General Trends in Group 14

Transcript

0:00Introduction

Hello everyone! Okay, a small puzzle first. Carbon dioxide is a gas you breathe out. Silicon dioxide is quartz, a hard solid with a very high melting point. Same group, same kind of formula, so why is one a gas and the other a rock? We will crack it today. Group fourteen trends come up often in JEE, so let's begin.

0:26Group 14: the carbon family

So, group fourteen: carbon, C on screen, silicon, S i, germanium, G e, tin, S n, and lead, P b. Carbon and silicon are non-metals, germanium is a metalloid, and tin and lead are metals. The valence configuration is n s two, n p two, where n is the outer shell and each raised two counts electrons. Okay? Now, size. Covalent radius, in picometres, the p m on screen: carbon seventy seven, silicon one hundred and eighteen, germanium one hundred and twenty two, tin one hundred and forty, lead one hundred and forty six. Anything odd about silicon and germanium? Yes, exactly, from silicon to germanium the radius grows only slightly. Germanium's three d electrons shield the nucleus poorly, so its outer electrons are pulled in, and the atom stays small. Right?

1:26Ionisation enthalpy and electronegativity

Next, the first ionisation enthalpy, in kilojoules per mole; mol to the minus one means per mole. Carbon one thousand and eighty six, silicon seven hundred and eighty six, germanium seven hundred and sixty one, tin seven hundred and eight, lead seven hundred and fifteen. So, does it fall all the way down? No, it falls from carbon to tin, then rises slightly at lead. Lead's four f and five d electrons shield poorly, so its outer electrons are held tighter. With me? Now electronegativity: carbon two point five, silicon, germanium and tin one point eight each, and lead one point nine. It usually falls down a group. Does it fall smoothly here? No, and this is a common trap. Below silicon it does not decrease regularly, and lead is even slightly higher. Clear?

2:24Melting point: the tin and lead trap

Next, melting points, in kelvin, the K on screen: carbon four thousand three hundred and seventy three, silicon one thousand six hundred and ninety three, germanium one thousand two hundred and eighteen, tin five hundred and five, lead six hundred. Lead is below tin, so lead melts lower, right? No, no, wait, read the numbers again: tin melts lower than lead. So the order, each pointed sign meaning greater than, is carbon, silicon, germanium, lead, tin. The first three are giant covalent solids; tin and lead, soft metals. Got it?

3:04Oxidation states: the inert pair effect

Now the key trend: the oxidation states plus four and plus two. So, down this group, which of the two gets more stable? Yes, exactly, plus two, and this is the inert pair effect. Okay? So G e two plus and S n two plus, the raised two plus being the charge, are reducing. The arrow means changes to: S n two plus readily becomes S n four plus. Right? For lead, it is the reverse: P b four plus is oxidising, and P b O two, the small two counting oxygen atoms, is a strong oxidant. Lead is most stable as P b two plus. With me? That is why P b I four and P b B r four do not exist: P b four plus would oxidise I minus, iodide, and B r minus, bromide. Clear?

3:56Catenation and multiple bonds

Next, catenation, or self-linking: atoms of one element joining into chains. The double pointed sign means much greater than, the wavy one roughly equal. Carbon is far ahead, and lead does not catenate at all. Why is carbon so far ahead? Look at the bond energies, in kilojoules per mole, the dash being a single bond: carbon carbon three hundred and forty eight, silicon silicon two hundred and ninety seven, germanium germanium two hundred and sixty, tin tin two hundred and forty. Carbon's is the strongest. Okay? Carbon also forms strong p pi, p pi multiple bonds, pi bonds between p orbitals: a double line for a double bond, as in C double bond C and C double bond O, and the triple line in C triple bond N is a triple bond. Can the larger silicon atom do the same? No, it cannot. So carbon dioxide is a small molecule, a gas, while silicon dioxide is a giant network of single silicon oxygen bonds, a solid. And that solves our puzzle. Right?

5:06Beyond four bonds, and hydrolysis

Next, covalency, the number of bonds an atom forms. Carbon has no d orbitals, so it stops at four, but silicon and the heavier members can go beyond four. Clear? For example, S i F six, G e C l six and S n with six O H groups, each written in square brackets as one ion carrying a two minus charge. But S i C l six does not exist: six large chlorine atoms cannot fit around silicon. Now, which reacts with water, C C l four or S i C l four? Yes, it is S i C l four. Carbon has no vacant orbital for water, so C C l four is not hydrolysed. But water attacks silicon's vacant three d orbitals: S i C l four plus four H two O gives silicon with four O H groups, plus four H C l. Got it?

6:07Example 1: SnCl2 vs PbCl2 reducing, SnO2 vs PbO2 oxidising

Now, example one. Which is the stronger reducing agent, S n C l two or P b C l two? And the stronger oxidising agent, S n O two or P b O two? Pause the video and try it first. Done? Okay, let's check. Lead is lower in the group, so P b C l two must be the stronger reducing agent, right? Oh no, careful, that is the trap. For tin, plus four is more stable, so S n two plus is readily oxidised, and S n C l two is the stronger reducing agent. With me? For lead, plus two is more stable, by the inert pair effect, so P b four plus is readily reduced, and P b O two is the stronger oxidising agent. Clear?

6:59Example 2: red lead as 2PbO.PbO2, mass of PbO2 from 6.86 g

Now a harder one. Show that red lead, P b three O four, behaves as a mixed oxide, two P b O, dot, P b O two; the middle dot joins two parts of one formula. Then find the mass of P b O two from six point eight six grams of it. Pause the video and try it first. Done? Okay, let's check. Think of red lead as two P b O plus one P b O two. Now we add nitric acid. Which part will dissolve? Yes, the basic P b O part dissolves, leaving P b O two: P b three O four plus four HNO three gives two lead nitrate, P b with two N O three groups, plus P b O two and two H two O, water. Okay? Now, M, the molar mass of red lead, is six hundred and eighty five point six grams per mole, so six point eight six grams is zero point zero one zero zero moles. How many moles of P b O two does each mole give? Just one, so zero point zero one zero zero moles of P b O two, molar mass two hundred and thirty nine point two, weighs zero point zero one zero zero times two hundred and thirty nine point two, which is two point three nine grams. Got it?

8:23Quick revision: inert pair effect, catenation, the traps

So, quick revision. Inert pair effect: S n two plus is reducing, P b four plus oxidising. Carbon: strong catenation and p pi, p pi bonds, but no d orbitals, so covalency four. And the traps: electronegativity one point eight for silicon, germanium and tin, and lead melting higher than tin. Now try the practice problems on this page. All the best!