When a small amount of iodine is added to a mixture of chlorin...

Question

When a small amount of iodine is added to a mixture of chlorine and methane, it prevents chlorination from occurring. Therefore, iodine is a free-radical inhibitor for this reaction. Calculate ΔH° values for the possible reactions of iodine with species present in the chlorination of methane, and use these values to explain why iodine inhibits the reaction. (The I―Cl bond-dissociation enthalpy is 211 kJ/mol or 50 kcal/mol.)

Accepted Answer

All right. Hello everyone. So this question says that iodine is a free radical inhibitor in a mixture containing chlorine and cycling chlorination is prevented from happening when iodine is added to the mixture, referring to the Delta H standard values for the possible reactions that might occur when iodine is present in the reaction mixture. Explain why iodine inhibits chlorination. Focus on the propagation step and assume that chlorine radicals are still for. All right. So here recall first and foremost that the heat of the reaction or Delta H standard can be calculated by finding the sum of the bond association energies of those that have been broken and subtracting from that the sum of bond association energies of those that have been formed. So with this in mind, recall that when delta H standard is a negative number, that indicates an exothermic reaction in which heat is released. On the other hand, a positive value means that the reaction is endothermic and therefore heat has to be absorbed. So to understand the trend that's being discussed in the introduction to this video, let's go ahead and draw the propagation steps for the chlorination of cyclops before and after iodine is added. So lets say that I have cyclops right here. And in this case, we're told to assume that the chlorine radical is still formed. So here in the first propagation step of the chlorination of cycling are chlorine radical is going to remove a hydrogen from one of the carbons of cyclops. This is going to result in the formation of an cul radical and HCL as a by-product. So let's discuss the value for delta H standard of the first propagation step in this reaction. In this case, the bond that was just broken is the bond between the secondary carbon of cyclops and its associated hydrogen atom. This has a bond association energy of 413 kilo joules. For more. On the other hand, the bond that just formed is a new bond between hydrogen and chlorine. This has a bond dissociation energy of 432 kilojoules per month. So I would calculate delta H standard by finding the difference between these two values. So that's equals 413 kg joules per mole subtracted by 432 kilojoules per m. And this means that delta, each standard for the first propagation step is equal to negative 10 kilo joules per m, which means that this is an exothermic process since delta H standard is negative. But with this in mind, lets go ahead and continue with the second propagation step in the second propagation step. The aforementioned are you radical is going to react with another equivalent of C two. This results in the formation of a new carbon chlorine bond and a regenerated chlorine radical. So now lets calculate delta H standard. The bond that is just formed or rather the bond that has just broken is the bond between the two chlorine atoms of cl two that has a bond dissociation energy of 240 kg joules per mole. And so the bond that just formed is a bond between a secondary carbon from cyclops and chlorine. This has a bond association energy of 356 klo joules per mole. So now delta H standard is equal to 240 kg joules per mole subtracted by 356 kg joules per mole, which equals negative 116 kg joules per mal. So these two reactions happen before iodine is added. After iodine is added to the mixture, one molecule of I two can react with the aforementioned chlorine radical as a result, this creates an iodine radical and IC L as a by product. So as for the bonds that have been broken. In this case, the bond that broke is the one between the two iodine atoms in I two that has an energy of 149 kg Joel per mole. As a result, the bond formed or the bond that formed is the one between iodine and chlorine in that has a bond association energy of 211 kilojoules per M. And so Delta H standard is equal to 149 kg joules per mole subtracted by 211 kg joules per mole. This means that Delta H standard is equal to negative 62 kilo joules per mole. And lastly, there is one more reaction to consider because iodine could react with our a radical as well. So instead of chlorinating copine, we would instead add iodine to the secondary carbon of cycling resulting in iodopen to hear a radical can react with I two. And as a result, we end up with a new carbon to iodine bond as well as an iodine radical. So in this particular case, the bond that has broken is once again, the bond between the two iodine atoms and I two. So that bond dissociation energy was 149 kilojoules per mole. But now a secondary carbon to iodine bond has formed. So that bond association energy is 238 kilojoules per mole. And so delta H standard is equal to 149 kilo joules per more, subtracted by 238 kg jules per m. This ultimately equals negative 89 kilo jewels per more. So first, let's compare the reaction forming chocy clop pentane to the reaction that forms iodent. We can see that the formation of chocy op pentane is more exothermic, then the formation of aural cycling. So that's not going to be the reason as to why chlorine or rather iodine inhibits chlorination. However, let's compare the formation of IC to the reaction between cyclops and ar chlorine radical. In this particular case, the formation of IC L in which the chlorine radical reacts with I two is more exothermic. Then the reaction between are chlorine radical and copine. So because of this, we've arrived at our final answer. The reason why chlorination is prevented from occurring is because the chlorine radical will react with iodine before it reacts with cyclops and there you have it. So here, I'm going to briefly pause the recording so I can write out the final answer. All right. So now the recording has resumed and the final answer reads as follows the reaction of chlorine or rather the reaction of the chlorine radical with iodine is more exothermic than the reaction of the chlorine radical with cyclops which consumes chlorine and prevents chlorination and there you have it. So with that being said, if you watch this video all the way through, thank you so very much. And I hope you found this helpful.

Key Concepts

Free Radical Mechanism

Bond Dissociation Enthalpy (BDE)

Thermodynamic Stability

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