Applications of Thermodynamics

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!