Chem 12 Reading and Homework Set 1

Chem 12 Reading and Homework set 1 Read (review) Ch 13. Also read the intro to Ch 16 and section 16.1. A few comments about the text… Sections 13.1 and .3 go through a lot of calculations of ion interaction energies and dipole moments. It's nice to be able to do this, but we're going to take a more qualitative approach to these issues. Section 13.5 goes through several "lattice energy calculations". Although these will not be especially important for us per se, they are nice illustrations of the use of energy diagrams to bring together various pieces of thermochemical data. (Although the overall energy change in these processes is correctly stated as the enthalpy of formation, ΔH°f, of the ionic solid, we're not going to make the distinction between enthalpy and "energy" just yet, so when you encounter a ΔH in the text, just treat that as a ΔE.) Section 13.8 uses bond energies ("D" values) to estimate energy changes in reactions. In the text's examples, it may make sense to chop everything into atoms and then put the atoms back together in a different way, but it's almost always more sensible to work only with the parts of the structure that change (unless you enjoy wasting time with unnecessary arithmetic). I cringe when I look at some of the structures in sections 13.10, .11, and .12 and the associated problems. The critical feature that's missing is formal charge — that's finally covered on p 630, but we've seen a much easier way to do this in class. Please keep in mind what we've learned in class about formal charge and drawing structures. You should assign formal charges to the atoms in all those structures in those sections of the text. Most of the homework sets will have a few problems that you should do on your own but are not to be turned in, followed by a few that will be turned in for credit. Problems will normally be posted about a week before they're due. Not for credit — A. The figure on p 595 shows the energy change involved in making H2 from two H atoms. On the previous page, Coulomb's law was discussed and illustrated with some calculations of ionion attraction energies. (a) Based on Coulomb's law, what is the repulsion energy between the two hydrogen nuclei in H2? (b) Of course, when two H atoms come together to make H2, there is a stabilizing electron-nuclear attraction energy (as well as a destabilizing increase in electron kinetic energy and a destabilizing increase in electron-electron repulsion). Given that H2 is 458 kJ/mol lower in energy than two H atoms, what is the total stabilization from those three electronic terms? (The total stabilization of H2 relative to two H atoms is just the sum of the nuclear-nuclear repulsion (positive) and all those other terms combined (negative).) B. At the bottom of p 600 there's a discussion about the surprisingly small dipole moment of carbon monoxide, CO. The author suggests that this is "likely caused by the lone pairs". Huh? See if you can find a more reasonable explanation by drawing the structure and thinking about resonance. Here's a hint… the negative end of the dipole is the C, not the O! C. Use the method for assigning formal charges that we learned in class, and quickly go through sections 13.10 - 13.12 and add the correct charges to the atoms in those structures. D. Do problems 14a,b,c, 29 (draw an energy change diagram like the ones in the text), 41 (this is a case where you have to chop everything up into atoms. Set your blender to "puree" and go for it), 54, 55 (you must absolutely put formal charges on those resonance structures —add them to the structures in the back of the text also), 74, 90 (this is a good review of VSEPR geom), 102. To be turned in for credit 20 points; due Wed, Feb 2, at the beginning of class — HW turned in after 10:10a will not be accepted for credit. These are all problems at the end of Ch 13 — 1. Problem 30 (you're estimating the "ΔE" for forming MgF2 from Mg metal and F2 gas here — don't worry about the distinction between ΔE and ΔH). 2. Problem 39 is a good example of the importance of formal charges. (a) Go ahead and do the calculation using bond energies. Note that you only need to break one bond and form one bond — don't waste time chopping up bonds that you're going to reform to make the product. (b) according to the NIST thermochemical database, the ΔH°f values — oops, I mean the Es — of methylisocyanide (reactant) and acetonitrile (product) are 164 and 74 kJ/mol, respectively. So what is the energy change for the isomerization shown? (this is the real energy change for the process). (c) Why was the bond energy calculation so far off in this case? 3. Problem 44a. (a) To start, please draw the structure of HCN. (b) To make our lives easier, we'll need the bond energy for just the π part of a C=C. Use the C=C and C–C bond energies in the table on p 617 to extract the energy of the CC π-bond. (c) Calculate the energy change for the two reactions in part a… But wait! Do not, under any circumstances, chop those molecules into atoms. That's a waste of your time and very painful for the molecules. Think like a surgeon, not a demolitions specialist. Which bonds need to be broken and which need to be formed? Work with those and leave everything else alone. (d) Now we really need to make a little adjustment to the calculation for that first reaction. A three-membered ring is a structural feature that you don't see every day, though many molecules have these rings. The bond angles inside a 3-membered ring are abnormally small, and this destabilizes (raises the energy of) the first reactant, ethylene oxide, by about 120 kJ/mol. Of course, bond energy calculations are oblivious to such things. So use that bit of information to adjust the ΔE that you calculated. (e) Based on NIST data, I calculated ΔE = –139 kJ/mol for the reaction: ethylene oxide + HCN —> acrylonitrile + water. How close is your adjusted ΔE to this? 4. Problem 56. This is a great opportunity to apply the method you learned in class. Draw both important resonance structures, with formal charges, as always. 5. Problem 70. Of course, step one is to add the formal charges to make those things valid structures, then we can ponder bond lengths and such.