March 6, 2010


Filed under: CHEMISTRY REVIEW NOTES — Tarebi @ 10:52 pm
  1. Postulates
    • Particles of matter are in a constant state of motion
    • These particles posses kinetic energy because of the constant motion
    • The temperature of a body/substance depend on the kinetic energy of its particles
    • Particles in solids, liquids and gases have different degree of motion
  1. Particles in solid state
    • Closely and firmly packed and held together by strong forces of cohesion
    • Only vibrate and rotate
    • Have definite shape, volume
    • Difficult to compress
  2. Particles in liquid state
    • Not closely packed as in solid state due to weaker forces of attraction
    • Can vibrate, rotate and translocate (i.e move from one place to another)
    • Fixed volume but no definite shape( assumes the shape of their container)
    • Difficult to compress
  3. Particles in gaseous state
    1. Have very high kinetic energy (i.e free to move about)
    2. Cohesive forces are negligible
    3. Have no definite shape ( occupy the shape of their container)
    4. Easily compressed
  1. Change in temperature
    • Main factor for change in the state
    • Particles gain more kinectic energy when heat is applied
    • The following processes can be explained by the kinetic theory of matter
    • Melting
    • Boiling
    • Freezing
    • Evaporation
  2. Melting
    • When heat is applied to a solid, its particles gain more kinetic energy and thus vibrate and rotate  more violently
    • A point is reached when the forces of vibration overcome the forces of cohesion
    • At this point or temperature (called the melting point) the crytalline structure of the solid suddenly breaks down
    • At the melting point a solid changes either into a liquid ( liquification) or gas(sublimation)
    • Note that the presence of an impurity decreases the melting point of the solid
  3. Evaporation
    • At a given temperature, some of the particles in a liquid have more kinetic energy than the others
    • These particles escape from the liquid into the air when  they get to the surface of the liquid
    • This process is called evaporation
    • If evaporation occurs in a closed container, the evaporated particles accumulate above the liquid and exact a pressure called vapour pressure. Some of the vapour particles collide with the liquid surface, loose their kinetic energy and re-enter the liquid. This is called condensation.
    • At equillibrium ( i.e when evaporation equals condensation) the vapour pressure becomes constant. This is called saturated vapour pressure of the liquid
    • The temperature of the liquid decreases if evaporation continues.
  4. Boiling
    • Like in melting, when heat is applied to a liquid, its particles gain kinetic energy and move more freely. Thus increasing the rate of evaporation
    • As the rate of evaporation increases, the saturated vapour pressure also increases
    • The liquid boils at a temperature when the saturated vapour pressure equals the atmospheric pressure
    • The temperature at which boiling ocurs is called the boiling point
    • Note that a rise in atmospheric pressure will increase the boiling point while a fall in atmospheric pressure will decrease the boiling point
    • Presence of impurities in the liquid will also increase the boiling point

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