CH353
- Physical Chemistry I Spring 2015, Unique 51170 Lecture Summary, 5 February 2015 |

Thermochemistry: We
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 open a bottle of perfume in a room, soon
perfume molecules will fill the entire room, 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, for example sweating. 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:delta S >= q/T.
We have already seen that the second law of thermodynamics defines the direction of spontaneous change of a thermodynamic process. However, in order to apply this, we have to know the total change in entropy of the universe, not just the
change in entropy of our system:delta S(tot) = deltaS(sys) +
deltaS(surr) >= 0The equality applies of the process is reversible, the inequality applies in all other cases. This means that we have to determine not only the entropy change of the system but also of the surroundings. Our rules for solving entropy problems are: 1. For any reversible process, delta S(tot)
= 02. To find delta S(sys), use a reversible
path from initial to final states. You can do this because
entropy is a state function and doesn't depend on the actual path.3. Determine delta S(surr)
independently. If the process is reversible, you already know
deltaS(surr) = -deltaS(sys). If the process
is irreversible, deltaS(surr) = q(surr)/T,
and you will have to figure out what q(surr) is. |