Hydrogen Bonding

Transcript

0:00Introduction

Hello everyone! Okay, a small puzzle to start. Water, H two O, boils at one hundred degrees Celsius. Now, H two S has sulphur in place of oxygen, a bigger, heavier atom. Heavier molecules usually boil higher, right? But H two S boils at minus sixty degrees Celsius! One hundred and sixty degrees lower. So why is water so different? Today, one tiny hydrogen atom explains it all. And this idea comes up often in JEE: boiling points, solubility, even why ice floats. So, let's begin.

0:37What is a hydrogen bond?

So, let's build it up. X, dash, H: the dash is an ordinary covalent bond, joining hydrogen to an atom we call X here. X is small and highly electronegative, so it pulls the shared electrons towards itself, and the hydrogen is left slightly positive. Okay? Now tell me. Which atoms are small enough, and electronegative enough, to play the role of X? Yes, exactly! Only three: fluorine, oxygen and nitrogen, written F, O or N on the screen. Remember this little list. Right? Now the full picture: X, dash, H, three dots, and Y on the right. The slightly positive hydrogen is attracted electrostatically to a lone pair on another electronegative atom, called Y here. That attraction, the three dots, is the hydrogen bond. Clear? Now, its strength: about ten to forty kilojoules per mole; on screen, k J mol to the minus one, which means per mole. Much weaker than a covalent bond, but stronger than the other forces between molecules. Got it?

1:39Which hydrogen bond is the strongest?

Now, all three of these atoms form hydrogen bonds. But which one forms the strongest? Fluorine, correct! It is the most electronegative of the three. So F H, three dots, F, is stronger than O H, three dots, O, and that is stronger than N H, three dots, N, the weakest. The pointed signs mean greater than; here, stronger than. Okay? And a very strong one: H F two minus. The small two below counts the fluorine atoms, and the minus on top is the charge. It is found in the potassium salt KHF two, with a very strong, symmetrical hydrogen bond. H bond on screen is short for hydrogen bond. With me?

2:22Boiling point, viscosity and solubility

Now, back to our puzzle. The letters b p mean boiling point, and the small circle with C means degrees Celsius. Water, one hundred; H two S, minus sixty. So tell me, why does water boil so much higher? Hydrogen bonding, exactly! In water, hydrogen sits on oxygen, one of our three. In H two S, it sits on sulphur, which is not. So water molecules hold each other with hydrogen bonds, and to boil water you must break them first. That takes extra energy. Puzzle solved! Intermolecular hydrogen bonds raise the boiling point. Okay? The same bonds explain more. Glycerol has three O H groups, so it forms a big network of hydrogen bonds and flows thick and slow: it is viscous. And the lower alcohols and amines mix with water, because they hydrogen bond with water molecules. Right? One more that comes up often. Carboxylic acids, in the vapour and in benzene, join in pairs through hydrogen bonds, called dimers. So the molar mass you measure comes out about double. Got it?

3:28Intermolecular and intramolecular

Now, two kinds of hydrogen bonds. First, intermolecular: between different molecules. On the left of the figure, the red dotted lines are hydrogen bonds, each joining two separate water molecules. Clear? Second, intramolecular: within one molecule. In the middle picture, the hexagon with a circle inside is a benzene ring, and the O H group and the nitro group sit right next to each other on it. So the hydrogen of O H bonds to an oxygen of the nitro group, the short dotted line at the top, in the same molecule, closing a ring. This is called chelation. Okay? That molecule is ortho nitrophenol; the small o stands for ortho, groups side by side. Its hydrogen bond is intra, short for intramolecular. On the right is para nitrophenol; the small p stands for para, groups at opposite ends. N O two is the nitro group; the small two counts the oxygens. Here the two groups are too far apart to bond to each other, so its O H reaches out to the next molecule: inter, short for intermolecular. Now, which one boils higher? Para, correct! If you said ortho, because it has a hydrogen bond built in, careful! That bond is used up inside the molecule, so ortho molecules do not hold on to each other. So ortho nitrophenol boils lower, and it is steam volatile: it distils with steam. And it is less soluble in water than para. Right?

4:53Water and ice

Now, water itself. How many hydrogen bonds can one water molecule form, at most? Up to four. Very good! Each water molecule has two H atoms, and its oxygen has two lone pairs. Got it? So, two hydrogen bonds through its own H atoms; see the two red dotted lines from the middle molecule in the figure. And two more through its two lone pairs. Perfectly balanced. With me? So in ice, each molecule is hydrogen bonded to four others, in an open tetrahedral cage with lots of empty space. So ice is less dense than liquid water. That's why ice floats! When ice melts, some hydrogen bonds break and the molecules pack closer. Clear? And one fact to remember: liquid water is densest at four degrees Celsius. Okay?

5:43Example 1: which of CH4, NH3, HCl, H2O, HF form H-bonds

Okay, Example one. Which of these form hydrogen bonds between their own molecules: C H four, N H three, H C l, H two O, and H F? The small numbers below count the atoms. Pause the video, and try it first. Done? Okay, let's check. One test: is the hydrogen bonded to N, O or F? In C H four, hydrogen sits on carbon, so no. Now, H C l. Chlorine is quite electronegative too. So H C l forms hydrogen bonds, right? No, no, careful! That is the trap. Chlorine is about as electronegative as nitrogen, but it is much larger, so its charge is spread out over a big atom. Low charge density, too weak a pull. So it is not on our list. H C l: no. Clear? So the answer: N H three, H two O, and H F, with hydrogen on nitrogen, oxygen or fluorine. One quick test, and done. Easy, isn't it?

6:42Example 2: boiling points of H2O, HF and NH3

Now a slightly harder one. Arrange water, H F and N H three by boiling point, and explain. Pause the video, and try it first. Done? Okay, let's check. F H, three dots, F is the strongest hydrogen bond. So H F boils the highest, right? No, no, wait! Each such bond is the strongest, true. But each H F molecule has only one H atom. Its fluorine has lone pairs to spare, but not enough H atoms to use them. So on average, each H F molecule forms only about two hydrogen bonds. Okay? Now water. Two H atoms and two lone pairs, so four hydrogen bonds per molecule. Twice as many! So water boils the highest. Right? Now N H three. Three H atoms, but only one lone pair on nitrogen, and that limits its hydrogen bonding. And N H, three dots, N is the weakest of the three anyway. So N H three comes last. Got it? So the order: water at one hundred degrees Celsius, then H F at nineteen point five degrees, then N H three at minus thirty three degrees. See? The number of hydrogen bonds per molecule beat the strength of one bond. Very good!

7:57Quick revision: H-bond rule, b.p., H-bonds per molecule

So, quick revision. One: a hydrogen bond needs hydrogen on F, O or N, attracted to a lone pair. Two: intermolecular hydrogen bonds raise the boiling point; intramolecular ones reduce association, that is, molecules holding on to each other. Three: count the hydrogen bonds per molecule; water's four beat H F's two. Now go and try the practice problems on this page. You can do them. All the best!