Oxoacids of Halogens

Transcript

0:00Introduction

Hello everyone! Okay, a small puzzle first. Chlorine is a stronger oxidising agent than iodine, so chlorine pushes iodine out of an iodide. So far, everything is normal. But now mix iodine with potassium chlorate, and out comes chlorine gas! Here, iodine pushes chlorine out. How is that possible? That sounds strange, right? Oxoacids of halogens come up often in JEE, and by the end of this lesson, you will know exactly why. So, let's begin.

0:34Four oxidation states, four kinds of acid

So, the halogens form oxoacids in four oxidation states. At plus one, we have HOC l, HOB r and HOI, the hypohalous acids. Okay? At plus three, there is only one acid, H C l O two, a halous acid. The small two is a subscript, it counts two oxygen atoms. Bromine and iodine form no stable acid at plus three. Right? At plus five, we have H C l O three, H B r O three and HIO three, the halic acids. With me? And at plus seven, H C l O four, H B r O four and HIO four, the perhalic acids. Now look again at all four rows. Where is fluorine? Nowhere, exactly! Fluorine forms only one oxoacid, HOF, so it has no place in this table. The reasons are its small size, the highest electronegativity, and no positive oxidation states.

1:31Structures of HOCl to HClO4: halogen always sp3

Clear? Now, the structures. In all these acids, the halogen is s p three hybridised; the three is written raised, as a superscript. Let us take chlorine, and walk through its four acids. Okay? In HOC l, the chlorine carries three lone pairs, and there is no C l double bond O at all. On screen, the two short lines mean a double bond. Right? H C l O two has two lone pairs on chlorine, and one C l double bond O, okay? See the pattern? One more oxygen, one lone pair less, and one double bond more. So for H C l O three, how many lone pairs, and how many double bonds? One lone pair and two double bonds, yes, exactly! And its anion, C l O three minus, with the minus charge written raised, is pyramidal. With me? Finally, H C l O four has no lone pair at all, and three C l double bond O bonds. So its anion, C l O four minus, is tetrahedral. Clear?

2:35Acid strength: more oxygen, stronger acid

Now, acid strength. It rises from HOC l to H C l O four; the pointed sign means less than. So H C l O four is the strongest of the four. But why should one more oxygen give a stronger acid? Look at the anion that is left when the acid loses its proton. More oxygen atoms delocalise, or spread out, its negative charge. So the anion is more stable, and the acid is stronger. Okay? So C l O four minus, with its charge spread over four oxygens, is a very weak base, even weaker than water. Right?

3:14Stability up, oxidising power down

Next, stability. From HOC l to H C l O four, thermal stability increases. Now here is a trap. In H C l O four, chlorine is plus seven, the highest of all four acids. So H C l O four must be the strongest oxidising agent, right? No, no, careful, it is the other way round! Oxidising power decreases from HOC l to H C l O four. HOC l, the least stable, gives up its oxygen most readily, so it is the stronger oxidant. Got it?

3:46Disproportionation

Now look at this reaction. Three HOC l gives two H C l plus H C l O three; the arrow means gives. Chlorine is plus one in HOC l. So what is it in H C l, and in H C l O three? Minus one in H C l, and plus five in H C l O three, that is correct! The same element is reduced and oxidised in one reaction. This is called disproportionation. With me? The anion, C l O minus, does the same thing on warming. Three C l O minus gives two C l minus plus C l O three minus. Okay? And potassium chlorate, K C l O three, heated without a catalyst, does it too. Four K C l O three gives three K C l O four plus K C l; the Greek capital delta over the arrow means heat. Clear?

4:37Example 1: acid strength order of HOCl, HOBr and HOI

Now example one, an easy one. Arrange HOC l, HOB r and HOI in decreasing acid strength. Pause the video and try it first. Done? Okay, let's check. Remember, among the hydrogen halides, H I is the strongest acid. So HOI should be the strongest here too, right? Oh no, careful, that is the trap! All three are in the same oxidation state, plus one, and the hydrogen sits on oxygen. So the halogen pulls electron density from the O H bond, the dash on screen, and the harder it pulls, the stronger the acid. So which halogen pulls the hardest? Chlorine, yes, the most electronegative of the three! Electronegativity goes chlorine, greater than bromine, greater than iodine; this pointed sign means greater than. Right? So the decreasing order is HOC l, greater than HOB r, greater than HOI, so HOI is the weakest. For the same oxidation state, acid strength falls with the electronegativity of the halogen. Got it?

5:47Example 2: why iodine liberates chlorine from KClO3

Now example two, and this one settles our puzzle. Why does iodine liberate chlorine from potassium chlorate, although chlorine is the stronger oxidant? Pause the video and try it first. Done? Okay, let's check. Two K C l O three plus I two gives two KIO three plus C l two. See, chlorine starts with the oxygen and ends up free, while iodine ends up with the oxygen. So what are the two halogens really competing for here? Oxygen, yes, exactly! Here they compete for oxygen, not for electrons. Chlorine wins the contest for electrons, but this is a different contest. Okay? And in the contest for oxygen, iodine wins. Iodine is the more electropositive element, and it forms the more stable oxoanion. Right? So the answer is this: the I O bonds, written with a dash, are stronger than the C l O bonds. And that solves our puzzle! Chlorine is the better oxidant, but iodine holds on to oxygen more strongly. Clear?

6:54Quick revision: acid strength, stability, oxidising power

So, quick revision. For one halogen, acid strength rises with the number of oxygens, and for the same oxidation state, it falls from chlorine to iodine. From HOC l to H C l O four, stability goes up and oxidising power goes down. And iodine beats chlorine in the contest for oxygen, which is why it frees chlorine from potassium chlorate. Now go and try the practice problems on this page. All the best!