CH353 - Physical Chemistry I
Spring 2013, Unique 52575

Lecture Summary, 22 February 2013

Chemical Potential:  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.  Looking ahead, we will define 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.