Ch1b2010_week5

Ch1b2010_week5 - What does this mean in terms of how ideal...

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1 What does this mean in terms of how ‘ideal’ gas particles will interact? 1. The particles are far apart and occupy zero volume (implies no repulsive interactions) 2. The particles couldn’t could care less about each other or the container that holds them (no attractive forces). Pot E Separation Distance (r) r 0 Pot E of interaction is zero everywhere – particles don’t repel, don’t attract, don’t do anything! Of course this is wrong! Of course this is wrong!
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2 kJ/mol r (Å) r Intermolecular (and interatomic) Potentials The interactions of atoms and molecules range can be fairly weak or very strong, but they always exist. Ar/Ar and HCl/HCl interactions are weak, non-bonding Attractions (long range) Repulsions (short range)
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3 Intermolecular Potentials Covalent (single) bonds have attractions (or bond strength values) on the order of 200 kJ/mol Ionic bonds are significantly stronger
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4 Van der Waals eqtn of state Lennard Jones Potential These two approaches towards describing real gases are equivalent – their goal is to capture the nature of intermolecular (or interatomic) interactions in a quantifiable way Chalk time
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5 Bad Assumption #1 1. The particles are far apart and occupy zero volume (implies no repulsive interactions) Here the particles are pretty far apart and they occupy very little of the total volume – probably assumption is ok Here the particles are closer together and occupy a significant fraction of the total volume – they can’t occupy same volume because they repel each other like billiard balls – our assumption is likely to be poor
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6 How might we correct for these bad assumptions? 1. The particles are far apart and occupy zero volume (implies no repulsive interactions) Prediction: Gases at low Pressure will be more ideal At high P: hypothesize a volume correction to the ideal gas law P (V-x) nRT Note: PV Note: PV -Px Px nRT nRT (when P is large, correction is large) (when P is large, correction is large)
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7 Bad assumption #2 Here the particles are pretty far apart and, if they are atoms, they behave as isolated particles At sufficiently high pressures (and/or low temperatures) particles will begin to aggregate – i.e. they attract one another (via van-der Waals attractions) Again, at high P (or low T), assumption 2 is likely to be poor. 2. The particles couldn’t could care less about each other or the container that holds them (no attractive forces). Particle aggregates
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8 Bad assumption #2 2. The particles couldn’t could care less about each other or the container that holds them (no attractive forces). Particle aggregates This implies that the gas is more compressible than it otherwise would be – i.e. the real Pressure is really a ‘reduced’ ideal gas Pressure (P+y) · (V-x) nRT C o r e c t i n a co u fo ttr cti s to p l
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9 Ideal Gases (P+y) · (V-x) nRT One way that people have tried to estimate what the corrections to the ideal gas law is to fit the deviations between Pressure/Volume/Temperature plots of ideal gases and those experimentally measured for real gases
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This note was uploaded on 10/27/2010 for the course CH 1b taught by Professor Natelewis during the Winter '09 term at Caltech.

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Ch1b2010_week5 - What does this mean in terms of how ideal...

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