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Properties of Gases

Properties of Gases. Kinetic-Molecular Theory Based on particle motion Gas particles are tiny compared to the great distances between them O 2 molecules travel 20,000 times diameter between collisions (6’ diameter – 22.7 miles)

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Properties of Gases

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  1. Properties of Gases • Kinetic-Molecular Theory • Based on particle motion • Gas particles are tiny compared to the great distances between them • O2 molecules travel 20,000 times diameter between collisions (6’ diameter – 22.7 miles) • Travel constantly, at extremely high velocity in all directions at many different speeds • Collisions constantly change their speed and direction

  2. Properties of Gases • Particles do not interact with each other or the walls of their container except during momentary collisions. • Chemical, gravitational, electrical forces can be ignored. Many collisions - 5,000,000,000/second at 0C. • Collisions between particles are elastic • Net kinetic energy unchanged • Average kinetic energy directly proportional to temp of gas in K • As temp rises, particles move faster, collide more

  3. Physical Properties of Gases • Diffusion – gases fill entire volume of container over time • Effusion – gas particles pass thru tiny opening into an evacuated space • Graham’s law of effusion • R = 1 √Molar mass

  4. Physical Properties of Gases • Permeability • Gas can pass thru another porous substance • Why balloons go flat and your lungs work • Fluidity • Can flow and take shape of container • Why there is wind • Compressibility and expansibility

  5. Physical Properties of Gases • Pressure – caused by the force of billions of collisions on a boundary • No boundary – no pressure • There is gas in space – but there is no boundary • Measure – force per unit area • Inside a balloon – many billions of collisions per second • Pressure always perpendicular to the surface

  6. Physical Properties of Gases • Measures of pressure • PSI – force (lbs) per unit area (in2) • Millimeters of mercury or torr • How high atmospheric pressure causes mercury to rise in a column • Atmosphere – 1 Atm = normal atmospheric pressure at sea level at latitude 45 • 2.0= double, 0.5 = ½

  7. Physical Properties of Gases • Measures of pressure • SI unit – Pascal • Force of one Newton acting on one square meter • Conversion factors • 1atm 1atm 760 torr 1 atm 760 torr 14.7 psi 14.7psi 101,325 Pa

  8. Physical Properties of Gases • Pressure, Volume and Temperature • Pressure up, volume down (inverse) temp same • Temp up, volume up—down, volume down • Volume same, temp increase – pressure increase • Temp down – pressure down • PRESSURE

  9. Gas Laws and Formulas • Boyle’s Law – Pressure and Volumes • The volume of a gas is inversely realted to the pressure if the temperature is held constant. • PV = k • P = pressure, V = volume, k = a constant • So, P1V1 = k = P2V2 • For any value of P, V will relate such that k remains constant and vice versa

  10. Gas Laws and Formulas • Charles’ Law– Temperature and Volumes • When the pressure on a sample of a dry gas is held constant, the Kelvin temperature and the volume are directly related. • V = k T • T = Temperature V = volume, k = a constant • So, V1 = k = V2 • T1 T2 • As V increases or decreases, T must do the same so that k will remain constant

  11. Gas Laws and Formulas • Gay-Lussac’s Law Temperature and Pressures • Pressure is directly proportional to temperature in kelvins for a fixed mass of gas held in a constant volume. • P = k T • T = Temperature P = Pressure, k = a constant • So, P1 = k = P2 • T1 T2 • As P increases or decreases, T must do the same so that k will remain constant

  12. Gas Laws and Formulas • Combined Gas Law • Boyle’s, Charles, and Gay-Lussac’s require something to be constant. • Boyle’s – temperature • Charles – pressure • Gay-Lussac – mass and volume • What to do when things aren’t constant? • Combined gas law • Combination equation taking into account all three variables • PV = k • T

  13. Gas Laws and Formulas • Dalton’s Law of Partial Pressures • Gases are seldom pure, there are always contaminants • The total pressure of a mixture of gases equals the sum of the partial pressures • Ptotal = P1+P2+P3+P…….. • Gas collection over water introduces water vapor • Pressure will equal gas collected plus vapor pressure of water at temperature collected • Since it is collected over water and displaces the water under 1atm, total pressure is always atmospheric pressure at your location (see barometer)

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