When energy is transferred between a system and its surroundings it can be transferred in the form of work or heat. To understand how work is involved a derivation may help. Consider a gas which is confined to a cylinder with a movable piston. Furthermore, consider a lead shot of some weight on top of the piston so that at rest the weight is balanced by the pressure of the gas. If we remove some lead shot, the gas will push upward through some differential displacement. We can relate this to the work by:

dV represents the differential change in volume. Integrating from an initial to final state to solve the integral yields the following equation which relates pressure and volume changes to work:

Don’t worry about having to derive this equation, just know the equation. This derivation tells us that when a gas expands the work done is positive and when the gas is compressed the work done is negative. Some books actually say the opposite. However, the important thing to remember is to use one sign convention and to assign positive and negative quantities appropriately–the answer will come out this way.

So how do we use this equation in thermodynamics?

There are many different processes but for MCAT purposes know the difference between a closed process and a open process. In a closed process, in the example above the piston would be locked to prevent the gas from expanding, no work can be done because the volume is held constant. Because the volume is held constant energy can be transferred in the form of heat. This means that the change in internal energy, from the first law of thermodynamics, is equal to the heat changes that occur. Furthermore, since no work is taking place, enthalpy changes are also equal to zero. However, in a open system, work does take place. A open system is one at constant pressure. Now a material, such as a gas, can expand or compress leading to changes in work. Furthermore, at constant pressure enthalpy changes can take place–this is because a gas can expand or compress (not only a gas but any material).

Ultimately, the difference between an open and a closed system is the work done. Energy is transferred between the system and surroundings in both

Thermodynamics is generally a tricky subject, so maybe an real world application of the material may help. (These are also the types of questions the MCAT might ask).

How do refrigerators work? This can be answered using the concepts of thermodynamics. The first law of thermodynamics states that the change in internal energy of a system is equal to the energy added to the system plus the work done on the system–basically a form of the conservation of energy. This is often written in equation form as U = Q + W, where U is the internal energy, Q is heat, and W is work. The internal energy of the system is a function of the kinetic energy of the system, which is a function of the temperature of the system. This means that an increase in the temperature of the system corresponds to an increase in the kinetic energy. Heat is the measure of energy moving into or out of the system. Before I move further, I will assign a sign convention to these quantities. Heat loss is negative, and heat gain is positive. Work done by the system on the surroundings is negative work because the system is losing internal energy, and work done by the surroundings on the system is positive work because the system is gaining internal energy. It doesn’t matter which sign convention you use as long as you keep it consistent; then the numbers will fall into place.

Calorimetry can be used to measure the change in heat of a system in two different ways–constant-volume and constant-pressure. In constant volume calorimetry, no work is performed on the system–it is a closed system. This means the first law of thermodynamics simplfies to: Q = change in internal energy—either heat will be lost or gained by the system. Since pressure is constant, the change in enthalpy is zero. However, a constant pressure system is an open system. The system is allowed to expand and contract so the first law can be rearranged: Q = change in internal energy - work done. The heat that is calculated is a function of the change in enthalpy since the system can change state.

So, how does this relate to how a refrigerator works? When you hold an ice cube in your hand, you feel cold, right? Why? This is because the ice cube absorbs heat from the surroundings, giving you this cold feeling. This idea relates to two very important concepts for the MCAT: expanding gases will cool, and compressed gases will warm. This practically is the basis of refrigeration. A basic refrigerator consists of a compressor, heat exchanging pipes, expansion valve, and refrigerant (typically ammonia gas, but CFCs as well). Basically, the compressor, usually located behind the refrigerator, compresses the refrigerant. When this happens, the pressure of the system increases and the temperature does as well. But, since this occurs in a closed process, there is no change in enthalpy and heat is lost–that is why it usually is warm behind your refrigerator. The gas is then allowed to cool, and it then passes through an expansion valve. The gas cools and encounters a pressure gradient because it is at high pressure on one end but at the other end it is at low pressure. This area is usually inside the refrigerator. As the gas passes through the expansion valve, it absorbs heat from the surroudings, thus making the surroundings cold. The amount of heat absorbed depends on what temperature you set the refrigerator at. This gas, with the heat, is then compressed again and the cycle repeats.

So, in the process, the internal energy of the gas increases and then decreases. The only way the gas will expand is if it absorbs heat. This process corresponds to a change in enthalpy since the state of the gas is changing. Thats it! I hope this helps. If you see a passage on air conditioners or refrigerators, you now have the tools to answer any types of question you might encounter. Good luck!

Electrons always go from the anode to the cathode. ALWAYS, no matter what kind of cell you have. This is because the cathode is defined as the reducing electrode, while the anode is being oxidized. Again, this is always the case, regardless of what kind of cell it is. What changes for galvanic vs. electrolytic cells is the SIGN of the electrodes. In a spontaneous (galvanic) cell, the negative electrons go from a negatively charged anode down their electrical gradient to a positively charged cathode. Makes sense, right? Negative charges would like to go to a positively charged electrode, given their druthers. On the other hand, in an electrolytic cell, you are forcing the negative electrons to go backward, against their electrical gradient, to a negative electrode. Thus, in electrolytic cells, the anode is now positive while the cathode is negative.

As for what flows to the anode, that would be the current. Recall that current is defined by physicists as the direction that imaginary positive charges would travel (though of course it is the electrons we are concerned with in chemistry, not the current). It is also possible if you were reading about a galvanic cell that the problem might mention ions from the salt bridge moving to the cathode and anode. (Since the electrodes are in separate cells for a galvanic cell, you need the salt bridge to complete the circuit or you won’t get any electron flow.) What happens? Well, when the electrons move from the anode, they leave behind positive charges. So negative charges from the salt bridge (anions) travel to the anode cell to counterbalance them. Conversely, the electrons going to the cathode side would build up a net negative charge if positive ions (cations) from the salt bridge didn’t travel to the cathode side to balance them. So salt bridge anions go to the anode side, and cations go to the cathode side.

Vapor pressure is the part of the total pressure that is accounted for by the vapor. Ideally, this means that if the total pressure is 12 atm, and the air is 25% vapor, then the vapor pressure is 25% of 12 atm = 3 atm. In a closed system, the amount of vapor in the gas phase will be set by the saturation vapor pressure. The central idea here is that of equilibrium.

Equilibrium is often thought of as the point where movement stops. In chemistry, a better way to think of it is that the rate in one direction and the rate in the other direction are equal and opposite, so there is no net movement.

In the case of the saturation vapor pressure, this means that for every molecule in the liquid that gets enough energy to break free of the liquid and enter the gas phase, there is a molecule already in the gas phase that strikes the surface of the liquid, and lacks the energy necessary to bounce back, hence remaining in the liquid. There is no net change in the ratio of molecules in the gas phase to molecules in the liquid phase.

For pressure in general, one must continue to think of this in terms of the single molecule colliding against others. If the pressure is high, then it will be harder (i.e., requires more energy) for a molecule to break free of the liquid and become a vapor, because the gas phase is crowded with other molecules. Under low pressure, there’s more space, and hence it is easier (i.e., requires less energy) for a molecule to break free of the liquid and become a vapor.

When you boil water, the water will get hot enough to vaporize, but no hotter–because if it got hotter than needed to vaporize, it would have already evaporated. So if the boiling point of water is 100 degrees, and you have boiling water, then you know that the water is 100 degrees–any more and it would vaporize, and any less and it wouldn’t be boiling. When the pressure is low, it takes less energy for a molecule to jump into the empty space above the liquid, meaning that the temperature required to make the water boil is less. In a pressure cooker, the water molecule needs more energy to get into the crowded space above the liquid, so the temperature required to make it boil gets higher.

Pressure is lower at higher altitude for a similar reason: imagine you are under a rug, and then imagine you are under ten rugs. The more layers of carpet on top of you, the higher the pressure is. The atmosphere is no different. There is no air in space, and the only reason that earth has an atmosphere is because gravity pulls the air down to earth. The lower your altitude is, the greater the thickness of the atmosphere above you, and the greater the pressure of the air weighing down on you. Conversely, the higher you go, the closer you are to space, and the less air there is above you weighing down on you.

Since the pressure at high altitude is low, water boils at a lower temperature. That means that the pot of boiling water at the top of Mt. Whitney is significantly cooler than the pot of boiling water at the bottom of Death Valley, and cooler still than the boiling water in the pressure cooker. The pressure cooker makes it possible for the water to remain a liquid at higher temperatures, so you can make boiling water hotter than normally possible (the water can’t vaporize because of the high pressure). Hence, due to the temperature differences, it would take longer to boil an egg at the top of a mountain, and less time to boil an egg in a pressure cooker.