Dioxygen, Oxides and Ozone

Transcript

0:00Introduction

Hello everyone! Okay, a small puzzle first. Take one litre of ozonised oxygen, heat it until all the ozone breaks down, then bring it back to the same temperature and pressure. Now there is more than one litre of gas, more than we started with! So where did the extra gas come from? Oxygen and ozone come up often in JEE, so let's begin.

0:26Dioxygen, O₂: preparation in the laboratory

First, dioxygen, O two, the small two written low meaning two atoms. In the laboratory, two K C l O three, potassium chlorate, gives two K C l plus three O two; the arrow means gives. Over the arrow are M n O two, manganese dioxide, and delta, the triangle, meaning heat. We can also heat a metal oxide. But which metals, reactive or less reactive? Less reactive, exactly! Their oxides break up on heating. Two H g O, mercury oxide, gives two H g plus O two, and two A g two O, silver oxide, gives four A g plus O two. Right? Two lead oxides work too: two P b three O four gives six P b O plus O two, and two P b O two gives two P b O plus O two. And two H two O two, hydrogen peroxide, gives two H two O plus O two. With me? Also, O two is paramagnetic, with two unpaired electrons in pi star: the Greek letter pi with a star, the antibonding pi orbitals. Clear?

1:34Oxides: acidic, basic, amphoteric, neutral

Now, oxides. Acidic ones first: S O two, C l two O seven, C O two, N two O five and M n two O seven, okay? Basic ones: N a two O, C a O and B a O, all metal oxides. Amphoteric ones react with both acids and bases: A l two O three, Z n O, S n O and P b O, right? Neutral ones: C O, N O and N two O, and two mixed oxides, F e three O four and P b three O four. Got it? The trend: across a period, oxides get more acidic; down a group, more basic. Now, M n two O seven in the first line: manganese is a metal, so its oxide must be basic, right? No, no, careful, it is acidic! Manganese here is plus seven, and for one element, the higher the oxidation state, the more acidic the oxide. Okay?

2:39Ozone: made by a silent electric discharge

Now, ozone, O three. Pass a silent electric discharge through dry oxygen: three O two gives two O three. Delta H nought, written as the Greek capital delta, then H, then a small circle, is the standard enthalpy change. It is plus one hundred and forty two kilojoules for each mole of ozone formed, so the reaction is strongly endothermic. So why a silent discharge, not an ordinary spark? Because a spark's heat would decompose the ozone as it forms, while a silent discharge gives energy without much heating. Even so, we get ozonised oxygen, only about ten percent ozone. Clear?

3:21Ozone: bent, with two equal bonds

Now, the structure. Ozone is bent, with a bond angle of one hundred and seventeen degrees, the small circle meaning degrees, and it is diamagnetic. With octets you get one double and one single bond, so the bonds differ in length, right? No, careful, that is not the full story, because ozone has two resonance structures. The double bond, the equals sign, sits on the left in one and on the right in the other, and the dash is a single bond. The raised plus and minus are charges, and the double-headed arrow means resonance. Okay? So the real molecule lies in between: both O O bonds are identical, each one hundred and twenty eight picometres, p m, with bond order one point five. Got it? Now count: in the first structure, how many lone pairs are there on all three oxygens? Six, exactly! The oxygen with the double bond has two, the central one has one, and the one with the minus charge has three, so two plus one plus three makes six. Right?

4:27Ozone, a powerful oxidising agent

Next, ozone is thermodynamically unstable: two O three gives three O two, an exothermic change. It readily gives nascent oxygen: O three gives O two plus an oxygen atom. That makes it a powerful oxidant. Okay? Black lead sulphide, P b S, becomes white lead sulphate: P b S plus four O three gives P b S O four plus four O two. Right? With iodide: two I minus, plus H two O, plus O three, gives two O H minus, plus I two, plus O two. We will use this reaction to estimate ozone in example two. With me? So, can such a strong oxidant oxidise K M n O four, potassium permanganate? No, it cannot, and this is a common trap! Manganese in K M n O four is already plus seven. Clear?

5:23Ozone in the stratosphere

In the stratosphere, ultraviolet light, the U V above the arrow, splits O two into two atoms, and an atom joins O two to give O three. Okay? But nitric oxide, N O, from supersonic jet exhaust, destroys it: N O plus O three gives N O two plus O two. Got it? Then CFC molecules, chlorofluorocarbons, are split by U V light, which sets free chlorine atoms. C l plus O three gives C l O plus O two, and C l O plus an oxygen atom gives back C l plus O two. The chlorine keeps returning, a chain, so one atom destroys many ozone molecules. Clear?

6:06Example 1: volume after heating 1 L of 10% ozonised oxygen

Now, example one, which solves our puzzle. One point zero zero litre, the capital L, of ozonised oxygen with ten percent ozone by volume is heated until all the ozone decomposes. What is the final volume, at the same temperature T and pressure p? Pause the video and try it first. Done? Okay, let's check. Ten percent of one litre is zero point one zero litre of ozone, leaving zero point nine zero litre of oxygen. At the same temperature and pressure, volumes go like moles. So how much oxygen does the ozone give? Zero point one five litre, exactly! Two O three give three O two, so multiply by three over two. Right? So the final volume is zero point nine zero plus zero point one five, which gives one point zero five litres. Puzzle solved: two ozone molecules become three oxygen molecules, so the gas grows. Got it?

7:08Example 2: mass of ozone from KI and thiosulphate titration

Now a harder one. Ozonised oxygen is passed through excess neutral potassium iodide, K I, and the iodine set free needs ten point zero millilitres of zero point one zero zero molar sodium thiosulphate, N a two S two O three; capital M means molar. What mass of ozone was present? Here is a hint: ozone to iodine to thiosulphate. Pause the video and try it first. Done? Okay, let's check. From the iodide reaction, one ozone sets free one iodine molecule, I two, and each iodine molecule takes two thiosulphate ions, S two O three, with a two minus charge. The sign with three lines is read as, is equivalent to. Okay? Thiosulphate is ten point zero times zero point one zero zero, which gives one point zero zero millimole. So the ozone is one millimole too, right? No, no, careful, that is the trap! Each ozone needs two thiosulphate ions, so ozone is one point zero zero over two, which gives zero point five zero zero millimole. Clear? Ozone's molar mass is forty eight grams per mole, or milligrams per millimole. So zero point five zero zero times forty eight gives twenty four point zero milligrams. Got it?

8:31Quick revision: O2 preparation, oxide trends, ozone

So, quick revision. Dioxygen comes from potassium chlorate with manganese dioxide, and it is paramagnetic. For oxides of one element, the higher the oxidation state, the more acidic the oxide. And ozone is bent, bond order one point five, a powerful oxidant, and one ozone sets free one iodine. Now try the practice problems on this page. All the best!