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1CHAPTER 5bThe Properties of Gases2The Kinetic-Molecular TheoryThe basic assumptions of kinetic-molecular theory are:Postulate 1Gases consist of discrete molecules that are relatively far apart.Gases have few intermolecular attractions.The volume of individual molecules is very small compared to the gas’s volume. (Gas molecules are infinitesimally small points.)Proof- Gases are easily compressible.3The Kinetic-Molecular TheoryPostulate 2Gas molecules are in constant, random, straight line motion with varying velocities.Proof - Brownian motion displays molecular motion.
4The Kinetic-Molecular TheoryPostulate 3Gas molecules have elastic collisions with themselves and the container.Total energy is conserved during a collision.Proof - A sealed, confined gas exhibits no pressure drop over time.5The Kinetic Model of GasesThe kinetic model of gases regards gas molecules as infinitesimal small points that travel in straight lines until they undergo instantaneous collisions.6The Kinetic-Molecular TheoryPostulate 4The kinetic energy of the molecules is proportional to the absolute temperature.The average kinetic energies of molecules of different gases are equal at a given temperature.Proof - Brownian motion increases as temperature increases.
7The Kinetic-Molecular TheoryThe gas laws that we have looked at earlier in this chapter are proofs that kinetic-molecular theory is the basis of gaseous behavior.Boyle’s LawP ∝1/V As the V increases the molecular collisions with container walls decrease and the P decreases.8The Kinetic-Molecular TheoryDalton’s LawPtotal= PA+ PB+ PC+ .....Because gases have few intermolecular attractions, their pressures are independent of other gases in the container.Charles’ LawV ∝T An increase in temperature raises the molecular velocities, thus the V increases to keep the P constant.9The Kinetic-Molecular TheoryThe quantitative kinetic molecular model is clearly defined on pages 156 -160. Please take a moment to read through this quantitative description of the theory.
10The Kinetic-Molecular TheoryPressure as defined by kinetic energy of gas molecules is given by the following equation (derived on pages 158-159).Where:P = pressure of a gasNA= Avogadro’s numbern = number of moles of gasm = mass of each gas particleV = volume of the containeru2= the average of the squares of the velocities of the particles22()2()3Amukinetic energy energy caused by motionnNmuPV==½½11The Kinetic-Molecular TheoryThe meaning of temperature.