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Gases

Gases. Kinetic Theory of Matter. Kinetic Theory of Matter. Matter exists as solids, liquids & gases under normal conditions. We can study the behavior of large groups of particles in all 3 states, but not individual particles. Review: Solids. Definite shape Definite volume

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Gases

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  1. Gases Kinetic Theory of Matter

  2. Kinetic Theory of Matter • Matter exists as solids, liquids & gases under normal conditions. • We can study the behavior of large groups of particles in all 3 states, but not individual particles

  3. Review: Solids • Definite shape • Definite volume • Particles close together, fixed • Particles move very slowly • High density • Hard to expand/compress

  4. Liquids • Indefinite shape, definite volume • Take the shape of container • Particles are close together, but mobile • Particles move slowly • High density • Hard to expand/compress

  5. Gases • Indefinite shape • Indefinite volume • Take the shape and volume of container • Particles are far apart • Particles move fast • Low density • Easy to expand and compress

  6. Kinetic Theory (late 19th century) • “Kinetic” from Greek kinetikos = moving • Based on idea that particles of matter are in constant motion---and this motion has consequences • Explains the properties of matter in terms of the energy the particles possess and the forces that act between them.

  7. Kinetic Theory • Model based on an ideal gas: • Imaginary gas that conforms perfectly to all of the assumptions of kinetic theory • Particles thought of as point masses with no volume or forces of attraction • A real gas can approach ideal behavior under conditions of high temperature and low pressure

  8. Five Assumptions • Gases consist of large numbers of tiny particles. • Volume occupied by a gas is 1000 times that of a liquid or solid of equal mass or number of particles.

  9. Particles of a gas are in constant, random, rapid, straight-line motion and thus possess kinetic energy • The collisions between particles of a gas and between particles and the container walls are elastic collisions (no net loss of kinetic energy)

  10. There are no forces of attraction or repulsion between the particles of a gas.

  11. The average kinetic energy of the particles of a gas is directly proportional to the Kelvin temperature of the gas K.E. = 1/2mv2 At absolute zero, all movement stops (no K.E.)

  12. Maxwell Boltzman Distribution Scale • In a sample of gas, particles have the same mass, but are moving at different speeds. • The particles have varying amounts of energy, much like students during the passing periods in the halls

  13. # of particles Change in conditions affects the distribution of particles

  14. The Kinetic Theory explains a number of observed properties of gases

  15. A gas has no definite shape or volume, but fills its container Why? Gas particles are independently moving in all directions, with no attraction or repulsion Expansion Open valve

  16. Ability of particles to flow, or glide past each other Why? No attractive forces; particles in constant motion Fluidity

  17. Gases are about 1000 times less dense than liquids or solids Particles are much further apart---10 diameters apart Low Density

  18. Ability to reduce the volume of a sample of gas Why? Great distance between particles results in lots of empty space. It is possible to crowd molecules closer together. Compressibility

  19. Spontaneous mixing of two substances Why? Particles are in random, constant motion; no forces of attraction Diffusion

  20. Process by which gas particles under pressure pass through a small opening Why? Particles in constant motion; No forces of attraction Effusion

  21. Deviations of Real Gases from Ideal Behavior • Why? • Particles dooccupy space • Particles do exert attractive and repulsive forces upon each other

  22. Greatest Deviations • Low temperature and high pressure • Low temp→low K.E. →low velocity → particles feel attractive and repulsive forces • High pressure →closer contact → particles feel attractive and repulsive forces

  23. Why do real gases approach ideal behavior at high temp and low pressure? • Greater speed • Particles far apart

  24. Real gases vary in attractive forces • Noble gases & diatomics: very little • Polar gases (NH3, H2O): great attraction, therefore great variance from ideal behavior

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