Kinetic Molecular Theory

1
Kinetic Molecular Theory

• Number of molecules
• Temp
• Volume
• Pressure
• Number of dancers
• Beat of music
• Size of room
• Number and force of collisions

2
Kinetic Molecular Theory

• Accounts for behavior of atoms and molecules
• Based on idea that particles are always moving
• Provides model for an ideal gas
• Ideal Gas Imaginary Fits all assumptions of
  the K.M theory
• Real gas Does not fit all these assumptions

3
5 assumptions of Kinetic-molecular Theory

1. Gases large numbers of tiny particles that are
   far apart.
2. Collisions between gas particles and container
   walls are elastic collisions (no net loss in
   kinetic energy).
3. Gas particles are always moving rapidly and
   randomly.
4. There are no forces of repulsion or attraction
   between gas particles.
5. The average kinetic energy of gas particles
   depends on temperature.

4
Physical Properties of Gasses

• Gases have no definite shape or volume they
  take shape of container.
• Gas particles glide rapidly past each other
  (fluid).
• Gases have low density.
• Gases are easily compressed.
• Gas molecules spread out and mix easily

5

• Diffusion mixing of 2 substances due to random
  motion.
• Effusion Gas particles pass through a tiny
  opening..

6
Real Gases

• Real gases occupy space and exert attractive
  forces on each other.
• The K-M theory is more likely to hold true for
  particles which have little attraction for each
  other.
• Particles of N2 and H2 are nonpolar diatomic
  molecules and closely approximate ideal gas
  behavior.
• More polar molecules less likely to behave like
  an ideal gas. Examples of polar gas molecules
  are HCl, ammonia and water.

7
Gas Behavior

• Particles in a gas are very far apart.
• Each gas particle is largely unaffected by its
  neighbors.
• Gases behave similarly at different pressures and
  temperatures according to gas laws.

8
Boyles Law

• Pressure goes up if volume goes down.
• Volume goes down if pressure goes up.
• The more pressure increases, the smaller the
  change in volume.

9
Boyles law

• Pressure is the force created by particles
  striking the walls of a container.
• At constant temperature, molecules strike the
  sides of container more often if space is
  smaller.
• V1P1 V2P2
• Squeeze a balloon If reduce volume enough,
  balloon will pop because pressure inside is
  higher than the walls of balloon can tolerate.

10
Charles Law

• As temperature goes up, volume goes up.
• Assumes constant pressure.
• V1 V2
• T1 T2

T , V
11
Charles law

• As temperature goes up volume goes up.
• Adding heat energy causes particles to move
  faster.
• Faster-moving molecules strike walls of container
  more often. The container expands if walls are
  flexible.
• If you cool gas in a container, it will shrink.
• Air-filled, sealed bag placed in freezer will
  shrink.

12
Gay-Lussacs Law

• As temperature increases, pressure increases.
• Assumes volume is held constant.
• P1 P2
• T1 T2
• A can of spray paint will explode near a heat
  source.
• Example is a pressure cooker.

13
Combined Gas Law

• In real life, more than one variable may change.
  If have more than one condition changing, use the
  combined formula.
• In solving problems, use the combined gas law if
  you know more than 3 variables.
• V1P1 V2P2
• T1 T2

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Using Gas Laws

• Convert temperatures to Kelvin!
• Ensure volumes and/or pressures are in the same
  units on both sides of equation.
• STP 0 C and 1 atm.
• Use proper equation to solve for desired value
  using given information.
• V1P1 V2P2 V1 V2 P1 P2 V1P1
  V2P2
• T1 T2 T1 T2
  T1 T2