Tests for Nitrogen and Sulphur

Transcript

0:00Introduction

Hello everyone! Let me start with a small puzzle. Take sulphanilic acid. It has an amino group, so it clearly contains nitrogen, right? Now fuse it with a little sodium, and run the test for nitrogen. The blue colour that should show nitrogen never comes. So where did the nitrogen go? Strange, isn't it? Today we learn the colour tests for nitrogen and sulphur, and by the end, you will solve this puzzle yourself. These tests come up often in JEE Main, so let's begin.

0:33Nitrogen, part one: the cyanide complex

So, let's start with nitrogen. The chart on the right shows the tests on the sodium fusion extract. Today we use its first three branches; the halogen and phosphorus boxes are for the next lesson. In the extract, nitrogen sits as cyanide: C N with a small minus sign written high, the cyanide ion. And the extract is alkaline, remember that. Okay? Step one: boil the extract with freshly prepared iron two sulphate, F e S O four. The small four written low is a subscript; it counts the oxygen atoms. With me? That word freshly is important. So let me ask you this: why can't we use an old bottle of iron two sulphate? Because an old solution has been oxidised and hydrolysed, and it gives brown iron three hydroxide: F e, then O H in round brackets, taken three times. We need iron two ions, not oxidised ones, to take up the cyanide. Clear? Here is what happens. Six cyanide ions join one F e two plus, the iron two ion; the two plus written high is its charge. The arrow means gives, and the square brackets hold one complex ion, with a charge of four minus: six negative charges and two positive leave four negative. Its name is hexacyanoferrate two: hexa means six, cyano is cyanide, and the Roman two in brackets says the iron is in the plus two state. Got it?

2:05Nitrogen, part two: Prussian blue

Now, Prussian blue also needs iron three ions, F e three plus. But we added only iron two sulphate. So where does the iron three come from? It comes from the air, yes! While we boil, the air oxidises part of the iron two to iron three. Or we simply add iron three chloride, F e C L three, as the chart says. Okay? Next, we acidify. But why do we need an acid? Remember, the extract is alkaline. So what do iron ions do in an alkaline solution? Yes, exactly, they come down as iron hydroxides, and these hide the colour. So we add concentrated sulphuric acid, H two S O four, which dissolves them, and then the blue can be seen. Right? Now the final step. Three hexacyanoferrate two ions join four iron three ions, and give F e four, then a square bracket holding F e and C N taken six times, with the whole bracket taken three times. Check the charge: three times minus four is minus twelve, and four times plus three is plus twelve, so the solid is neutral. With me? This is Prussian blue, iron three hexacyanoferrate two. The iron outside the bracket is iron three, and the iron inside is iron two. So a Prussian blue colour or precipitate shows nitrogen: the first green box in the chart. Clear?

3:34Sulphur: two tests

Now, sulphur, the second branch of the chart. In the extract, sulphur sits as sulphide. Test one: acidify the extract with acetic acid, and then add lead acetate. Okay? Lead ions meet sulphide ions, and we get lead sulphide. So tell me, what colour is lead sulphide? Yes, it is black! S two minus, the sulphide ion, plus P b two plus, the lead ion, gives P b S, lead sulphide. The little arrow pointing down means a precipitate. So a black precipitate with lead acetate shows sulphur. Got it? Test two: add sodium nitroprusside to the extract. Sulphide gives a violet colour, which some books call purple. Right? Here is the reaction. Nitroprusside is F e, C N five, N O, in square brackets, with a charge of two minus. The sulphide adds to the N O ligand, and we get the violet ion, with a charge of four minus. Two minus and two minus make four minus, so the charge fits again. With me?

4:41Nitrogen and sulphur together

Now, the interesting case: nitrogen and sulphur in the same compound. With only a little sodium, the sodium, carbon, nitrogen and sulphur all combine into sodium thiocyanate, N a SCN, the formula after the arrow. The ion SCN minus is called thiocyanate. Okay? So, nitrogen is present. Then the nitrogen test should give Prussian blue, right? No, no, careful, that is the trap! When iron three meets thiocyanate, we get F e SCN in square brackets, with a charge of two plus. It has a blood red colour. So iron three chloride, added directly to the extract, turns it blood red: the third branch of the chart. Clear? And what about Prussian blue? It needs free cyanide, but here the nitrogen is locked up in thiocyanate. So there is no free cyanide, and no Prussian blue. Keep this in mind; it solves our puzzle soon. Got it? But what if we use excess sodium? The extra sodium breaks the thiocyanate: sodium thiocyanate plus two sodium gives sodium cyanide plus sodium sulphide. Right? Now there is free cyanide and free sulphide, so the separate nitrogen and sulphur tests both work. So the amount of sodium really matters. With me?

6:02Example 1: balancing the Prussian blue equation

Now, Example one, an easy one to warm up. Complete and balance this: sodium hexacyanoferrate two, the first formula on screen, plus iron three chloride, gives Prussian blue. Pause the video and try it first. Done? Okay, let's check. Start from the product. Prussian blue has four iron three ions outside the bracket, and three hexacyanoferrate two ions, the bracket taken three times. So we need four iron three chlorides, and three of the sodium salt. Okay? Now the leftovers. Three sodium salts carry twelve sodium ions, and four iron three chlorides carry twelve chloride ions. So what do they make? Yes, exactly, twelve sodium chloride! So three of the sodium salt, plus four iron three chlorides, give one Prussian blue, plus twelve sodium chloride. Right? Always check by counting. Sodium: twelve on each side. Chlorine: twelve and twelve. Iron: three plus four on the left, and four plus three in Prussian blue, seven each. So the equation is balanced. Clear?

7:19Example 2: which give Prussian blue? Sulphanilic acid trap

Now a slightly harder one, Example two. Which of these give Prussian blue? Aniline, benzenesulphonic acid, urea, and sulphanilic acid, each fused with a small amount of sodium. Pause the video and try it, and think of our puzzle. Done? Okay, let's check. Aniline and urea contain nitrogen, and no sulphur. So fusion gives sodium cyanide, and both give Prussian blue. The double arrow on screen means so. Okay? Benzenesulphonic acid has sulphur, but no nitrogen at all. No nitrogen means no cyanide, so no Prussian blue. Right? Now sulphanilic acid. It has nitrogen, so it gives Prussian blue, right? No, no, wait, it has sulphur too! With a small amount of sodium, nitrogen and sulphur go together into sodium thiocyanate, so there is no free cyanide. It gives blood red with iron three chloride, not Prussian blue. And that solves our puzzle: the nitrogen was hiding in thiocyanate! With me? So the answer is aniline and urea. Two out of four, and the trap was sulphanilic acid. Got it?

8:34Quick revision: Prussian blue, PbS, nitroprusside, blood red

So, quick revision. Nitrogen: boil with fresh iron two sulphate, then iron three and sulphuric acid, and look for Prussian blue. Sulphur: a black precipitate of lead sulphide with lead acetate, or a violet colour with sodium nitroprusside. Nitrogen and sulphur together, as thiocyanate, give a blood red colour with iron three chloride, and no Prussian blue. Now go and try the three practice problems on this page. All the best!