Overview of The Second Law of Thermodynamics and Heat Engines
There are a number of ways to express the Second Law of Thermodynamics. One expression of the Second Law by Lord Kelvin is that it is impossible to engineer a transformation whose only result would be to convert heat from a source at constant temperature into work. Another expression of the Second Law is by Rudolf Clausius, that heat cannot of itself pass from a colder to a hotter body. What can unify these expressions expressions conceptually is to see that they are both ways of saying that the entropy of an isolated system will increase over time, approaching a maximum value at equilibrium. Entropy is a function which can only increase for a system and its surroundings. Entropy is time's arrow. A function which is always greater at a later time. While it is useful to think of entropy as 'disorder', make sure you learn to see this in an abstract statistical sense. Entropy is the number of possible microscopic configurations of a system. Think about that. As spontaneous change occurs, and a system approaches equilibrium, as entropy increases, differences in temperature or chemical potential smooth out. Because that direction of change is statistically more likely, it occurs spontaneously.
The Second Law of Thermodynamics and Heat Engines on the MCATThe Second Law appears in direct fashion with moderate frequency on the MCAT. For example, a physical sciences passage on a chemical reaction or thermodynamic process may pose a question as to whether a particular change corresponds to an increase or decrease of entropy. Bear in mind, though, that the direct importance for the test, with regard to this topic, is dwarfed by the significance of the Second Law of Thermodynamics for understanding the physical and biological sciences in a rich way.
This is an important conceptual bridge which in crossing you will not only seize an advantage over the competition, but also gain a foothold into a much deeper understanding of science. Try to make thinking about the 2nd law a walking around habit. Walk around with it and be patient. It takes a careful thought process to proceed step by step into a deeper understanding of this material, but it will be worth it both for the test and for your appreciation of science in general.