Kaplan MCAT General Chemistry Review

The next time you’re leaving a birthday party, holiday celebration, bar or bat mitzvah, christening, wedding, or funeral, snag a helium balloon on your way out. (Balloons at funerals? Hey—some people like to go out smiling!) Tie the helium balloon to the gearshift lever between the front seats, making sure that the balloon is floating freely. Once you’re on an open road, accelerate the car abruptly, and as you do, watch the balloon. What do you predict will happen to the balloon as the car accelerates? What do you observe? You might think, based on how you feel when you are sitting in a car accelerating forward , that the balloon will be pushed toward the back of the car due to its inertia. After all, isn’t this what happens to you? You feel pushed back into your seat as a result of your inertia, which is resisting the car’s forward force and acceleration. However, the balloon’s movement isn’t what we might predict. In fact, it’s the opposite: The balloon shifts forward as the car accelerates forward because the balloon is filled with helium, one of the noble gases. The molecular weight of helium is 4 grams/mole, while that of air, which is mostly nitrogen and oxygen, is about 29 grams/mole. This means that air is about seven times denser than helium. Because the denser air in which the balloon is floating has more mass than the helium-filled balloon, the air will have greater inertia. In fact, we can approximate the balloon’s inertia as practically nonexistent. Therefore, as the car accelerates forward, everything that has significant mass, including the air in the car, resists the forward motion (has inertia) and shifts toward the back of the car (even though, of course, everything in the car is accelerating forward, just not as quickly as the car itself ). As the air shifts toward the back, a pressure gradient builds up such that there is greater air pressure in the back of the car than in the front, and this pressure difference results in a pushing force against the balloon that is directed from the back toward the front. Responding to this force, the balloon shifts forward in the direction of the car’s acceleration. Who would think that general chemistry and physics could be so much fun? Well, if you’ve been paying attention: We do!

The Gas Phase

Matter can exist in three different physical forms, called phases or states: gas, liquid, and solid. We will discuss liquids and solids in Chapter 8, Phases and Phase Changes. The gaseous phase is the simplest to understand, since all gases display similar behavior and follow similar laws regardless of their particular chemical identities. Like liquids, gases are classified as fluids because they can flow. The atoms or molecules in a gaseous sample move rapidly and are far apart from each other. In addition, only very weak intermolecular forces exist between gas particles; this results in certain characteristic physical properties, such as the ability to expand to fill any volume and to take on the shape of a container. (This last characteristic defines fluids—liquids and gases—generally.) Gases are also easily, although not infinitely, compressible, which distinguishes them from liquids.

We can define the state of a gaseous sample generally by four variables: pressure (P), volume (V ), temperature (T ), and number of moles (n). Gas pressures are usually expressed in units of atmospheres (atm) or in units of millimeters of mercury (mm Hg), which are equivalent to torr. The SI unit for pressure, however, is the pascal (Pa). The mathematical relationships among all of these units are

1 atm = 760 mm Hg = 760 torr = 101.325 kPa

On the MCAT, you’ll encounter any or all of these units, so become familiar with them through your practice problems. The volume of a gas is generally expressed in liters (L) or milliliters (mL). Temperature is usually given in Kelvin (K). Many processes involving gases take place under certain conditions, called standard temperature and pressure, or STP, which refers to conditions of 273.13 K (0°C) and 1 atm.

Please carefully note that STP conditions are not identical to standard state conditions. The two standards involve different temperatures and are used for different purposes. STP (273 K and 1 atm) is generally used for gas law calculations; standard state conditions (298 K and 1 atm) are used when measuring standard enthalpy, entropy, free energy changes, and voltage.

MCAT Expertise

On the MCAT, remember that STP is different from standard state. Temperature at STP is 0°C (273.15 K). Temperature at standard state is 25°C (298.15 K).

Ideal Gases