Page 49 - Instant notes

B1
THE FIRST LAW

Key Notes
Thermodynamics is the mathematical study of heat and its
relationship with mechanical energy and other forms of work. In
chemical systems, it allows determination of the feasibility,
direction, and equilibrium position of reactions.
The sum of all the kinetic and potential energy in a system is the
internal energy, U. Because it includes nuclear binding energy,
and mass-energy equivalence terms, as well as molecular
energies, it is not practical to measure an absolute value of U.
Changes in the value of U and its relationship to other
thermodynamic quantities are therefore used.
If the value of a thermodynamic property is independent of the
manner in which it is prepared, and dependent only on the state of
that system, that property is referred to as a state function. A path
function is a thermodynamic property whose value depends upon
the path by which the transition from the initial state to the final
state takes place.
The energy of an isolated system is constant. An alternative,
equivalent expression is that energy may be neither created nor
destroyed, although the energy within a system may change its
form. It is a result of the first law that energy in an open system
may be exchanged with the surroundings as either work or heat
but may not be lost or gained in any other manner.
Work is energy in the form of orderly motion, which may, in
principle, be harnessed so as to raise a weight. The most common
forms of work are pressure-volume work and electrical work. The
work done by a system against a constant external pressure is
given by w=−p ex ∆V. The maximum amount of volume expansion
work which a system may accomplish under reversible conditions
is given by w=−nRTln(V f /V i ).
When a system takes up or gives out energy in the form of heat,
the temperature change in the system is directly proportional to
the amount of heat. At constant pressure,