Another example of irreversible change is the conversion of mechanical work into frictional heat there is no way, by reversing the motion of a weight along a surface, that the heat released due to friction can be restored to the system.Ī reversible change is one carried out in such as way that, when undone, both the system and surroundings (that is, the world) remain unchanged. 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 perform work on the gas. The most widely cited example of an irreversible change is the free expansion of a gas into a vacuum. Similarly, heat can be transferred reversibly between two bodies by changing the temperature difference between them in infinitessimal steps each of which can be undone by reversing the temperature difference. 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 infinitessimal 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 infinitessimal 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 the other state functions. 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 manner in which a process is carried out. 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 the extent of this energy spreading. We now need to understand how the direction and extent of the spreading and sharing of energy can be related to measurable thermodynamic properties of substances that is, of reactants and products. The preceding page explained how the tendency of thermal energy to disperse as widely as possible is what drives all spontaneous processes, including, of course chemical reactions.
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