Kaplan MCAT General Chemistry Review

Styrofoam cups are such good insulators that they can be used as holding containers for certain calorimetry experiments. “Coffee-cup calorimetry,” which uses Styrofoam cups to measure heats of solution and specific heats of metals and other materials, is low-tech, yet it can produce remarkably accurate results, as long as care has been taken to calibrate the calorimeter and to minimize heat loss through the top. The next time you are at your favorite overpriced coffee chain, standing at the milk and sugar station, think about what you are doing when you mix the cold milk into the hot coffee. What you have there in your hand is really a little science experiment. If you took the time to measure the masses and temperatures of the hot coffee and the cold milk before mixing them, measured the drink’s temperature after you had stirred them up, and then looked up the specific heats for water and milk, you would have enough information to calculate the amount of heat exchanged between the hot coffee and the cold milk. (And if you really wanted to be accurate, you would include in your calculation the mass of the little wooden stirrer and its specific heat as well!) Of course, if you actually used the milk and coffee station as your lab bench, you may be putting your life at risk in the hands of the angry, under- and overcaffeinated mob standing behind you impatiently waiting their turn.

This chapter will review the basic principles of thermochemistry, which is the study of the energy changes that accompany chemical and physical processes. Starting from the first law of thermodynamics, which states that energy is never created or destroyed but at most simply changed from one form to another, we will analyze the calculations that are done to quantify the various changes or exchanges in energy as a system moves from some initial state to a final state. As we go along, we will define what is meant by system and surroundings, state functions, heat, enthalpy, entropy, and Gibbs free energy.

Systems and Processes

For whatever reason, students seem to have some anxiety over what, exactly, constitutes a system and what, by exclusion from the system, constitute the surroundings or environment. Perhaps the problem isn’t so much with the definitions, which are fairly straightforward, but the way in which the boundary between the two can be “moved” to suit the interests of the experimenter or observer. Simply put, the system is the matter that is being observed. It’s the total amount of reactants and products, say, in a chemical reaction. It’s the amount of solute and solvent used to create a solution. It could even be the gas inside a balloon. Then the surroundings, or environment, are everything outside of what you’re looking at. However, the boundary between system and surroundings is not permanently fixed, and it can be moved. For example, you might consider the mass of coffee in your coffee cup to be the system and the cup containing it to be the environment. If this is the way you set your boundary, then you’re probably interested in determining, say, the amount of heat transferred from the hot coffee to the cooler coffee cup. Alternatively, you might define the system as the hot coffee and the cup and the environment as the air surrounding the coffee cup. If this is the way you’ve defined your boundary, then you’re probably interested in the heat exchange between the hot coffee/cup system and the cooler surrounding air. The boundary can be extended out farther and farther, until ultimately the entire mass of the universe is included in the system, at which point there are no surroundings. Where you place the boundary is really a decision based on what matter you’re interested in studying.

Systems can be further characterized by whether or not they can exchange heat and/ or matter with the surroundings. A system may be characterized as follows:

• Isolated—The system cannot exchange energy (heat and work) or matter with the surroundings; for example, an insulated bomb calorimeter.

• Closed—The system can exchange energy (heat and work) but not matter with the surroundings; for example, a steam radiator.

• Open—The system can exchange both energy (again, heat and work) and matter with the surroundings; for example, a pot of boiling water.