Ionisation Enthalpies

Transcript

0:00Introduction

Hello everyone! Let me start with a small puzzle. Manganese and iron are next door neighbours, and iron has one more proton in its nucleus. So iron should hold its electrons more tightly, right? Now take the third ionisation enthalpy, the energy to pull out the third electron. Manganese needs three thousand two hundred and sixty kilojoules per mole, and iron needs only two thousand nine hundred and sixty two. More protons, and yet iron lets go more easily? Strange, isn't it? Today we study the ionisation enthalpies of the transition elements. This is an important topic for JEE, and by the end you will solve this puzzle in one line. So, let's begin.

0:44First ionisation enthalpy: a slow rise

Let us start with the first electron. The symbol on screen, capital delta with a small i below it, then H with a subscript one, stands for the first ionisation enthalpy, okay? And k J mol to the power minus one is read kilojoules per mole. Scandium, six hundred and thirty one. Titanium, six hundred and fifty six. Vanadium, six hundred and fifty. Chromium, six hundred and fifty three. And manganese, seven hundred and seventeen. Then iron, seven hundred and sixty two. Cobalt, seven hundred and fifty eight. Nickel, seven hundred and thirty six. Copper and zinc we keep aside for Example one. With me? So, look at the trend. Across the series these values rise overall, with a few small dips, but slowly, much less steeply than in the p block. Right? But wait. At every step the nuclear charge goes up by one, so the pull on the outer electron should grow a lot. Then why is the rise so slow? What is cancelling that extra pull? Yes, exactly, it is the new electron itself! At each step the added electron is a d electron, and the added d electron screens the four s electron from the extra nuclear charge. More pull, but more screening too, so the four s electron feels only a little more. Clear?

2:11Second ionisation enthalpy: Cr and Cu stand out

Now the second electron, and two facts from the configuration chapter. First, ions lose their four s electrons first. Second, d five, a half filled set, and d ten, a completely filled set, are extra stable. The five and the ten are written small and raised, okay? Now chromium. It loses its four s electron first. So, what is left in chromium one plus, the ion with a single positive charge? Yes, three d five, exactly half filled! So the second electron must break that stable d five set, and that costs a lot. The second ionisation enthalpy, with subscript two, is one thousand five hundred and ninety two kilojoules per mole, high. Okay? Copper is the same story, one level up. Copper one plus is three d ten, completely filled, so the second electron must come out of that filled set. Its value is one thousand nine hundred and fifty eight, high again. With me? Now zinc comes right after copper, with one more proton. So its second ionisation enthalpy should be even higher, right? No, no, careful, it is lower! Look at zinc one plus, three d ten, four s one. It still has a four s electron to give. So its second electron comes from four s, and the filled d set stays untouched. The value is one thousand seven hundred and thirty four, lower than copper. Got it?

3:43Down a group: 5d higher than 4d

Now, one more trend, down a group. The five d elements have one more shell than the four d elements. So you would expect them to be easier to ionise, right? No, it is the opposite! The five d elements have higher first ionisation enthalpies than the four d elements. Surprising, isn't it? The reason sits just before the five d series. Fourteen four f electrons are filled there, and four f electrons screen the nucleus poorly. Okay? So the outer electrons feel a higher effective nuclear charge, and they are held more tightly. Clear? And the name to write in the exam is the lanthanoid contraction, which comes from this poor screening by the four f electrons. With me?

4:29Example 1: higher first ionisation enthalpy, Cu or Zn?

Now, Example one, with the copper and zinc we kept aside. Which has the higher first ionisation enthalpy, copper or zinc? And why? Pause the video, and try it first. Done? Okay, let's check. Many students jump straight to copper. Copper's second ionisation enthalpy was one thousand nine hundred and fifty eight, huge! So copper is higher here too, right? No, no, wait, that was the second electron, and here we want the first! So, start from the atoms. Copper is three d ten, four s one, and zinc is three d ten, four s two. Okay? Copper gives up its single four s electron. But zinc must take an electron out of a filled four s two subshell, a stable, completely filled pair. That is harder. Right? And zinc has one more proton, so a larger nuclear charge holds that electron. Clear? So the answer is zinc, at nine hundred and six, against copper's seven hundred and forty five kilojoules per mole. Got it? And now see the flip. For the second electron, copper is higher, at one thousand nine hundred and fifty eight, against zinc's one thousand seven hundred and thirty four. So always ask which electron is leaving, and from which set. That is the whole trick!

5:52Example 2: third ionisation enthalpy, Mn high, Fe low

Now, Example two, and this is our puzzle from the start. Why is the third ionisation enthalpy of manganese unusually high, and that of iron comparatively low? Pause the video and try it first. Here is a hint: write the two plus ion first. Done? Okay, let's check. The symbol with subscript three is the third ionisation enthalpy. Manganese, three thousand two hundred and sixty, and iron, two thousand nine hundred and sixty two kilojoules per mole. Okay? Now, the third electron comes out of the two plus ion; the raised two plus is its charge. So tell me, what is the configuration of manganese two plus? Correct, it is three d five, the half filled set! So removing one more electron breaks the stable half filled set. That costs extra, and that is why manganese is unusually high. Right? Now iron. Iron two plus is three d six, just one electron more than half filled. So when it loses that one electron, what is left? Yes, exactly, three d five! For iron, removing one electron gives the stable three d five set. So the cost is comparatively low. Clear? So the answer in one line: d five is lost for manganese, and reached for iron. There is our puzzle solved. The number of protons is not the whole story; the stable d five set matters too. With me? And the same idea works for zinc. Zinc two plus is three d ten, completely filled, and its third ionisation enthalpy is very high too, three thousand eight hundred and thirty seven. Okay?

7:36Quick revision: screening, d5 and d10 ions, 5d vs 4d

So, quick revision. First of all, the first ionisation enthalpy rises slowly across a series, because the added d electron screens the four s electron. Second, write the ion first: breaking d five or d ten costs extra, and reaching d five costs less. And third, the five d elements have higher first ionisation enthalpies than the four d elements, because of the lanthanoid contraction. Now go and try the two practice problems on this page. Just write the ion first, and you will do well. All the best!