# Postulate 2 gas particles are in constant random

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Postulate 2: Gas particles are in constant, random, straight-line motion except when they collide with each other or with the container walls. Postulate 3: Collisions are elastic, meaning that colliding particles exchange energy but do not lose any energy due to friction. Their total kinetic energy is constant. Between collisions the particles do not influence each other by attractive or repulsive forces.
5-53 Figure 5.14 Distribution of molecular speeds for N2at three temperatures.
5-54 Figure 5.15 Pressure arise from countless collisions between gas particles and walls.
5-55 Figure 5.16 A molecular view of Boyles law. Pextincreases, Tand nfixedAt any T, Pgas= Pextas particles hit the walls from an average distance, d1. Higher Pextcauses lower V, which results in more collisions, because particles hit the walls from a shorter average distance (d2< d1). As a result, Pgas= Pextagain.
5-56 Figure 5.17 A molecular view of Daltons law.
5-57 Figure 5.18 A molecular view of Charless law. AtT1, Pgas= Patm.Higher Tincreases collision frequency, so Pgas> Patm.Thus, Vincreases until Pgas= Patmat T2.
5-58 Figure 5.19 A molecular view of Avogadros law. For a given amount, n1, of gas, Pgas = Patm. When gas is added to reach n2the collision frequency of the particles increases, so Pgas> Patm. As a result, Vincreases until Pgas= Patmagain.
5-59 Kinetic Energy and Gas Behavior At a given T, all gases in a sample have the same average kinetic energy. 12 Ek= mass x speed2Kinetic energy depends on both the mass and the speed of a particle. At the same T, a heavier gas particle moves more slowly than a lighter one.
5-60 Figure 5.20 The relationship between molar mass and molecular speed.
5-61 Grahams Law of EffusionEffusionis the process by which a gas escapes through a small hole in its container into an evacuated space.Grahams law of effusionstates that the rate of effusion of a gas is inversely proportional to the square root of its molar mass. A lighter gas moves more quickly and therefore has a higher rate of effusion than a heavier gas at the same T.Rate of effusion 1 M
5-62 Figure 5.21 Effusion. Lighter (black) particles effuse faster than heavier (red) particles.
5-63 Sample Problem 5.14 Applying Grahams Law of Effusion PROBLEM: A mixture of helium (He) and methane (CH4) is placed in an effusion apparatus. Calculate the ratio of their effusion rates. SOLUTION: Mof CH4= 16.04 g/mol Mof He = 4.003 g/mol CH4 He rate rate = 16.04 4.003 = 2.002 PLAN: The effusion rate is inversely proportional to Mfor each gas, so we find the molar mass for each substance using its formula and take the square root. The ratio of the effusion rates is the inverse of the ratio of these square roots.
5-64 Real Gases: Deviations from Ideal Behavior The kinetic-molecular model describes the behavior of ideal gases. Real gases deviate from this behavior.
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