Question #32: For the level of Chemistry generally covered on the MCAT, the difference between bond energy and bond dissociation energy is generally not significant.
For a more detailed answer: bond dissociation energy is a very specific definition that it refers to splitting the enthalpy change during the process of a bond “equally” being split into two radicals (homolysis) at 0 K.
This energy is always the same under these conditions assuming that the particle is in a vacuum. However, real life is more complicated, and bond energy reflects that. For example, breaking a bond will have slightly different energy if it is broken heterolytically – that is if you deprotonate water to OH-, that bond energy is not the same as the bond dissociation. In short, bond dissociation energy and bond energy both generally track the same way with bond length and strength but vary slightly in their exact values.
Question #42: By convention, a wedge bond signifies an atom coming out of the page and a hash (dash) bond signifies the bond going into the page. A wedge looks like a bold triangle and a hash bond a set of parallel lines.
For this question, the challenge is that the molecule is flipped over in the answer in addition to the displacement taking place. Keep in mind that an Sn2 is an inversion of the stereocenter, and when you flip over a molecule, you change hash bonds to wedge bonds and vice versa.
Question #44: To know a molecule is symmetrical, we look for a mirror plane within it. For this example, it is easier to see that B is symmetrical using the Fischer projection. Recall that you cannot rotate the stereocenters within a fischer projection, but you can rotate the ends of it (here there are only ends). Often to be extra tricky, we’ll show a molecule that needs to be rotated to clearly see its mirror plane. I have shown you this rotation here in a picture.
mirror plane imageFor Fischer projections, a molecule is bent over like a bridge. To deconvolute one, you imagine flying over the top of the fischer projection. The horizonal lines come out to hug you and the vertical lines are curved away from you. In addition, anything to your left becomes a wedge, and anything to your right becomes a dash, to keep track of your rotating point of view.