Chem 205 lecture 3
Quick review of important points from lecture 2:
• U of gas depends on Utrans + Urot + Uvib
o At normal temepratures, no Uvib
o Urot has df 3 for nonlinear, 2 for linear, each=1/2RT
o Utrans=3/2nRT since 3 df
• Because molecules are non-linear and have Urot in ADDITION to Utrans, they have
o Means they have higher heat capacity
o Can be thought of as: requires more U to get from 0k to whatever temperature it
• VdW equation as shown from last class is not that important, just know that the an^2/V^2
represents intermolecular attraction while –nb compensates for the volume that gas
Virial Equation *note, compressibility factor is 1 for ideal gas at all temperatures and at all pressures.
For real gases, the dips you see are when the gas really wants to condense into a liquid. For
example, in the left graph, O dips but H doesn’t. Why doesn’t H? Because H has extremely low
boiling point so it doesn’t want to condense.
In graph to right, it is a certain substance at different temperatures. At low temperatures we see
it dip, but not at high temperatures.
At 1 atmosphere, most gases act ideal
General virial equation predicts Z: When p is small, all other terms are negligible, so equation reduces to 1.
• Laws of thermal dynamics
o Where U is internal E, q Is heat, w is work. Positive q, w=work done on system,
heat going into system
• State functions=path INDEPENDENT
o Include: P, V, T, m, U, H, G, S
o Q, w are NOT state functions since they are path dependent
However, their difference is U, which is a state function
• Systems not always in equilibrium, in that case, we have an irreversible process
o Irreversible: system goes through cycle to return to initial state, but surroundings
have experienced a permanent change