Kaplan MCAT General Chemistry Review

While it is true that mercury is much more dense than alcohol, the thermometer content should not make a difference in the student’s results within this temperature range [(A) and (B)]. There is no information to suggest the relative precision of the instruments (C). (Had the question asked about a digital thermometer, this would be relevant.) (D) is correct. There should be negligible differences in the T obtained by each respective thermometer, though initial and final temperature measurements may vary slightly for each instrument.

14. A

Styrofoam is an excellent, though imperfect, insulator. Calibration accounts for heat loss to the calorimeter. The amount of water is irrelevant in a specific heat measurement, and that the calibration is supposed to account for the properties of the styrofoam (B). Calibration (C) accounts for a heat transfer, not a temperature change. The water temperature equilibrates in order to calibrate the thermometer, but that doesn’t suggest anything about the reaction of other calorimeter contents inside (D).

15. A

The addition of salt ions to pure water disrupts hydrogen bonding between water molecules. When an aqueous solution is more disordered, there are weaker forces between component molecules. Compared to a highly stable hydrogen bond network like water, the salt water solution is disordered enough that its intermolecular bonding is much easier to break, lowering its specific heat.

16. B

Pressure must remain constant; paragraph 3 implies that bomb calorimeters are useful because they maintain constant pressure. Though specific heat is an intensive quantity, it is important to keep track of mass for the overall calorimetry calculation, which can involve different substances with different heat capacities. Heat should not enter or exit the system in a precise measurement. Items I and III are correct so the answer is (B).

17. C

Bomb calorimeters operate at high pressure, so they can accommodate temperature changes in a gas. Coffee cup calorimeters are no longer useful when the water boils. (A), (B), and (D) are distractors. Saltwater (A), as an aqueous ionic solution, is analogous to the fruit punch solution described in the passage. While ethanol (B) boils at a lower temperature than water, its structure is highly similar to that of pure water and it is relatively stable at room temperature. Coffee cup calorimeters can measure the specific heat of a metal like copper (D) if it is placed in water.

QUESTIONS 18–21

18. B

For every 2 mol of HCl, 1 mol of hydrogen gas is produced (assuming excess magnesium). Multiplying both sides of the ratio by 1.5 means that 3 mol HCl under the same conditions should produce 1.5 mol hydrogen gas. At STP, 1 mol of a gas (assuming it to be ideal) occupies 22.4 L so 1.5 mol HCl should occupy 33.6 L.

19. C

Savvy test takers will note that (A) and (D) can be eliminated because the answer is internally inconsistent; favoring a reaction almost always means increasing its rate. To decide between (B) and (C), reason via Le Châtelier’s principle. The stress is heat. An endothermic reaction requires heat and so is more likely to consume the heat (i.e., reduce the stress) than an exothermic reaction, which will add to the stress by producing heat.

20. C

By definition, gamma radiation is a stream of energy that in addition to creating the Hulk has neither mass nor charge. No such thing as delta radiation (D) has been defined. Alpha particles and beta particles have both charge and mass.

21. A

Rust, or corrosion, is the oxidation of a substance when it comes into contact with both water and oxygen. (B) can be eliminated because, if true, this would enhance the reactivity of Al or Zn. (C) can be eliminated because reducing agents are oxidized and so that would make Al or Zn more likely to rust. (D) is incorrect, because Al and Zn still rust, just more slowly than iron. As a metal rusts it oxidizes and thus function as a reducing agent. Self-protective oxides (a common way of finding alkali metals as well) prevent further oxidation by complexing the atom with oxygen.

PASSAGE III

22. C

Compound C is shown to promote the reaction without affecting the overall yield of product. It is not consumed or produced in the net reaction, either. This is enough information to identify compound C as a catalyst, which decreases the activation energy of a reaction by definition. In this case, (C) is more likely than (D). Step 1 of the reaction is the fast step and step 2 is the rate-determining, slow step. Even though the catalyst may have an effect on the fast step, its primary purpose is to speed up the slow step. The bulk of the energy input in the overall reaction is in step 2. The catalyst decreases the activation energy, so it is likely to have its primary effect on the step that requires the greatest energy input.

23. D