Hydrogenation reactions are used to add hydrogen across double bonds in hydrocarbons and other organic compounds. Use average bond energies to calculate ΔHrxn for the hydrogenation reaction. H2C'CH2( g) + H2( g)¡H3C¬CH3( g)

Video transcript

Welcome back everyone. To another video hydrogenation reactions are used to add hydrogen across double bonds in hydrocarbons and other organic compounds use average bond energies to calculate the entropy change of the reaction. For the hydrogenation reaction. We're given our reaction specifically propane reacts with hydrogen gas to produce propane and therefore answer choices. A negative 7884 B negative 475 C negative 739 D negative 128. All of them are given in kilojoules per mole. We need to use the average bond energies from the tables. But before we do so, we just want to understand which bonds are being broken and which bonds are being formed. So we are going to redraw this reaction by expanding all of our bonds for protein. We are going to draw CH three which is carbon with three carbon hydrogen bonds. It's bonded to another carbon which has a one hydrogen, that carbon is double bonded to another carbon, which has to Hagen Adams bonnet tip. And now we are performing a reaction. We have hydrogen gas. So we have hydrogen hydrogen single bond and that we are producing propane. So we are going to draw a three membered carbon chain and we are going to add all of our hydrogens one two, 34, 5678. OK. So that makes sense. Now, we, first of all have to understand which bonds are being broken and which bonds are being formed. So, in this case, first of all, we can say that we are breaking 123456 CH bonds. OK. Then we also have a one hh bond. So we're also breaking one hh bond. In addition to that, what we notice is that we're breaking one CC bond, one CC single bond as well as a one CC double bond. OK. So those are all of our bonds. Now, the question is which bonds are we forming? Well, essentially we have eight hygen. So we are forming eight CH bonds and then we have 12, right? So there are two CC single bonds formed. Now, let's remember that the entropy change of the reaction is equal to the sum of the piece of the bonds broken minus the sum of the piece of the bonds formed. So what we're going to do, we're going to, first of all, focus on the bonds broken and we'll start with ch we're saying, OK, we have six moles of CH bonds broken. OK? And we're going to multiply that by the bond entropy of ach single bond, OK. So from the tables, we know that this is 414 414. I'm sorry, kilojoules per mole. This is how we get the total amount of energy needed to break six moles of ch bonds. In addition to that, we need to add one mole of hydrogen bonds. So we are saying one mole multiplied by hh right. So we want to identify the bond entropy for the HH bond which is 436 killer Jules, Broman. Now we have CC single bond. So we are adding one mole of CC according to the table. The bond entropy for ac C single bond is 347 killer jules. From now, we want to finally add one mole of CC double bonds. Now, the bond entropy for the CC double bond would be 611 kilojoules per mole. So we are done with our bonds on the left side. Now, we need to subtract the sum of the bond entropies on the right side. So first of all, we have eight moles of C single bonds. So we're taking eight moles multiplied by ch bond entropy. We already know what it is right. Previously, we said that it was 414 kilojoules per mole. And we're also adding in parenthesis, of course, two moles of CC single bonds. Now, once again, we said it was 347 kill Jules Reman. This is our setup if we calculate the result, we end up with negative 128 kilojoules, just kilojoules if you look at the units. But if we're considering one mole of our reaction, right? Essentially, let's say relative to one mole of propane, we can say that we can modify our units per mole of this reaction. So the answer in this case, what correspond to choice D negative 128 kilojoules per mole? That's the estimate of the entropy change of this reaction. Thank you for watching.