Need help preparing for the General Chemistry section of the MCAT? MedSchoolCoach expert, Ken Tao, will teach everything you need to know about State Functions and Thermodynamic Laws. Watch this video to get all the mcat study tips you need to do well on this section of the exam!

As MCAT test takers, our view of thermodynamics centers on its dealings with the movement of heat and energy through systems. The changes in the enthalpy and entropy of a system are path independent. Given any starting state of a system, the change in enthalpy of that system depends only on the difference between the final value of enthalpy and the starting value of enthalpy, no matter the steps taken in between. Because the terms energy, enthalpy, and entropy depend only on final and initial values, we call them state functions.

This is in contrast to what are called path functions. Path functions are path dependent, and the details of intermediate steps indeed make a difference when calculating the total change to the system. Consider the work (W) done on a box. If you desired to lift a box on top of a shelf, you could pick it straight up or you could push it up a ramp. Clearly, the force of friction acting on the box will be greater when pushing it up a ramp, so more work must be done in that situation. Therefore, work is not a variable that is path independent, and is unlike our state functions.

Thermodynamic laws

For the MCAT, there are three key thermodynamic laws to study: the zeroeth law, the first law, and the second law. The zeroeth law introduces the concept of temperature. It states that objects in thermal equilibrium are at the same temperature – that is, objects in thermal equilibrium are defined as having no heat transfer between the objects. Conversely, if you have two objects in contact with one another and heat is transferred from one object to the other object, the heat-source is at a higher temperature and the heat-sink is at a lower temperature. The first law says that the total energy of the universe is conserved. Energy can move between systems, or be converted from one form to another, but can never be created or destroyed. The second law of thermodynamics states that the total entropy of the universe is always increasing. Note that by taking a low-level perspective, we can find a plethora of examples of ordered systems. For example, a single sperm and egg develop into a highly ordered human being. This is not in contradiction with the second law – note that the second law states that it is the sum of the entropy in all systems in the universe that is increasing. Some systems may appear ordered, but there are many other systems that are highly disordered, such that the net disorder of the universe is constantly increasing.

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