- Physical Chemistry I
Spring 2012, Unique 52135
Lecture Summary, 29 February 2012
So far we have only discussed how the energy (usually free
energy) of a system changes as a function of pressure, temperature, and
volume. Thus far we have kept the final variable in the ideal gas
law, n, constant.
However, in a chemical reaction taking place in a closed or
isolated system, although no matter is exchanged between the system and
the surroundings, the energy of the system will most definitely change
as reactants are converted to products. We therefore need a way
of understanding how free energy changes as a function of the amount
and chemical identity of the subtances that fill the system.
To begin this, we introduced the concept of chemical potential (mu), which is the change in free energy of a system caused by the change in the moles of the reactants and products in that system. It is the potential energy held by a system as a function of the chemical bonds contained by and intermolecular interactions between molecules in that system. A chemical reaction will reach equilibrium when the sum of all chemical potentials in the system equals 0. If the chemical potentials of the reactants are greater than the chemical potentials of the products, the reaction will proceed spontaneously in the forward direction until the system reaches equilibrium. We defined a reaction coordinate, z, to tell us at what composition of reactants and products this will occur. Finally, we defined an equilibrium constant, K(p), which is the product of the partial pressures of every component in the system raised to their signed stoichiometric coefficient, nu.