Chem 112 Exam AID Course Pack

Ethan newton barry zhang for sos winter 2012 21

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Unformatted text preview: ses with temperature, this process is linked to temperature. The temperature at which this occurs is called the boiling point (and the normal boiling point refers to the temperature associated with P = 1 atm). − This is analogous for freezing (though in this case, intermolecular forces are generally not broken; it is rather the entropy/randomness of the molecules that is increasing as heat is applied). − Also note that boiling and freezing can occur due to both increases in temperature and decreases in atmospheric pressure. (Also, if the pressure is low enough, the transition would be solid to gas instead). Ethan Newton & Barry Zhang for SOS Winter 2012 19 € Trouten’s Rule: the entropy of vaporization for most liquids is very similar. (Exception: liquids that form hydrogen bonds. Why might this be?) Vapour pressures are related to temperatures by the Clausius- Clapeyron equation: º P2* ΔHV $ 1 1 ' ln * = − & − ) P1 R % T2 T1 ( (Given the enthalpy of vaporization and the vapour pressure at one temperature, the vapour pressure at any temperature can be obtained). − In general, liquids cannot exist at temperatures above their boiling point (whatever the boiling point might be given the pressure). Any increase in the energy of the molecules will just put more of them into the gaseous phase, taking the extra energy with them. − Superheating can occur when there are no nucleation points in the solution where bubbles can form. (A liquid can supercool for the same reason). Note that phase transitions are also associated with entropy (liquids have more entropy than solids, and gases have more entropy than liquids). Relative Humidity: the pressure of water in the air divided by the vapour pressure of water at the current temperature: RH = P(H2O)/P*(H2O) - as temperature drops, P*(H2O) also drops while P(H2O) remains the same, so RH increases - the dew point is the temperature at which P*(H2O) = P(H2O) and RH = 100% (and the air is saturated with water vapour) - if the temperature is below the dew point, fog or dew will form - as temperature rises, P*(H2O) also rises while P(H2O) remains the same, so RH increases Phase Diagrams - a graph with temperature on the horizontal axis and pressure on the vertical axis - shows what phase a substance is in, given any temperature and pressure - approximately, solid tends to be in the top left, gas in the top right, li...
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