Kaplan MCAT General Chemistry Review

To answer this question, you must apply Le Châtelier’s principle, which states that adding a compound to a system will shift the system reaction such that less of that compound is produced. Adding compound A decreases the overall yield of the system. This change pushes the equilibrium in step 1 to the left to produce more of compound AB and less of compound A. Unbound compound B is not involved in any step of the reaction, so adding it will have no effect on any of the steps. Compound C is a catalyst; increasing the concentration of a catalyst may increase the rate of the reaction, but will not affect the equilibrium constant at any given temperature. The correct answer is (D), as compound D is present on the left side of step 2. Adding more of compound D will push the reaction to the right, increasing the amount of product.

24. A

In terms of concentration, K_eq is equal to [products]/[reactants]. An increased concentration of products signifies an increased equilibrium constant, so K_eq of this reaction will increase along with the equilibrium concentration of BD. The data suggest that this concentration increases with increasing reaction temperature, but the concentration starts to stabilize after the temperature reaches approximately 100° C. Only (A) contains a graph demonstrating a K_eq which initially increases with temperature but starts to level off around 100° C. (B) shows a graph which follows the opposite of the correct path. (C) suggests that the equilibrium constant rises faster and faster as temperature increases, which is incorrect. (D) represents an equilibrium constant that does not change with temperature.

25. C

If a reaction is first-order with respect to each of the reactants, the overall rate is directly proportional to the concentration of each reactant. Doubling the concentration of compound AB will double the rate of reaction. In this new experiment, the concentration of compound AB is twice as high as the concentration used in the first experiment, so the rate of formation is also twice as high.

26. D

The data in the two tables shows that the second experiment, which omitted the catalytic compound C, was significantly slower than the first at every temperature except 150° C. Recall that under normal circumstances, a catalyst will speed up a reaction by decreasing its activation energy, thereby allowing the reaction to reach its activation energy more easily. Consequently, increasing the amount of heat will also help the reaction achieve its activation energy. In this particular experiment, the catalyzed experiment was not much faster than the uncatalyzed experiment at 150°C. This suggests that the reaction reached its activation energy without the help of a catalyst at this temperature range.

27. B

Single-replacement reactions involve the direct replacement of one constituent of a molecule with another constituent molecule. In this case, compound C replaced compound A to turn AB into BC. Double replacement reactions, on the other hand, involve the replacement of two different species. An example of such a reaction is AB + CD AC + BD. Combination reactions require two or more molecules to combine into one molecule, as in step 2. Decomposition reactions require one molecule to break down into two or more molecules, as in step 3.

28. A

Gases are highly compressible, while solids and liquids are not. For this reason, changes in pressure will not have a substantial effect on solids and liquids. An increase in pressure will push the reaction toward the side with less gas molecules, while a decrease in pressure will do the opposite. Because step 1 is the only part of the reaction mechanism that involves gases, it is also the only part that will be affected by a change in pressure.

29. B

This system starts out with two gases (AB and C) and one solution (D) and ends with one gas (A) and two solutions (BD and C). Gases have higher average kinetic energies than solutions do, so they are more disordered (i.e., they have a higher overall entropy, which is a thermochemical measure of the relative disorder in a system). The entropy of the products is lower than the entropy of the reactants, so S for the overall reaction is negative. Based on the information given, it is impossible to determine whether H is positive or negative. Although heat is added to stimulate the reaction, the passage does not specify how much heat is released at the end. G is typically calculated from H and S. H is not known, so G is also impossible to determine.

PASSAGE IV

30. A

Newlands’ table suggests that platinum is heavier than gold. According to the modern periodic table, gold is heavier than platinum. (B), (C), and (D) represent pairs of elements that were arranged correctly by Newlands.

31. B