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chapter 5b - CHAPTER 5b The Properties of Gases 1 The...

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1 CHAPTER 5b The Properties of Gases 2 The Kinetic-Molecular Theory The basic assumptions of kinetic-molecular theory are: Postulate 1 Gases 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. 3 The Kinetic-Molecular Theory Postulate 2 Gas molecules are in constant, random, straight line motion with varying velocities. Proof - Brownian motion displays molecular motion.
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4 The Kinetic-Molecular Theory Postulate 3 Gas 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. 5 The Kinetic Model of Gases The kinetic model of gases regards gas molecules as infinitesimal small points that travel in straight lines until they undergo instantaneous collisions. 6 The Kinetic-Molecular Theory Postulate 4 The 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.
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7 The Kinetic-Molecular Theory The 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 Law P 1/V As the V increases the molecular collisions with container walls decrease and the P decreases. 8 The Kinetic-Molecular Theory Dalton’s Law P total = P A + P B + P C + ..... Because gases have few intermolecular attractions, their pressures are independent of other gases in the container. Charles’ Law V T An increase in temperature raises the molecular velocities, thus the V increases to keep the P constant. 9 The Kinetic-Molecular Theory The quantitative kinetic molecular model is clearly defined on pages 156 -160. Please take a moment to read through this quantitative description of the theory.
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10 The Kinetic-Molecular Theory Pressure as defined by kinetic energy of gas molecules is given by the following equation (derived on pages 158-159). Where: P = pressure of a gas N A = Avogadro’s number n = number of moles of gas m = mass of each gas particle V = volume of the container u 2 = the average of the squares of the velocities of the particles 2 2 ( ) 2 ( ) 3 A mu kinetic energy energy caused by motion nN mu P V = = ½ ½ 11 The Kinetic-Molecular Theory The meaning of temperature.
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