Bond Parameters

Transcript

0:00Introduction

Hello everyone! Okay, a small puzzle before we start. Hydrogen gas and chlorine gas combine to give hydrogen chloride. Is energy given out, or taken in? And how much? Could you estimate it with just three numbers from a table, without doing the experiment yourself? Sounds impossible, right? By the end of this video, you will do it in three short lines. The secret is four quantities that describe a covalent bond: the bond parameters. And in JEE, questions comparing bond lengths and bond strengths come up often. So, let's begin.

0:39Bond length and bond angle

So, the first parameter: bond length. It is the equilibrium distance between the two nuclei, the distance at which the two bonded atoms settle down. Nucleus to nucleus, okay? Now think. Can we estimate a bond length without measuring it at all? Yes, we can! Every atom has a covalent radius, so just add the two covalent radii. The wavy equals sign on screen means approximately equal to. It is an estimate, not an exact value. Right? For example, carbon and chlorine. The short dash between C and Cl is a single bond. Carbon's covalent radius is seventy seven, and chlorine's is ninety nine. Seventy seven plus ninety nine: one hundred and seventy six. The unit, p m, means picometres. Clear? The second parameter is the bond angle: the angle between the bonds coming out from a central atom. So, a bond length needs two atoms, and a bond angle needs a central atom with two bonds. With me?

1:41Bond enthalpy: the energy to break a bond

Now the third parameter: bond enthalpy. It is the energy needed to break one mole of a bond, with everything in the gas phase. Remember that condition: gas phase. More energy to break means a stronger bond. Okay? Now take water. It has two O H bonds, and they look exactly alike. So breaking the first one and breaking the second one must cost the same energy, right? No, careful! The first O H bond needs five hundred and two, and the second needs only four hundred and twenty seven. The unit on screen, k J mol to the power minus one, is read as kilojoules per mole. Why different? After the first bond breaks, the second bond is in O H, a different chemical environment. Got it? So which number do we call the O H bond enthalpy? We take the average. Five hundred and two plus four hundred and twenty seven, over two. Four hundred and sixty four point five kilojoules per mole. Clear? And that is the rule for polyatomic molecules, molecules with more than two atoms: we use the average bond enthalpy.

2:50Bond order: more bonds, shorter and stronger

Right? And the fourth parameter: bond order, the number of bonds between two atoms. Single bond, bond order one. Double bond, two. Triple bond, three. Simple, okay? Now think. Between the same two atoms, if the bond order goes up, does the bond get longer, or shorter? Shorter, yes, exactly! And stronger too. More shared pairs pull the two nuclei closer, and hold them more strongly. Right? Look at carbon. One dash, the single bond: one hundred and fifty four picometres. Two lines, the double bond: one hundred and thirty four. Three lines, the triple bond: one hundred and twenty. The length keeps falling. With me? Now nitrogen, and this time the bond enthalpies. Single: one hundred and sixty. Double: four hundred and eighteen. Triple: nine hundred and forty six kilojoules per mole. The triple bond is nearly six times as strong as the single bond! So, length goes down, enthalpy goes up. Clear?

3:52A trap: fluorine versus chlorine

Now, a trap. Usually, a larger atom gives a longer and weaker bond. Iodine is much larger than chlorine, so H I is longer and weaker than H Cl, okay? Now compare fluorine and chlorine. Both molecules have a single bond, and fluorine is the smaller atom. So the F F bond must be the stronger one, right? No, no, wait! Look at the numbers. F two, with a small two below for two atoms: one hundred and fifty eight kilojoules per mole. Cl two: two hundred and forty two. The F F bond is the weaker one! Surprised? Here is why. The F F bond is short, so the lone pairs on the two small fluorine atoms come very close. They repel each other strongly, and that weakens the bond. Remember this exception; questions on it come up often. Got it?

4:44Example 1: C-O bond length from covalent radii

Alright, example one. The covalent radii of carbon and oxygen are seventy seven and sixty six picometres. Estimate the length of the carbon oxygen single bond. Pause the video, and try it first. Done? Okay, let's check. Bond length is approximately the sum of the covalent radii. Seventy seven plus sixty six: about one hundred and forty three picometres. Easy, right? And here is the nice part. The measured C O bond length in methanol is one hundred and forty three picometres. Our quick estimate matches it. Perfect!

5:23Example 2: enthalpy of H2 + Cl2 from bond enthalpies

Now example two, and this is our puzzle from the start. Estimate delta H for this reaction. Delta, the Greek letter that looks like a small triangle, with H, means the enthalpy change. On screen: H two gas plus Cl two gas gives two H Cl gas. The little g in brackets means gas, and the arrow means gives. The bond enthalpies are: H H, four hundred and thirty five; Cl Cl, two hundred and forty two; and H Cl, four hundred and thirty one kilojoules per mole. Pause and try. Hint: which bonds break, and which bonds form? Done? Okay, let's check. Breaking a bond costs energy; that is exactly what bond enthalpy measures. On the left, we break one H H bond and one Cl Cl bond. Four hundred and thirty five plus two hundred and forty two: six hundred and seventy seven kilojoules taken in. Okay? Now, forming a bond gives that same energy back. H Cl is four hundred and thirty one. So the bonds formed give back four hundred and thirty one kilojoules, right? Oh no, careful! Look at the equation again: two H Cl. Two moles of H Cl bonds are formed. So two times four hundred and thirty one: eight hundred and sixty two kilojoules given out. Clear? Now, delta H equals the energy for bonds broken, minus the energy for bonds formed. Six hundred and seventy seven minus eight hundred and sixty two: minus one hundred and eighty five kilojoules. With me? The minus sign tells us more energy comes out than goes in, so energy is given out. Puzzle solved! Three numbers from a table, three short lines of working, and we have the answer. See how useful bond enthalpies are?

7:08Quick revision: bond length, bond enthalpy, bond order

So, quick revision. One: bond length is about the sum of the covalent radii, and the bond angle is the angle between the bonds at a central atom. Two: bond enthalpy is the energy to break one mole of a bond in the gas phase, and for polyatomic molecules we use the average. Three: a higher bond order means a shorter and stronger bond, but remember, F two is weaker than Cl two. Now go and try the practice problems on this page. You can do them. All the best!