Potassium Permanganate

Transcript

0:00Introduction

Hello everyone! Let me start with a puzzle. Potassium permanganate has a molar mass of one hundred and fifty eight. One book says its equivalent mass is thirty one point six. Another says fifty two point seven, and a third says one hundred and fifty eight. So, which book is wrong? Hold on to your answer; we will settle it in Example two. Today we see how potassium permanganate is made, what its ion looks like, and how it oxidises in acid, neutral and alkaline solution. It comes up often in JEE, so let's begin.

0:37From pyrolusite, MnO₂, to permanganate

So, how do we make it? We start from the ore pyrolusite, manganese dioxide, M n O two, and fuse it with KOH, potassium hydroxide, in air. Two M n O two, plus four KOH, plus O two, give two K two M n O four, plus two water. The arrow means gives, and the small numbers written low count the atoms. Now tell me, manganese is plus four in M n O two. What is it in K two M n O four? Yes, exactly, it goes from plus four to plus six! Two potassiums give plus two, four oxygens give minus eight, so manganese must be plus six. This compound, K two M n O four, is potassium manganate, and it is green. Okay? Next, in neutral or acid solution, the green manganate disproportionates: the same element is both oxidised and reduced. Three M n O four two minus, plus four H plus, give two M n O four minus, plus M n O two, plus two water; the small raised signs are the charges. So two manganese atoms go up from plus six to plus seven, and one comes down to plus four. Clear? Commercially, the manganate is oxidised electrolytically instead. M n O four two minus gives M n O four minus, plus one electron, written e minus. Here every manganese goes from plus six to plus seven. Right?

2:10Manganate and permanganate ions

Now let us compare the two ions. Manganate, M n O four two minus, has manganese at plus six. That leaves just one d electron, written d one; the raised one counts the d electrons. One unpaired electron makes manganate paramagnetic, and it is green. Okay? And permanganate, M n O four minus, has manganese at plus seven. That leaves d zero, no d electrons at all. So it is diamagnetic, and yet it is purple. But wait a moment: with no d electrons, there can be no d to d transition. So where does the purple colour come from? It comes from charge transfer, not from a d to d transition. This is a common trap in the exam. Having no d electrons does not always mean colourless. Clear? Both ions are tetrahedral, with four oxygens around the manganese. And both have pi bonds, the Greek letter pi on screen. They form by overlap of oxygen p orbitals with manganese d orbitals. With me? One more reaction to remember. Heat K M n O four, potassium permanganate, at five hundred and thirteen kelvin; the K after the number stands for kelvin. Two K M n O four give K two M n O four, plus M n O two, plus O two, which is oxygen gas. Got it?

3:37Permanganate as an oxidant: three media

Now the most important part: permanganate as an oxidising agent. In acid, permanganate ends up as M n two plus, so manganese goes from plus seven to plus two. How many electrons does each permanganate ion gain? Yes, it gains five electrons! M n O four minus, plus eight H plus, plus five e minus, give M n two plus, plus four water. We write n equals five, the number of electrons gained. Okay? Its E naught is one point five one volt; the small circle means standard, and the V stands for volt. In acid, it oxidises iron two plus to iron three plus, and iodide, I minus, to iodine, I two. It also oxidises oxalate, C two O four two minus, to carbon dioxide, at sixty degrees Celsius. Right? In neutral or faintly alkaline solution, the product is M n O two instead. M n O four minus, plus two water, plus three e minus, give M n O two, plus four O H minus. Plus seven to plus four, so n is three here. Clear? Now here is a trap for you. In acid, iodide gave iodine. So in neutral solution, does iodide give iodine again? No, careful, it goes further, to iodate, I O three minus! In neutral solution, thiosulphate, S two O three two minus, also goes to sulphate, and M n two plus goes to M n O two. So the medium changes the product. Got it? And in strongly alkaline solution, permanganate takes just one electron and becomes manganate, M n O four two minus. So n is one. With me? And one practical point. In titrations, permanganate is its own indicator. The first excess drop turns the solution pink, and that pink is the end point. Okay?

5:41Example 1: why KMnO4 titrations are not done in HCl

Now, Example one, an easy one. Why are permanganate titrations not done in H C l, hydrochloric acid? Pause the video and try it first. Done? Okay, let's check. Many students say, H C l is a strong acid, so it should work fine, right? But look at what H C l brings with it: chloride ions. So what can permanganate do to chloride? Yes, exactly, it oxidises chloride to chlorine! Two M n O four minus, plus sixteen H plus, plus ten C l minus, give two M n two plus, plus five C l two, plus eight water. Each chlorine goes from minus one to zero. Okay? So here is the answer. H C l is oxidised to chlorine, so extra titrant is consumed. Right? That is why we acidify with sulphuric acid, H two S O four. And not nitric acid, HNO three, either, because it is itself an oxidant. Clear?

6:47Example 2: equivalent mass of KMnO4 in three media

Now, Example two, and it is our puzzle from the start. Find the equivalent mass of K M n O four, with molar mass M equal to one hundred and fifty eight, in acidic, neutral and strongly alkaline media. Pause the video and try it first. Done? Okay, let's check. Equivalent mass is M over n, where n is the number of electrons each permanganate ion gains; the slash means divided by. So, in acid, what is n? That is correct, n is five, because manganese goes from plus seven to plus two. One hundred and fifty eight over five gives thirty one point six. Okay? In neutral solution, M n O two forms, plus seven to plus four, so n is three. One hundred and fifty eight over three gives fifty two point six six and so on, about fifty two point seven; the wavy equals sign means approximately. Right? And in strongly alkaline solution, only manganate forms, plus seven to plus six, so n is one. The equivalent mass is the full one hundred and fifty eight. With me? So, which book was wrong? Not a single one of them; all three are correct! The medium decides n. So never use n equals five blindly; check the medium first. Got it?

8:14Quick revision: MnO2 to KMnO4, n = 5, 3, 1, never HCl

So, quick revision. First, M n O two gives green manganate, and then purple permanganate. Second, n is five in acid, giving M n two plus, three in neutral solution, giving M n O two, and one in strongly alkaline solution. And third, acidify with H two S O four, never H C l; permanganate is its own indicator. Now go and try the two practice problems on this page. You will find them easy. All the best!