Since the gas does no work on the surrounding in a free expansion (the external pressure is zero, so PΔ V = 0,) there will be a permanent change in the surroundings. Although the system can always be restored to its original state by recompressing the gas, this would require that the surroundings The most widely cited example of an irreversible change is the free expansion of a gas into a vacuum. Reversibly between two bodies by changing the temperature difference between them in infinitesimal steps each of which can be undone by reversing the temperature difference.
#ENTROPY DEFINITION CHEMISTRY SERIES#
The way around this is to restrict our consideration to a special class of pathways that are described as reversible.Ī change is said to occur reversibly when it can be carried out in a series of infinitesimal steps, each one of which can be undone by making a similarly minute change to the conditions that bring the change about.įor example, the reversible expansion of a gas can be achieved by reducing the external pressure in a series of infinitesimal steps reversing any step will restore the system and the surroundings to their previous state. This means, of course, that the quotient q/ T cannot be a state function either, so we are unable to use it to get differences between reactants and products as we do with Manner in which a process is carried out. It turns out that we can generalize this to other processes as well, but there is a difficulty with using q because it is not a state function that is, its value is dependent on the pathway or You will recall that when a quantity of heat q flows from a warmer body to a cooler one, permitting the available thermal energy to spread into and populate more microstates, that the ratio q/ T measures Sharing of energy can be related to measurable thermodynamic properties of substances – that is, of reactants and products. Now we need to understand how the direction and extent of the spreading and We have explored how the tendency of thermal energy to disperse as widely as possible is what drives all spontaneous processes, including chemical reactions. Rudolf Clausius originated the concept as energy gone to waste in the early 1850s, and its definition went through a number of more precise definitions over the next 15 years. It is also widely misrepresented as a measure of disorder, as we discuss below. Absolute entropies of most common substances are tabulated, allowing us to calculate the entropy of a reaction in the same way we can calculate enthalpy of reaction from standard enthalpies of formation.Įntropy is one of the most fundamental concepts of physical science, with far-reaching consequences ranging from cosmology to chemistry. The second section discusses the meaning of entropy, and what disorder means on a microscopic level.Įntropy is a state function, which means we can apply Hess' Law to it. A reversible process can be modeled as a series of tiny steps, while an irreversible process must be modeled as a single large change.
The first section explains the difference between reversible and irreversible processes. Because of this, the second law of thermodynamics explains why a perpetual motion machine can never exist. This is because some of the energy from your car engine is lost as heat. If you touch the hood of your car while the engine is running, the hood of the car will feel hot. One consequence of the second law of thermodynamics is that in any engine there will be some energy lost as heat that cannot be harnessed to do work. The second law of thermodynamics states that the entropy of the universe is always increasing. Entropy is a measure of the disorder of a system, measured in joules (J). The second law of thermodynamics involves a thermodynamic quantity we call entropy (S).
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