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Real Gas Behavior Pideal Videal = Nrt Holds for Ideal Real gas behavior Pideal Videal = nRT holds for ideal gas behavior. Ideal gas behavior is based on KMT, p. 345: 1. Size of particles is small compared to the distances between particles. (The volume of the particles themselves is ignored) So a real gas actually has a larger volume than an ideal gas. Vreal = Videal + nb n = number (in moles) of particles Videal = Vreal - nb b = van der Waals constant b (Table 10.5) Pideal (Vreal - nb) = nRT But this only matters for gases in a small volume and with a high pressure because gas molecules are crowded and bumping into each other. CHEM 102 Winter 2011 Ideal gas behavior is based on KMT, p. 345: 3. Particles do not interact, except during collisions. Fig. 10.16, p. 365 Particles in reality attract each other due to dispersion (London) forces. They grab at or tug on each other and soften the impact of collisions. So a real gas actually has a smaller pressure than an ideal gas. 2 Preal = Pideal - n a n = number (in moles) 2 V real of particles 2 Pideal = Preal + n a a = van der Waals 2 V real constant a (Table 10.5) 2 2 (Preal + n a/V real) (Vreal - nb) = nRT Again, only matters for gases in a small volume and with a high pressure because gas molecules are crowded and bumping into each other. 2 CHEM 102 Winter 2011 Gas density and molar masses We can re-arrange the ideal gas law and use it to determine the density of a gas, based on the physical properties of the gas. PV = nRT molar mass = MM = M = mass/moles = m/n => n = m/MM PV = (m/MM)RT m/V = P * MM/RT density = d = mass/volume = m/V = P*MM/RT m/V = P*MM/RT If we don’t know the identity of a gas (don’t know its MM), then we can measure its mass, volume, pressure, and temperature to solve for its MM – and its probable identity. 3 CHEM 102 Winter 2011 Phase change between liquid and gas Fig. 11.16, p. 391. dynamic equilibrium: forward and reverse processes occur simultaneously and at the same rates. liquid and gas (vapor) in dynamic equilibrium Fig. 11.19, p. 383. vaporization (evaporation): molecules at the surface of a liquid gain energy and move into the gas phase (become gaseous molecules). condensation: molecules in the gas phase lose energy and enter the liquid phase (become liquid molecules). vaporization-condensation equilibrium Example: capped water bottle 4 CHEM 102 Winter 2011 equilibrium vapor pressure (vapor pressure): the pressure due to molecules leaving the liquid state and entering the gaseous state. When saying “vapor pressure,” a person is actually saying “vapor pressure of a liquid.” The presence of a liquid is implied by the term vapor pressure. What is the difference between evaporation and boiling? evaporation boiling 5 CHEM 102 Winter 2011 boiling: the condition of the vapor pressure of a liquid being equal to the external pressure above the liquid’s surface; molecules throughout the liquid gain energy and move into the gas phase (bubbles). Fig. 11.6, p. 380. normal boiling point: the temperature at which boiling occurs with 1 atm of external pressure. Fig. 11.5, p. 380. 6 CHEM 102 Winter 2011 Why does rubbing alcohol evaporate so quickly as compared to water? Rubbing alcohol, aka, C3H7OH, CH3CH(OH)CH3, (CH3)2CHOH, and isopropyl alcohol. What IMF’s exist? What happens to vapor pressure? See Fig. 11.5, p. 380. 7 CHEM 102 Winter 2011 Properties of a liquid due to IMF’s cohesion: attractive forces between molecules in a substance. Impacts boiling point, spherical droplets surface tension viscosity (has shape/entanglement factors, too) adhesion: attractive forces between molecules of different substances. Impacts … Gecko sticky forces capillary action (dispersion, London) and meniscus 8 .
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