Permanganate against Oxalic Acid

Transcript

0:00Introduction

Hello everyone! Something strange happens in the lab. We run purple potassium permanganate into warm oxalic acid, and the first few drops lose their colour slowly. But a little later, every drop turns colourless the moment it falls in. Why should a reaction speed up as it goes? And here is a second surprise: we add no indicator, yet the end point is easy to see. Let us solve both puzzles. Redox titrations come up often in JEE practical chemistry, so let's begin.

0:34The reaction and the 2 : 5 ratio

So, here is the reaction. Permanganate carries one minus charge and oxalate two minus; the signs up top are charges, and the small numbers below are subscripts counting atoms. Two permanganate ions, five oxalate ions and sixteen hydrogen ions, H plus, react; the arrow points to what they give. We get two manganese ions of charge two plus, ten carbon dioxide and eight water. Okay? Where do the two and the five come from? So, what is the oxidation number of manganese in permanganate? It is plus seven, and it ends as plus two, so each manganese, M n on screen, gains five electrons; the arrow means goes to. Carbon, the C on screen, goes from plus three in oxalate to plus four in carbon dioxide. One oxalate has two carbons, so it loses two electrons. With me? So two permanganates gain two times five, ten electrons, and five oxalates lose five times two, also ten. That is why potassium permanganate, K Mn O four, and oxalic acid react in the mole ratio two to five. Clear? Now the formula, with molarity, capital M, and volume, capital V, okay? Many students say permanganate takes five electrons, so divide its side by five. Which number really goes under permanganate, two or five? It is two, its own coefficient in the equation, and five is the trap. Molarity times volume of permanganate, over two, equals molarity times volume of oxalic acid, over five; the small labels below M and V name the solution, and ox means oxalic acid. Got it?

2:19Setting up: burette, flask and temperature

Now the set-up, in the course figure: the burette above, the conical flask below. Permanganate goes in the burette, and the label says we read the lower meniscus, but the upper one for dark K Mn O four. Why the upper one? Because the solution is so dark that the lower meniscus cannot be seen, so we read the upper one. Right? The flask holds the oxalic acid, with an equal volume of dilute sulphuric acid, H two S O four, which supplies the H plus ions. Okay? The flask is also warmed. Why warm it, and why not simply boil it? In the cold, the reaction is too slow. But above about seventy degrees Celsius, oxalic acid begins to decompose into carbon dioxide, carbon monoxide and water. So we warm to sixty to seventy degrees Celsius, but never boil; the little circle is the degree sign. Got it? And the indicator? We need none, because permanganate is a self-indicator. While oxalic acid is left, each purple drop is decolourised; the first extra drop stays, and the solution turns a permanent pale pink; here the arrow means changes to. The white tile under the flask in the figure makes this colour change easy to see. That solves our second puzzle. Clear?

3:40Speed-up, the right acid, and a warning

Now back to our first puzzle: the first drops are decolourised slowly, and the later ones at once. Which product could speed the reaction up? It is the manganese two plus ions. At the start there are none, so the reaction is slow; once some have formed, they catalyse it. When a product catalyses its own reaction, we call it autocatalysis. With me? Now, why sulphuric acid in the flask? Why not hydrochloric acid? Because permanganate oxidises hydrochloric acid, H C l, to chlorine, so some permanganate is wasted on it. And nitric acid, HNO three, is itself an oxidising agent, so it is not used either. Okay? And one warning sign. If a brown turbidity, a brown cloudiness, appears, it is hydrated manganese dioxide, M n O two. It means too little acid, or permanganate added too fast, so manganese was reduced only to plus four, not to plus two.

4:41Equivalent masses

Right? The same electron counts give the equivalent masses. Potassium permanganate, molar mass one hundred and fifty eight, gains five electrons, so one hundred and fifty eight over five gives thirty one point six. Oxalic acid dihydrate, with two water molecules, has molar mass one hundred and twenty six and loses two electrons, so one hundred and twenty six over two gives sixty three.

5:08Example 1: molarity and strength of KMnO4 from the titre

Okay? Alright, example one. Twenty five point zero millilitres of zero point zero four molar oxalic acid decolourises sixteen point zero millilitres of permanganate. Find the molarity and strength of the K Mn O four. Here m L is millilitres, and capital M means molar, moles per litre. Pause the video and try it first. Done? Okay, let's check. Millilitres times molarity gives millimoles, the m mol on screen. Oxalic acid: twenty five point zero times zero point zero four, that is one point zero millimoles. Right? Now, how much permanganate does that need? Is it two fifths of it, or five halves? It is two fifths, because two permanganates react with five oxalates. Two fifths times one point zero gives zero point four millimoles, all in sixteen point zero millilitres. Clear? So the molarity is zero point four over sixteen point zero, which gives zero point zero two five molar. With me? Strength means grams per litre, the g L to the power minus one on screen. Molarity times molar mass: zero point zero two five times one hundred and fifty eight gives three point nine five grams per litre. Okay?

6:32Example 2: find x in hydrated oxalic acid H2C2O4.xH2O

Now a harder one. Zero point six three grams of hydrated oxalic acid is dissolved to make one hundred millilitres. The dot in the formula joins the water of crystallisation, and the number of water molecules is x, the value we must find. Ten point zero millilitres needs ten point zero millilitres of zero point zero two molar permanganate. Pause the video and try it first. Hint: watch the volumes. Done? Okay, let's check. Permanganate used: ten point zero times zero point zero two, that is zero point two millimoles. Right? From permanganate to acid, we take five halves. Five halves times zero point two gives zero point five millimoles of acid, in the ten millilitres we titrated. Okay? Now be careful. Is zero point five millimoles all the acid in our zero point six three grams? No, and this is the trap. We titrated only ten millilitres of the hundred, just a tenth. So the whole sample holds zero point five times ten, which is five millimoles. Miss this, and your molar mass comes out ten times too big. Got it? Zero point six three grams is six hundred and thirty milligrams, the m g on screen. So the molar mass, M, is six hundred and thirty milligrams over five millimoles, which gives one hundred and twenty six grams per mole. Clear? Oxalic acid without its water has molar mass ninety, and each water adds eighteen. So one hundred and twenty six equals ninety plus eighteen times x, and the double arrow on screen means therefore, so x equals two. It is the dihydrate we met before. Beautiful, isn't it?

8:18Quick revision: 2:5 ratio, self-indicator, autocatalysis

So, quick revision. Permanganate and oxalic acid react in the ratio two to five, and each molarity times volume is divided by its own coefficient. Permanganate goes in the burette, read at the upper meniscus, and oxalic acid in the flask with dilute sulphuric acid, at sixty to seventy degrees Celsius. No indicator is needed: colourless to a permanent pale pink, and the manganese two plus ions speed it up. Now go and try the practice problems on this page. All the best!