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CHEM 205 Lecture 3

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CHEM 205
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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 more energy o Means they have higher heat capacity o Can be thought of as: requires more U to get from 0k to whatever temperature it is at • 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 actually occupies 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 ΔU=q+w 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 Lecture 4 • ΔU=q+w • 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
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