To understand the meaning and applications of the second law of thermodynamics, to understand the meaning of entropy, and perform some basic calculations involving entropy changes. Entropy Entropy can be thought of as a measure of a system's disorder. A lower degree of disorder implies lower entropy, and vice versa. For example, a highly ordered ice crystal has a relatively low entropy, whereas the same amount of water in a much less ordered state, such as water vapor, has a much higher entropy. Entropy is usually denoted by S, and has units of energy divided by temperature (J/K). For an isothermal process (the temperature of the system remains constant as it exchanges heat with its surroundings), the change in a system's entropy is given by The first law of thermodynamics (which states that energy is conserved) does not specify the direction in which thermodynamic processes in nature can spontaneously occur. For example, imagine an object initially at rest suddenly taking off along a rough horizontal surface and speeding up (gaining kinetic energy) while cooling down (losing thermal energy). Although such a process would not violate conservation of energy, it is, of course, impossible and could never take place spontaneously. AS = . where Q is the amount of heat involved in the process and T is the absolute temperature of the system. The heat Q is positive if thermal energy is absorbed by the system from its surroundings, and is negative if thermal energy is transferred from the system to its surroundings. Using the idea of entropy, the second law can be stated as follows: The second law of thermodynamics dictates which processes in nature may occur spontaneously and which ones may not. The second law can be stated in many ways, one of which uses the concept of entropy. The entropy of an isolated system may not decrease. It either increases as the system approaches equilibrium, or stays constant if the system is already in equilibrium. Any process that would tend to decrease the entropy of an isolated system could never occur spontaneously in nature. For a system that is not isolated, however, the entropy can increase, stay the same, or decrease. Part A What happens to the entropy of a bucket of water as it is cooled down (but not frozen)? O It increases. O It decreases. O It stays the same.