- Principles of Chemistry II: Honors
Spring 2014, Unique 51880
Lecture Summary, 30 January 2014
then went on and discussed the topic of thermochemistry, the science
of the flow of heat due to chemical transformations:
Reactant --> Product +/- heat
In chemical thermodynamics we almost always are interested in energies at constant pressure, therefore in reaction enthalpies of a system. Because enthalpy is a state function, by defining a standard state of P = 1 bar and T = 298 K, we can tabulate enthalpies of formation for any substance under that standard, then use that information to calculate a reaction enthalpy under any set of circumstances, even those far away from the standard state.
Second Law of Thermodynamics: We have spent quite a bit of time developing the first law of thermodynamics, which we have expressed as energy = heat + work. We have found that we can find heat, work, internal energies, and enthalpies for a variety of different systems and different paths. But clearly we are missing something. For example, an an isothermal expansion of an ideal gas, deltaU = 0, but under some conditions, w < 0; i.e. the system does work on the surroundings. How is this possible? How can we do work without a change in internal energy? Furthermore, we have seen that some transformations occur spontaneously only in one direction, even if deltaU = 0. For example, if we heat water in an open container to > 100˚C, the water will evaporate, but the reverse process will never happen. Finally, we know of simple examples in which a reaction occurs spontaneously even if it requires energy from the surroundings, such as the cold pack I passed around in class.
We are clearly missing a kind of energy that will provide a driving force for spontaneous events to occur even with no change in internal energy. We developed a state function, called entropy, S, which defines the direction of spontaneous change (deltaS > 0), and which will quantify this driving force:
deltaS >= q/T.
Thermochemistry problem 1