- A spontaneous process is one that could occur naturally (without any ongoing external action) under a given set of conditions.
- A reversible process is one where the system remains in equilibrium throughout the entire process.
- Entropy (S) increases as energy is dispersed (spread out).
- At constant temperature, the change in entropy is given by
- Spontaneity does not imply the rate of a reaction. Spontaneous reactions may be fast or slow.
Up to this point, we’ve discussed processes and reactions in terms of the system’s energy and energy exchanges with the surroundings using the first law of thermodynamics. The first law does not indicate, however, whether a process will happen spontaneously. A spontaneous process is one that could occur naturally (without any ongoing external action) under a given set of conditions.
Avoid this common error
Thermodynamic spontaneity does not imply anything about the rate of the process. Iron oxidizing into iron oxide (rust) is a thermodynamically spontaneous process, but it is not quick. Thermodynamics cannot predict the rate of a reaction. In order to discuss reaction rates, we would have to study the reaction kinetics (an upcoming topic).
Changes in internal energy or enthalpy aren’t the only factors that dictate whether (or how) something happens. We also must consider how the energy is distributed, and how that distribution changes during a process. This distribution of energy is measured using the state function introduced in the previous section, entropy (S). Entropy can be thought of as a measure of disorder. Entropy increases as energy is dispersed (spread out). The units of entropy are J/K, and the units for molar entropy are J/Kmol. Entropy is an extensive state variable, whereas molar entropy is intensive.
The previous section discussed entropy from a statistical mechanics perspective. Entropy can equivalently be quantified using thermodynamics. The thermodynamic definition of entropy is
where dS describes a small change in entropy, dqrev is a small change in heat flow for a process that is carried out reversibly (i.e. one where the system remains in equilibrium throughout the entire process), and T is the temperature.
In CHEM 123, we’ll only calculate changes in entropy for constant temperature processes, where
If you take further physical chemistry courses, you will revisit this topic and calculate ΔS for processes where the temperature can change (and use integrals to do so).