ps4 - Chem120B Problem Set 4 Due 1 Problems 18-37 and 18-39...

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Chem120B Problem Set 4 Due: February 22, 2008 1. Problems 18-37, and 18-39 in Simon and McQuarrie’s Physical Chemistry . 2. Rotations about a covalent bond, such as the carbon-carbon bond in ethane, are hindered by forces between other chemical groups attached to the bonded atoms. The potential energy U due to these forces typically looks something like this: Your task is to sketch the heat capacity for this rotational degree of freedom as a function of tempera- ture. Here are some things to keep in mind: (a) When rotational energy is much less than the barriers sep- arating potential energy minima ( Δ E ), the motion is oscillatory. In other words, it is like a harmonic degree of freedom. (b) When rotational energy is much greater than Δ E , the motion is essentially unhindered. In other words, it is like a free rotation. (c) Imagine that the spacing between “vibra- tional” energy levels is small compared to Δ E , so that the high-temperature limit of harmonic motion is reached before rotations become unhindered.
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