Iron and Its Compounds

Transcript

0:00Introduction

Hello everyone! Let me start with a small puzzle. Put an iron nail in dilute hydrochloric acid, and it dissolves, giving off hydrogen gas. Now put the same nail in concentrated nitric acid instead. It does not dissolve at all! Concentrated acid, and no reaction? How can that be? Hold on to that; we will answer it soon. Today we study iron and its compounds, from acids and oxides to the colour tests with iron ions. These come up often in JEE, so let's begin.

0:35Iron with acids

So, first, iron with acids. In dilute hydrochloric acid, H C l, or dilute sulphuric acid, H two S O four, iron dissolves. F e, plus two H plus from the acid, gives F e two plus, plus H two, which is hydrogen gas. The arrow means gives, the small numbers written low count the atoms, and the small raised signs, like two plus, are the charges on the ions. Okay? Now, our puzzle. In concentrated nitric acid, HNO three, iron does not dissolve; we say it becomes passive. So let me ask you this: what could stop the acid from reaching the iron? Yes, exactly, something sitting on the surface! The acid forms a thin, protective film of oxide on the iron, and this film shields the metal underneath. That is passivity, and our puzzle is solved.

1:28Green vitriol and ferric chloride

Clear? Now, two compounds of iron. First, green vitriol, F e S O four, dot, seven H two O, where the dot means seven water molecules come with each F e S O four. On heating, it first loses its water. With me? Then, on strong heating, the anhydrous sulphate breaks down. Two F e S O four give F e two O three, plus S O two, plus S O three. So we get iron oxide, sulphur dioxide and sulphur trioxide. Okay? Next, ferric chloride. Its hydrate, F e C l three, dot, six H two O, holds six water molecules. So, to make anhydrous F e C l three, why not just heat the hydrate and drive the water off? Oh no, that does not work! On heating, the hydrate is hydrolysed: the water reacts with the salt itself, and you never get anhydrous F e C l three. Got it? So how is it made? We heat iron in a stream of dry chlorine. Two F e, plus three C l two, give two F e C l three. No water anywhere, so no hydrolysis. Right? And in water, F e C l three is hydrolysed, so its solution is acidic. It also slowly deposits brown hydrated ferric oxide. Clear?

2:48Oxides of iron; ferrous and ferric

Now the oxides of iron. F e O is black and basic. F e two O three is red-brown. And F e three O four is black and magnetic; keep this one in mind, because it comes back in Example one. Okay? And rust is F e two O three with water attached. The letter x, in front of H two O, means the number of water molecules can vary. Right? Now, ferrous means iron two plus, and ferric means iron three plus. Let me ask you this: which of these two ions polarises an anion more? Yes, exactly, iron three plus! It is smaller and more highly charged, so it polarises more. With me? So ferrous compounds are more ionic, more basic, less volatile, and less easily hydrolysed than ferric ones. Learn this with its reason. Got it?

3:42Tests for iron ions

Now, the tests for iron ions. Add thiocyanate, SCN minus, to iron three plus, and you get a blood-red complex. The square brackets hold the complex ion: one iron, one thiocyanate in round brackets, and five waters, with an overall charge of two plus. Okay? Now add fluoride ions, and the blood-red colour vanishes! Why does that happen? Fluoride forms a more stable complex, F e F six, three minus, and this one is colourless. So the red complex is removed, and the colour goes. Clear? Next, iron three plus with K four F e C N six, potassium ferrocyanide, gives Prussian blue. Its formula is F e four, then F e C N six taken three times; the round brackets group the C N, six of them. Right? And iron two plus with K three F e C N six, potassium ferricyanide, gives Turnbull's blue. Students often mix up these two, so match them carefully: iron three plus with K four, and iron two plus with K three. With me?

4:48Brown ring test for nitrate

Iron two plus is also the reagent in the brown ring test for nitrate. First, N O three minus, plus three F e two plus, plus four H plus, give N O, plus three F e three plus, plus two water. So nitrate is reduced to N O, nitric oxide, and iron two plus is oxidised. Okay? Then the N O joins the hydrated iron two plus ion. F e, H two O six, two plus, plus N O, gives F e, H two O five, N O, two plus, plus water. One water is swapped for N O, and this complex gives the brown ring. Right?

5:26Example 1: oxidation states of iron in Fe3O4

Now, Example one, an easy one. What are the oxidation states of iron in F e three O four? Pause the video and try it first. Done? Okay, let's check. Four oxygens give minus eight, so the three irons share plus eight. Call each one x, then three times x equals eight, so x is eight over three. But wait, can a single iron atom really be at plus eight over three? No, it cannot, and that is the trap! F e three O four is really F e O, dot, F e two O three. The raised Roman numerals two and three give each iron's oxidation state, so one iron is at plus two, and two irons are at plus three. Clear? Let us check the charges. Plus two, plus two times plus three, gives plus eight, which balances the four oxygens at minus two each. Right? So the answer is plus two and plus three. Plus eight over three is only the average. Got it?

6:28Example 2: iron with cold, very dilute nitric acid

Now, Example two, a little harder. Concentrated nitric acid did nothing to iron, remember? Now write the equation for the action of cold, very dilute nitric acid on iron. Pause the video and try it first. Done? Okay, let's check. Dilute hydrochloric acid gave hydrogen gas. So dilute nitric acid should give hydrogen too, right? Oh no, careful, that is a classic trap! Here the nitrate itself is reduced, and it goes all the way down to ammonium, N H four plus. Okay? Now count electrons. Nitrogen goes from plus five to minus three, so it gains eight electrons; e minus stands for an electron. Iron goes from zero to plus two, so it loses two. So tell me, how many iron atoms do we need for each ammonium ion? Yes, we need four, because four times two is eight, and that matches the eight electrons one nitrogen takes. With me? So here is the equation. Four F e, plus ten HNO three, give four F e, N O three taken twice, which is ferrous nitrate, plus N H four N O three, ammonium nitrate, plus three water. Count the nitrogens: eight in the ferrous nitrate, and two in ammonium nitrate, ten in all.

7:51Quick revision: passivity, anhydrous FeCl3, Fe3+ tests

Clear? So, quick revision. First, dilute hydrochloric or sulphuric acid gives iron two plus, but concentrated nitric acid makes iron passive. Second, anhydrous F e C l three comes from iron and dry chlorine; heating the hydrate only hydrolyses it. And third, iron three plus gives blood-red with thiocyanate, and Prussian blue with K four F e C N six. Now go and try the two practice problems on this page. All the best!