Boron: Borax, Boric Acid and the Trihalides

Transcript

0:00Introduction

Hello everyone! Two small puzzles first. Boric acid has three O H groups, so you would expect it to give three protons. Yet it is monobasic, and even that one proton is not its own! And fluorine is the most electronegative halogen, yet boron trifluoride is the weakest Lewis acid of the boron trihalides. In this video we solve both, and meet borax on the way. These ideas come up often in JEE, so let's begin.

0:28Borax: the true formula

Let us start with borax, N a two B four O seven, dot, ten H two O, okay? The small numbers below count the atoms, and the dot attaches water to the salt. So, are all ten really present as water? No, and here is the interesting part! The true formula is N a two, then in square brackets B four O five, O H in round brackets four, then dot eight H two O, okay? The atoms of two water molecules went into the anion itself, so only eight remain as water. Look at this anion; the small raised two minus is its charge. Two of its borons are s p three and two are s p two, the three and two written small and raised. And each boron carries one terminal O H group.

1:17Borax in water and on heating

Now, put borax in water: borax plus seven H two O gives two N a O H plus four H three B O three, boric acid. The arrow means gives, okay? Sodium hydroxide is a strong base and boric acid is weak, so the solution is alkaline. And borax is used as a primary standard for acids, as example two will show. On heating, borax swells up, and at red heat melts to a transparent glass. The little triangle on the arrow, delta, means heating. Borax gives two N a B O two plus B two O three, the key to our next test.

1:53The borax bead test

Now the borax bead test. In the hot bead, B two O three and a transition metal oxide give a coloured metaborate. Cobalt oxide, C o O, gives cobalt metaborate, C o, B O two in brackets, taken twice, which is blue. Okay? The bead is heated in the oxidising flame, which is non luminous, and the reducing flame, which is luminous. Cobalt gives blue in both, and chromium green in both. Easy to remember, right? But copper changes. It is blue green in the oxidising flame, and colourless, or red and opaque, in the reducing flame. Got it?

2:28Boric acid: three O–H groups, one proton

Now boric acid, B, O H in brackets, three. Back to our puzzle: with three O H groups, how many protons can it give away? Careful, it gives away none of its own! Its boron is electron deficient, so it accepts a hydroxide ion, O H minus, from water instead of giving its own H plus. Accepting a hydroxide like this makes it a Lewis acid. Right? So, BOH three plus two H two O gives BOH four minus, in square brackets, plus H three O plus; the double half arrows mean equilibrium. One hydroxide taken, one H three O plus released, so it is monobasic. With me? And it is weak: its p K a, with a small a below, is about nine point two; the wavy equals sign means approximately. So boric acid is a weak monobasic Lewis acid.

3:19Titrating boric acid, and its layers

Now, can we titrate such a weak acid directly with sodium hydroxide, and get a sharp end point? No, we cannot. It is too weak, so there is no sharp end point. Okay? The trick is to add a cis diol, such as glycerol or mannitol. It forms a very stable chelate with the borate ion, the equilibrium shifts right, and boric acid behaves as a strong monobasic acid. Clear? In the solid, flat BOH three units are hydrogen bonded into sheets, which slide over one another. So boric acid feels soft and soapy.

3:55Boron trihalides: electron deficient

Now the boron trihalides, B X three, with X a halogen. They are planar and s p two, with an empty two p orbital on boron, so they are electron deficient. Okay? So they are Lewis acids: B F three plus a fluoride ion gives B F four minus. With ammonia, it gives F three B, arrow, N H three, as ammonia donates its lone pair to boron. Right?

4:19Lewis acid strength: the reverse order

Now the second puzzle. Fluorine is the most electronegative halogen, so it pulls electrons from boron the most. So B F three should be the strongest Lewis acid, right? Oh no, careful, it is the opposite! The order is B F three, less than B C l three, less than B B r three, less than B I three, each less than meaning weaker. Electronegativity alone would suggest the reverse. The reason is back bonding: the halogen's filled p orbital donates electrons back into the empty two p orbital of boron. We call it p pi, p pi back bonding, pi being the Greek letter. Okay? This overlap is best for orbitals of similar size: two p with two p in B F three, and poorest for two p with five p in B I three. So B F three has its electron deficiency relieved the most, making it the weakest. Puzzle solved, clear?

5:15Example 1: hybridisation change of boron when BF3 meets NH3

Now, example one. What is the change in hybridisation of boron when B F three reacts with N H three? Pause the video and try it first. Done? Okay, let's check. B F three is trigonal planar, so boron is s p two. Now ammonia donates its lone pair. How many electron pairs does boron have now? Yes, exactly, boron now has four electron pairs: three in its bonds to fluorine, and one from ammonia. Okay? Four electron pairs means s p three. So boron goes from s p two to s p three, as the arrow shows, and from planar to tetrahedral. Very good, you got it!

6:01Example 2: volume of 0.20 M HCl to titrate 0.953 g borax

Now example two. Zero point nine five three grams of pure borax, molar mass capital M equal to three hundred and eighty one point four, is titrated with H C l using methyl red. Find the volume of zero point two zero molar H C l needed; this second M means molar. Pause the video and try it, writing the equation first. Done? Okay, let's check. Here is the reaction: borax plus two H C l plus five H two O gives two N a C l plus four H three B O three. But boric acid is an acid too. Will it disturb the end point? No, because boric acid is too weak to affect methyl red. So we count only the H C l used by borax. Right? Moles of borax: zero point nine five three over three hundred and eighty one point four gives two point five zero times ten to the power minus three moles. One borax needs two H C l, so H C l is two times that, five point zero zero times ten to the power minus three moles. Oh no, careful here: if you take it one to one, you get only half the volume, a very common slip. Volume is moles over molarity: five point zero zero times ten to the power minus three, over zero point two zero, gives zero point zero two five zero litres. That is twenty five point zero millilitres, the m L on screen. Well done, you got it right!

7:30Quick revision: borax, boric acid, BX3 Lewis acidity order

So, quick revision. Borax is N a two B four O five, O H four, dot eight H two O, with two s p two and two s p three borons; it is alkaline in water. Boric acid is a weak monobasic Lewis acid that accepts hydroxide. And Lewis acidity rises from B F three to B I three, due to back bonding. Now try the practice problems on this page. All the best!