Reaction Enthalpies from Bond Enthalpies

Transcript

0:00Introduction

Hello everyone! Okay, a small puzzle first. When gas burns on your kitchen stove, the gas molecules and the oxygen molecules break apart, and a lot of heat comes out. So breaking bonds gives out heat, right? Hmm, wait. To pull two bonded atoms apart, you have to put energy in. So if breaking bonds costs energy, where does all that heat come from? Strange, isn't it? Today we solve this, and we learn to estimate the heat of a reaction using only a table of bond enthalpies. This comes up often in JEE, so let's begin.

0:37Break bonds, make bonds

So, let's settle the puzzle. Think of the bond between two atoms. When you break it, is energy taken in, or given out? Taken in, yes! You must pull the atoms apart, so you have to supply energy. The little triangle is the capital Greek letter delta, meaning change in, and H is enthalpy. So for breaking a bond, delta H is greater than zero: positive. Okay? And the reverse: when two atoms join to form a bond, energy comes out, so delta H is less than zero: negative. And there is our puzzle, solved! On the stove, breaking the old bonds costs energy, but the new bonds that form give out even more. The extra comes out as heat. Right? Now, let's make a formula. With enthalpies of formation, delta f H, with a small f for formation, we always did products minus reactants. So here also, bonds formed in the products, minus bonds broken in the reactants, right? No, no, careful! It is the other way round. Breaking is energy in, so it counts plus; forming is energy out, so it counts minus. So delta r H, with a small r for reaction, equals the sum of the bond enthalpies of the bonds broken, minus the sum of the bond enthalpies of the bonds formed. The big zigzag is the capital Greek letter sigma, meaning sum of. Clear? So notice: the broken bonds sit in the reactants. Reactants first, products second: the reverse of the delta f H order. Mix these two up, and the sign of your answer flips. Got it? And one condition. Bond enthalpies are measured for gaseous molecules, so this formula is for reactions in the gas phase. For liquids or solids, the enthalpies of vaporisation or sublimation must also be taken into account, to bring everything to the gas phase. With me?

2:37Count only the bonds that change

Now, a big molecule may have ten or twenty bonds, but in a reaction only one or two of them change. So, do we have to break every single bond, on both sides? You could, and the answer would be the same! But see why. Every bond that survives the reaction gets added once as broken, and subtracted once as formed. Plus and minus the same number: they cancel. Right? So, the topper's way: count only the bonds that actually change. Less writing, and fewer mistakes. Clear? One more thing. The bond enthalpies in your table are mean values, averages. So this method gives an estimate: roughly right, but not exact. Okay?

3:22Resonance energy: extra stability

Now, one more idea that comes from bond enthalpies. Some molecules, like benzene, have more than one contributing structure. Take the most stable one, and calculate its enthalpy from bond enthalpies. That is our calculated value. Got it? Then we measure the actual enthalpy of the real molecule. Now tell me. Is the real molecule more stable than our calculation says, or less stable? More stable, yes! Its actual enthalpy is lower. And this gap is called the resonance energy: calculated minus actual. It is the extra stability of the real molecule. Right? This extra stability comes from delocalisation. In benzene, the pi electrons, pi is the Greek letter you see on screen, are delocalised: spread out over the ring instead of fixed in place. For benzene, the resonance energy is about one hundred and fifty kilojoules per mole; the k J mol to the power minus one on screen is just kilojoules per mole. With me?

4:23Example 1: delta H for chlorination of methane

Now, Example one. Estimate delta H for this reaction: C H four plus C l two gives C H three C l plus H C l; the arrow means gives. The short dash between two atoms means a single bond. Bond enthalpies: C H, four hundred and fourteen; chlorine chlorine, two hundred and forty two; carbon chlorine, three hundred and thirty; and hydrogen chlorine, four hundred and thirty one, all in kilojoules per mole. Pause the video, and try it. Done? Okay, let's check. First, which bonds break? Methane has four C H bonds. Do all four of them break? No, only one! Three C H bonds go straight into C H three C l, unchanged, so we leave them out. So one C H bond breaks, and the chlorine chlorine bond breaks. Four hundred and fourteen plus two hundred and forty two: six hundred and fifty six, taken in. Okay? Now the bonds formed: one carbon chlorine bond, and one hydrogen chlorine bond. Three hundred and thirty plus four hundred and thirty one: seven hundred and sixty one, given out. Right? So delta H is broken minus formed: six hundred and fifty six minus seven hundred and sixty one, which is minus one hundred and five kilojoules. Negative, so the reaction is exothermic. The new bonds give out more than the old bonds cost, exactly like our stove. Perfect!

5:59Example 2: delta H for dehydration of ethanol

Now a harder one. Estimate delta H for the dehydration of ethanol, that is, removing water from it. C H three C H two O H gives C H two, double bond, C H two, plus H two O, water. The double line means a double bond, and the small g in brackets means gas, so everything is gaseous and our formula works. Data: C H, four hundred and fourteen; C O, three hundred and sixty; C C single, three hundred and forty eight; C C double, six hundred and twelve; and O H, four hundred and sixty three. Pause and try. Hint: count only the bonds that change. Done? Okay, let's check. Ethanol loses a hydrogen from one carbon, and the O H group from the other carbon. So one C H bond and one C O bond break. And the carbon carbon single bond becomes a double bond, so we break the C C single bond too. Four hundred and fourteen plus three hundred and sixty plus three hundred and forty eight: one thousand one hundred and twenty two. With me? Now, the bonds formed. One C C double bond, for sure. And water has two O H bonds, so we form two of them, right? No, no, wait! One of those O H bonds was already in ethanol. The O H group just moves across, unchanged. Only one new O H bond forms, with the hydrogen that left the carbon. So formed: six hundred and twelve plus four hundred and sixty three, one thousand and seventy five. Clear? So delta H is one thousand one hundred and twenty two, minus one thousand and seventy five: plus forty seven kilojoules. Positive, so this dehydration is endothermic: heat must be supplied. And if you had counted two O H bonds, even the sign would have come out wrong. That's it!

8:01Quick revision: bonds broken minus formed, resonance energy

So, quick revision. One: delta r H equals the sum of the bond enthalpies of the bonds broken, minus those of the bonds formed, for reactions in the gas phase. Broken first! Two: count only the bonds that actually change; the rest cancel. Three: resonance energy is the calculated enthalpy minus the actual one, about one hundred and fifty kilojoules per mole for benzene. Now go and try the practice problems on this page. All the best!