Kaplan MCAT General Chemistry Review

The activation energy of a reaction is equal to the distance on the y-axis from the energy of the reactants to the peak energy prior to formation of products. The activation energy of the first step of the forward reaction, for example, is equal to the distance along the y-axis from point A to point B. The largest energy increase on this graph occurs during the progress between point E and point D, which represents the first step of the reverse reaction.

13. B

This energy diagram presents a two-step system. The first reaction proceeds from point A to point C, and the second reaction proceeds from point C to point E. This means that the reactants predominate at point A, the intermediates predominate at point C, and the products predominate at point E. Point B, which is between points A and C, is the energy threshold at which most of the reactant starts to be converted into intermediate, so the reactant and the intermediate will both be present at this point. No product is produced until after point C, so it will not be present in the reaction mixture. Catalysts may be present in the mixture at any point, depending on the nature and the quantity of the catalyst.

14. C

Reaction 2 is simply the reverse of reaction 1. Because K_eq of reaction 1 is equal to [products]/[reactants], K_eq of reaction 2 must be equal to [reactants]/[products]. This means that K_eq for reaction 2 is the inverse of K_eq of reaction 1, so the answer is 1/0.1 = 10.

15. A

A negative H value always indicates an exothermic reaction, meaning that the forward reaction produces heat. Visualize that as follows:

A + B C + D + heat

This means that removing heat by decreasing the temperature is similar to removing any other product of the reaction. To compensate for this loss, the reaction will shift to the right, causing an increase in the concentrations of C and D as well as a decrease in the concentrations of A and B.

16. C

K_a is equal to the ratio of products to reactants in a dissociated acid. A compound with a K_a greater than 10^-7 contains more H⁺ cations than HA^- anions, which makes it a weak acid (unless K_a is several orders of magnitude higher than 1, which would indicate a strong acid). This means that the compound in question is acidic and that it is likely to react with a compound that is basic. Of the four answer choices, NH₃ is the only base. Also remember that a K_a of 10^-4 leads to a pKa of 4, which is a good approximation for the pKa of several organic acids.

17. D

To answer this question, you will need to recall that the slow step of a reaction is the rate-determining step. The rate is always related to the concentrations of the reactants in the slow step, so NO₂ is the only compound that should be included in the correct answer. The concentration of NO₂ is squared in the rate law because, according to the question, the reaction obeys second-order kinetics.

18. D

The faster a reaction can reach its activation energy, the faster it will proceed to completion. Because this question states that all conditions are equal, the reaction with the lowest activation energy will have the fastest rate. (D) illustrates the smallest difference between the initial and peak potential energies, so that reaction can overcome its activation energy more easily than the other proposed scenarios on the energy diagram. (A), (B), and (C) have higher activation energies.

Chapter 6: Thermochemistry

Back in the “bad old days,” before people became aware of the environmental havoc wreaked by Styrofoam, we used to drink our coffee from Styrofoam cups. Now, of course, coffee is usually served in paper cups consisting of some percentage of post-consumer recycled material, enveloped in a thin cardboard sleeve (also typically made from recycled material) for added insulation against the hot contents. In those rare instances when coffee is served in Styrofoam, the bearer of the cup may receive a few withering glances from those especially passionate about environmentalism and at the very least will probably feel a little guilty for using it.

While Styrofoam might be bad for the environment, it is certainly a good insulator. That’s why it was the material of choice for disposable coffee cups for so long. It keeps hot things (like coffee) hot and cool things (like your hands—relative to the coffee) cool. The layer of Styrofoam protects your hands from too quickly absorbing the energy from the hot coffee and getting burned. Paper coffee cups just aren’t as effective at insulation as Styrofoam cups are; hence, the need for the cardboard sleeve as an extra layer of protection.