The system is initially partitioned into two halves, such that the molecules from one half cannot move into the other half. 1 shows the initial state of a hypothetical closed system that contains four molecules. Entropy is best understood by examining a very simple example at the microscopic scale. Entropy increases as T decreases or Q increases.Įntropy is related to the likelihood that equilibrium will be reached. Entropy is a state function, in which changes during a reversible process in a closed system are given by the ratio Q/ T. Entropy is a measure of the energy degradation or disorder of the system.Įntropy is a thermodynamic property just like temperature and pressure. Energy is degraded when heat transfers from one system to another of lower temperature. For example, the ocean contains an immense amount of energy, but it is not very useful because of its low temperature. Furthermore, energy at higher temperatures is more useful than energy at lower temperatures. Thus, work is a more valuable form of energy than heat-work has a high quality. Heat and work are not of the same quality in that work can be efficiently converted to thermal energy (e.g., frictional heat losses), but thermal energy can be only partially converted into mechanical energy (e.g., steam power plants). The second law also has implications for the efficiency of processes. The second law of thermodynamics introduces a new thermodynamic property, entropy, and provides a mathematical statement that describes this unidirectional nature of processes. Processes have a natural direction to them in that spontaneous processes tend to dissipate gradients in the system until equilibrium is reached, e.g.:Ī system that is not subject to forced flows of mass or energy from its surroundings will evolve to a time-invariant state that is uniform or composed of uniform subsystems-the equilibrium state. Processes that satisfy these conservation equations may not be physically possible that is, the process of a cold cup of coffee spontaneously heating up on your dinner table would satisfy the first law of thermodynamics but has a near zero probability to occur. Entropy itself is traditionally described with the units of J/K.Conservation of total mass and energy are insufficient to solve many phase-equilibrium problems. Standard entropies of formation are given in molar quantities because they assume the process is taking place to create 1 mole of the substance. But the magnitude of the change is related to the amount of energy the system currently has (which is directly related to its temperature in kelvin). We associate adding heat with an increase in entropy. If you want to think conceptually, think what adding heat will do to the system. So we look at the amount of heat in joules and compare that to the temperature where we applied the heat. So this allows us to measure $ \Delta S$ directly by looking at how much heat we apply to cause this process to proceed. At 273 K ice and liquid water are in a state of equilibrium, but if we apply heat we can cause ice to melt. So if you take for example ice melting at 273 K, this process is thermodynamically reversible. Entropy doesn't depend on the pathway that we take. The best explanation I can give is that in order to measure entropy for a process we can exploit the fact that it's a state function.