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States of Matter. I will use the kinetic-molecular theory to explain the physical properties of gases, liquids and solids I will compare types of intermolecular forces I will explain how kinetic energy and intermolecular forces combine to determine the state of a substance

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States of Matter

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States of matter

States of Matter

  • I will use the kinetic-molecular theory to explain the physical properties of gases, liquids and solids

  • I will compare types of intermolecular forces

  • I will explain how kinetic energy and intermolecular forces combine to determine the state of a substance

  • I will describe the role of energy in phase changes

13 1 gases

13.1 Gases

  • I will use the Kinetic-molecular theory to explain the behavior of gases

  • I will describe how mass affects the rates of diffusion and effusion

  • I will explain how gas pressure is measured and calculate the partial pressure of a gas

  • Vocabularykinetic-molecular theoryelastic collision

  • temperatureGraham’s Law of effusiondiffusion

  • Pressurebarometerpascalatmosphere

  • Dalton’s law of partial pressures



  • Substances that are gases at room temperature

  • usually display similar physical preperties despite

  • their different compositions.

  • Why is there so little variation in behavior among gases?

Kinetic molecular theory

Kinetic-Molecular Theory

Kinetic molecular theory1

Kinetic-Molecular Theory

  • Describes the behavior of gases in terms of

  • particles in motion

  • Makes several assumptions about gas particles:

  • Size

  • Motion

  • Energy

Gas assumptions

Gas Assumptions

  • Particle Size

  • Separated by empty space

  • Volume of particle = small

  • Volume of empty space = large

  • NO significant attractive or repulsive forces

  • Particle Motion

  • Constant random motion

  • Move in a STRAIGHT line

  • Collide with walls or other particles

  • Collisions are elastic (NO kinetic energy is lost, just transferred)

Gas assumptions cont

Gas Assumptions Cont...

  • Particle Energy

  • Affected by mass and velocity

  • KE = ½ mv2

  • Single gas

    • Particles have same mass

    • Particles have different velocity

    • Particles have different KE

  • Temperature = measure of the average KE of the particles in a sample of matter

    • At a given temperature, ALL gases have the SAME average KE

Gas behavior

Gas Behavior

Gas behavior1

Gas Behavior

  • Low Density

  • Compressible

  • Expandable

  • Diffuse

  • Effuse

Gas behavior2

Gas Behavior

  • Low Density

  • Density = mass/volume

  • Large space between gas particles

  • Fewer gas molecules than solid or liquid molecules in the same volume

Gas behavior3

Gas Behavior

Compression & Expansion

  • Large amount of empty space between gas particles

  • Allows particles to be squished into a smaller volume

  • Stop squishing; random motion of particles fills the available space, expands to original volume

Gas behavior4

Gas Behavior


Particles diffuse

from areas of high concentration to areas of low concentration

Rate of diffusion

Depends on mass of particles

Lighter = faster

Heavier = slower

  • The movement of one material through another material

  • Gas particles have no significant forces of attraction

  • Particles can slide past each other

  • Mix until evenly distributed

Gas behavior5

Gas Behavior

Gas behavior6

Gas Behavior


  • Related to diffusion

  • When a gas escapes through a tiny opening

  • Ex. Puncture a balloon or tire

  • Inverse relationship between effusion rates and molar mass

Gas behavior7

Gas Behavior

Graham’s Law of Effusion

Also applies to rate of diffusion

Set up a proportion to compare the diffusion rates of 2 gases

  • The rate of effusion for a gas is inversely proportional to the square root of its molar mass

Finding a ratio of diffusion rates

Finding a Ratio of Diffusion Rates

  • Let’s Try!

Practice problems

Practice Problems

  • RN2/RNe = 0.849

  • RCO/RCO2 = 1.25

  • 2.5mol/min

Calculate the ratio of effusion rates for nitrogen (N2) and Neon (Ne).

Calculate the ratio of diffusion rates for carbon monoxide (CO) and carbon dioxide (CO2)

What is the rate of effusion for a gas that has a molar mass twice that of a gas that effuses at a rate of 3.6 mol/min?

Use to

find ratios

Gas pressure

Gas Pressure

Gas pressure1


Gas Pressure

  • Force per unit area

  • Gas particles exert pressure when they collide with:

  • The walls of their container

  • Each other

  • Exert pressure in ALL directions (because particles move in all directions)

Individual particles can only exert little pressure

Many particles colliding can exert substantial pressure

pressure of gases is what keeps our tires inflated, makes our basketballs bounce, makes hairspray come out of the can, etc.

Gas pressure2

Gas Pressure

  • Pressure increases when temperature increases because the molecules are moving with greater speed and colliding against the sides of their containers more often.

  • Therefore, the pressure inside that container is greater, because there are more collisions.

Gas pressure3

Gas Pressure

Atmospheric Pressure

Varies by elevation

Mountains = less

Sea Level = more

  • Air pressure at Earth’s surface

  • Equal to:

  • Pressure exerted by 1 kg mass on a square centimeter

Gas pressure4

Gas Pressure

Measuring Air Pressure

Air pressure varies because of:

Changes in air temperature

Changes in humidity

Increase in air pressure = Hg rises

Decrease in air pressure = Hg falls

  • Barometer

  • Measures atmospheric pressure

  • Mercury in it is always about 760mm

    • Exact amount determined by 2 forces

      • Gravity- downward force

      • Air pressure- upward force (air presses on surface of Hg)

Gas pressure5

Gas Pressure

Measuring Enclosed Gas Pressure

  • Manometer

  • Measures pressure of enclosed gas

  • The difference in height of the mercury in the 2 arms is used to calculate the pressure of the gas in the flask

Gas pressure6

Gas Pressure

Units of Pressure

SI Unit = Pascal (Pa)

Atmosphere = atm (used to report air pressure)

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