Part C: Concept Checks

Transcript

0:00Introduction

Hello everyone! Okay, one small puzzle first. Party balloons are filled with helium, not hydrogen, right? So helium must be the lighter gas, lighter even than hydrogen. Is that statement true or false? Hold that thought; Example one will settle it. Today we do six quick concept checks on the p block. Each sounds obvious; that is how JEE statement questions catch students. So, let's begin.

0:30Check 1: does diborane have a B–B bond?

First, group thirteen. Diborane, B two H six: the small two and six below count the atoms. Each boron brings three valence electrons, and each hydrogen brings one, so two times three, plus six times one, gives twelve. Okay? Now, a classical structure for eight atoms needs seven bonds of two electrons each, so fourteen electrons. We have only twelve, so diborane is electron deficient. So, is there a boron to boron bond? No, there is not, and that is the trap here! The two borons are held together only through two bridges, BHB, with a hydrogen between the borons; each whole bridge is a single bond shared by three atoms, as we will see. Right? In the figure, on the left, the four terminal hydrogens sit on normal bonds, written two c, two e: two centres, two electrons. They lie in one plane, and they are one hundred and nineteen picometres long. With me? The two bridging hydrogens sit on the curved orange bonds. Each is a three-centre two-electron bond, three c, two e, or banana bond, and it is longer, one hundred and thirty four picometres. The angles at boron are ninety seven degrees for the bridges, and one hundred and twenty for the terminal bonds. Ignore the aluminium chloride dimer on the right. Clear?

1:53Check 2: graphite or diamond, which is stable?

Check two, from group fourteen, is about carbon. On the left is diamond: every carbon is s p three, with a small raised three, bonded to four others in a rigid network, with bonds of one hundred and fifty four picometres. It is the hardest natural substance. Okay? On the right is graphite: layers of s p two carbons, bonds of one hundred and forty one point five picometres, and layers three hundred and forty picometres apart. So which form is thermodynamically stable at room conditions? Diamond is so hard, so it must be diamond, right? Oh no, careful, hardness is not the test; energy is. Delta f H, with a small f for formation and a circle for standard, is the standard enthalpy of formation. It is taken as zero for graphite, and plus one point nine kilojoules per mole for diamond; the minus one on mol means per mole. Right? So diamond sits higher in energy. Graphite is the thermodynamically stable form of carbon, and diamond is metastable: it lasts, but it is not the lowest in energy. Got it? Then why don't diamonds turn into graphite? They do, but extremely slowly, far too slowly to notice. Stable means lowest in energy; it says nothing about speed. Clear?

3:13Check 3: is PbO2 a peroxide?

Check three is from group fourteen again. P b O two has one lead and two oxygens; the small two below counts the oxygens. Two oxygens, so it must be a peroxide, right? No, it is not: P b O two is lead four oxide, and the Roman numeral four in brackets is the oxidation state of lead. It holds P b four plus and O two minus ions; the raised signs are charges. So each oxygen is at minus two. Okay? Now, a true peroxide has the O two, two minus ion, with an O O bond and oxygen at minus one. Only these give H two O two, hydrogen peroxide, with acids. P b O two has no O O bond, so no hydrogen peroxide. With me? So what does it do with acids? By the inert pair effect, lead is more stable as P b two plus, the plus two state, so P b O two is a strong oxidising agent. Right? With hydrochloric acid, H C l, it oxidises chloride to chlorine: P b O two plus four H C l gives P b C l two, plus C l two, plus two H two O; the arrow means gives. Got it? With concentrated nitric acid, it releases oxygen: two P b O two plus four HNO three gives two lead nitrate, P b, N O three in brackets, twice, plus two H two O, plus O two. So it is chlorine or oxygen, but never hydrogen peroxide. Clear?

4:39Check 4: is SF6 hydrolysed by water?

Check four, from group sixteen, starts with S F four, a see-saw shaped molecule with an open site on sulphur. Water attacks it: S F four plus two H two O gives S O two, plus four H F, hydrogen fluoride. Okay? Now S F six, with six fluorine atoms around one sulphur atom, in an octahedral shape. That is even more fluorines than S F four. So S F six should be hydrolysed even faster, right? No, no, wait, it is not hydrolysed at all. The six fluorine atoms crowd around the sulphur and shield it completely, so a water molecule simply cannot reach it. Right? And the sulphur already holds six atoms, with no open site left; we call it coordinatively saturated. So S F six is not hydrolysed, not even by steam.

5:29Example 1: is helium lighter than hydrogen? 4 vs 2 g/mol

Got it? Now, the two examples. Example one is our puzzle from the start. Is helium lighter than hydrogen? Pause the video and try it first. Here is a hint: compare their molar masses. Done? Okay, let's check. Helium, H e, is a monatomic gas, so each particle is a single atom. Its molar mass, capital M, is four grams per mole. Right? Now hydrogen gas is H two, with two atoms in each molecule. So its molar mass is two times one, which is two grams per mole. Okay? Four is greater than two; that is the greater-than sign. So helium is heavier, twice as heavy per mole as hydrogen. Our opening statement is false, and that was the trap! With me? Then why fill balloons with helium? Because helium is light and non-inflammable, so it is safe; hydrogen is lighter, but explosive. This exact trap does come up in statement questions. Clear?

6:30Example 2: is boric acid a Lewis or a Bronsted acid?

And now Example two, a little deeper. Is boric acid a protonic acid, that is, a Bronsted acid? Pause the video and try it first. Here is a hint: ask where the proton comes from. Done? Okay, let's check. Boric acid is B, O H three, with O H in brackets, the same as H three B O three. It has three O H groups, so it simply gives away their protons, like any acid, right? No, and here is the key idea: acid has two meanings here. Bronsted acid means a proton donor. Lewis acid means an electron-pair acceptor. Okay? Boron in boric acid is electron deficient. So it accepts an O H minus ion, a hydroxide ion, from water. With me? Here is the equation: BOH three, plus two H two O, gives BOH four minus, the borate ion, in square brackets, plus H three O plus; the double half arrows mean equilibrium. So the proton comes from water, not from boric acid. Right? So its own O H bonds do not ionise. Boric acid is a weak monobasic Lewis acid; monobasic, because it accepts just one hydroxide ion. So the answer is no, it is not a protonic acid. Got it?

7:49Quick revision: six p-block true or false statements

So, quick revision. Diborane has no boron to boron bond, only two bridges; graphite is stable, diamond metastable. P b O two is not a peroxide, and S F six is not hydrolysed, its sulphur fully shielded. Helium is heavier than hydrogen, and boric acid is a Lewis acid, not a Bronsted acid. Now try the practice problems on this page. All the best!