Mixed Problems: Set 2 (Groups 15 and 16)

Transcript

0:00Introduction

Hello everyone! Let us start with a small puzzle. Take some ozone and let it break down completely into ordinary oxygen, at the same temperature and pressure. Nothing is added, and nothing escapes, okay? So the volume of gas should stay the same, right? And yet, the volume goes up! So where does the extra gas come from? This is Set two of the mixed problems on the p block: eight quick questions on groups fifteen and sixteen, in the JEE Main style. First we sharpen three tools, with a quick question from the set, and then we solve two more problems, our puzzle included. So, let's begin.

0:41Oxoacids of phosphorus: count the P–OH groups

First tool, with a quick question from Set two. How many moles of sodium hydroxide, N a O H, neutralise one mole of hypophosphorous acid, H three P O two? The m o l on screen is short for mole, and the small numbers below the letters are subscripts; they count the atoms. Three hydrogen atoms, so three moles of N a O H, right? No, no, careful, that is the trap here! In an oxoacid of phosphorus, only a hydrogen on oxygen, in an O H group, can ionise. But a hydrogen bonded directly to phosphorus, in a P H bond, does not ionise at all; the dash on screen is the bond. Okay? So the basicity equals the number of POH groups, not the number of hydrogen atoms. Really simple, and yet it catches so many students. Clear? Hypophosphorous acid has just one POH group, and two P H bonds. The double arrow means so: it is monobasic, and one mole of N a O H is enough. The answer is one, not three. Got it?

1:50Oxidation states and disproportionation

Second tool: oxidation states. Take hydrogen as plus one and oxygen as minus two. Then phosphorus is plus one in H three P O two, plus three in H three P O three, phosphorous acid, and plus five in H three P O four, orthophosphoric acid. Okay? And group fifteen shows oxidation states all the way from minus three to plus five. In phosphine, P H three, phosphorus sits at the bottom, at minus three. Right? Now a word that comes up often in these questions: disproportionation. It means one element, in one oxidation state, is both oxidised and reduced in the same reaction. Part of it goes up, and part of it comes down. With me? So let me ask you this. In nitrous acid, HNO two, nitrogen is plus three, with room above it and room below it. So what can nitrous acid do? Yes, exactly, it can disproportionate! The arrow means gives: in acid, three HNO two give HNO three, plus two N O, plus H two O, water. One nitrogen goes up from plus three to plus five, and two come down from plus three to plus two. Clear?

3:07Ozone and the volumes of gases

Third tool, for group sixteen: ozone, O three. Ozone is thermodynamically unstable. Two O three give three O two, ordinary oxygen, and this change is exothermic: it gives out heat. Okay? Now think about gases. In this equation, what do the coefficients two and three tell us about the volumes? Yes, exactly, they give the ratio of the volumes! The capital T and capital P on screen are the temperature and the pressure. At the same T and P, the coefficients of the gases give their volume ratio; this is Gay Lussac's law in practice. Right? So two volumes of ozone become three volumes of oxygen. Hold on to this line; it is the key to our puzzle. Got it?

3:55Example 1: volume increase when 30 mL of ozone decomposes

Now, Example one from Set two, and this is our puzzle! Thirty millilitres of ozone, the m L on screen, is completely decomposed. By how much does the volume increase, under the same conditions? Pause the video, and try it first. Done? Okay, let's check. Two volumes of ozone give three volumes of oxygen, so the volume is multiplied by three over two. Three over two times thirty gives forty five millilitres of oxygen; the cross means times. And there is our puzzle solved: more moles of gas means more volume. With me? So the answer is forty five millilitres, right? Oh no, careful, read the question again! It asks by how much the volume increases. Forty five minus thirty gives fifteen millilitres. So fifteen is the answer, and forty five is the trap. Clear?

4:52Example 2: H3PO3 disproportionation and NF3 vs NH3 basicity

Now, Example two from Set two: a statement pair, and a little harder. The Roman numerals one and two on screen just number the statements. Statement one: H three P O three disproportionates on heating. Statement two: N F three, nitrogen trifluoride, is a stronger base than N H three, ammonia. Decide whether each one is true or false. Pause the video, and try it first. Done? Okay, let's check, statement one first. In H three P O three, phosphorus is plus three, right between minus three and plus five. So on heating, what can it turn into? Yes, exactly, it goes both up and down! Four H three P O three give three H three P O four, plus P H three. Three phosphorus atoms go up to plus five, in orthophosphoric acid, and one comes down to minus three, in phosphine. So statement one is true. Okay? Now statement two. Ammonia and nitrogen trifluoride both have a lone pair on nitrogen. So which one is the stronger base? It is ammonia, by a long way, and here is why. The base strength of ammonia comes from the lone pair on nitrogen, which it can give away. Clear? Now put three fluorine atoms on that nitrogen. Fluorine pulls electron density away from nitrogen, so the lone pair is much less available. So N F three is a very weak base, and statement two is false. With me? So the answer: statement one is true, and statement two is false. In a statement pair, always judge each statement on its own first. Got it?

6:39Quick revision: basicity, disproportionation, gas volumes

So, quick revision. The basicity of an oxoacid of phosphorus is the number of POH groups, not the number of hydrogen atoms. Disproportionation means one element, in one oxidation state, is both oxidised and reduced, like phosphorous acid on heating. And for gases at the same temperature and pressure, the volumes go in the ratio of the coefficients. Now go and try the practice problems on this page. You can do it. All the best!