cyanide groups in square brackets. See, ferrous goes with ferricyanide; the names cross over. Right? And iron three plus, the ferric ion, goes with potassium ferrocyanide, K four, the Prussian blue test we just saw. Ferric with ferro, ferrous with ferri; do not mix them up in the exam. Clear? One more difference: iron two plus gives no colour with thiocyanate. So a blood-red colour points to iron three plus. Got it?

5:11Example 1: reddish-brown precipitate, blood red with KSCN

Alright, example one, an easy one. We get a reddish-brown precipitate in group three. It dissolves in hydrochloric acid, and the solution turns blood red with KSCN, potassium thiocyanate. Identify the cation. Pause the video and try it first. Done? Okay, let's check. In our syllabus, group three gives only two precipitates, and the reddish-brown one is ferric hydroxide, not the white aluminium hydroxide. Right? Next, the blood-red colour with thiocyanate is the iron thiocyanate complex, with a charge of two plus. Clear? So the cation is iron three plus. Iron two plus would give no colour with thiocyanate, so the blood red settles it. Got it?

5:59Example 2: solubility of Al(OH)3 at pH 9 from Ksp

Now example two, with numbers. Find the solubility of aluminium hydroxide in a solution of p H nine, given that its K s p, the solubility product, is one point three times ten to the power minus thirty three; the small s p below the K is just a label. Pause the video and try it first. Done? Okay, let's check. Each aluminium hydroxide gives one aluminium ion and three hydroxide ions. So K s p equals the concentration of aluminium three plus, times the concentration of hydroxide ion, cubed; square brackets mean concentration, and the raised three means cubed. And the solubility is simply the aluminium ion concentration. With me? Next comes the hydroxide concentration. The p H is nine, so it must be ten to the power minus nine, right? That is exactly the trap, because ten to the minus nine is the hydrogen ion concentration. For hydroxide we need p O H, fourteen minus nine, which gives five. So the hydroxide concentration is ten to the power minus five moles per litre; the capital M stands for moles per litre. Okay? Now cube it: ten to the minus five, cubed, is ten to the minus fifteen. So one point three times ten to the minus thirty three, over ten to the minus fifteen, gives one point three times ten to the power minus eighteen moles per litre. That is the solubility, an extremely small number. Very good if you got that!

7:36Quick revision: Prussian blue, blood red and the lake test

So, quick revision. First, in group three, iron gives reddish-brown ferric hydroxide, and aluminium gives white gelatinous aluminium hydroxide. Second, iron three plus gives Prussian blue with ferrocyanide, and a blood-red colour with thiocyanate. Third, aluminium hydroxide dissolves in excess sodium hydroxide, and the lake test gives a floating blue mass. Now go and try the four practice problems on this page. All the best!