Kaplan MCAT General Chemistry Review

The effective nuclear charge refers to the strength with which the protons in the nucleus can “pull” additional electrons. This phenomenon helps to explain electron affinity, electronegativity, and ionization energy. In Cl, the nonionized chlorine atom, the nuclear charge is balanced by the surrounding electrons: 17+/17-. The chloride ion, in contrast, has a lower effective nuclear charge, because there are more electrons than protons: 17+/18-. Next, elemental potassium also has a “balanced” effective nuclear charge: 19+/19-. K⁺, ionic potassium, has a higher effective nuclear charge than any of the other options do, because it has more protons than electrons: 19+/18-. Thus, the potassium ion, (D), is the correct answer.

15. D

Ionic bonds are bonds formed through unequal sharing of electrons. These bonds typically occur because the electron affinities of the two bonded atoms differ greatly. For example, the halogens have high electron affinities because adding a single electron to their valence shells would create full outer octets. In contrast, the alkaline earth metals have very low electron affinities and are more likely to be electron donors because the loss of two electrons would leave them with full outer octets. This marked difference in electron affinity is the best explanation for the formation of ionic bonds between these two groups. (A) states the opposite and is incorrect, because the halogens have high electron affinity and the alkaline earth metals have low affinity. (B) is incorrect because equal sharing of electrons is a classic description of covalent bonding, not ionic. (C) is a true statement but is not relevant to why ionic bonds form.

16. C

In the first period, all elements have only an s-orbital. In the second period onwards, a 2p-orbital is present. In the third period, we find 3s-, 3p-, and 3d-orbitals. Though 3d appears to be part of the fourth period, it still shares the same principal quantum number as 3s and 3p (n = 3) and is therefore still applicable.

Chapter 3: Bonding and Chemical Interactions

The Maillard reaction is one of the more important chemical processes that occur in the process of cooking and baking. (Yes, here we go with food preparation, again. This is, in fact, relevant to your MCAT preparation.) The reaction mechanism itself is one with which you are (or will be) closely familiar from your studies of organic chemistry: a nucleophilic reaction between the amino terminus of the peptide chain of a protein and the carbonyl functionality of a sugar to form an N-substituted glycosylamine. This compound then undergoes a complex series of rearrangements and other reactions to produce a set of compounds that gives cooked food its pleasing brown color and delectable flavor. For no food preparation is this reaction more important than for meat (or meat substitute for all you vegetarians out there).

When the surface of the meat (or meat substitute) comes into contact with the hot surface of a pan or grill, the proteins and sugars on the meat exterior begin interacting through the Maillard reaction. The pan must be sufficiently hot to bring the exterior of the meat to a temperature of around 155°C (310°F), which is the optimal temperature for the reaction to occur. Of course, those willing to shell out $50 or more for a steak at a fine steakhouse are expecting perfection and good taste through and through, not just on the surface. So how does a grill master achieve the impossible: generating very high heat for the exterior but not overcooking the interior? The answer, or at least part of it, is this: drying the meat. When meat that has a lot of water on its exterior surface hits the hot pan, the first process that takes place is the boiling of the water. Boiling is a phase change from liquid to gas and occurs at a constant temperature; water’s boiling point is 100°C (212°F). Because this temperature is considerably lower than that necessary for the Maillard reaction, no browning will occur and the flavor compounds so sought after will not form. Rather, the meat will essentially steam, and the end product will be a lifeless, overcooked hunk of toughened proteins unworthy of its hefty price tag. The lesson here is, if you want a tasty steak, always dry your meat!