IB Chem Unit 7 (Gas and KMT)

Kinetic Molecular Theory (KMT)

• Gas particles are in constant, random, straight line motion

• There are a lot of space between gas particles

• Gas particles are so small, they occupy NO volume on their own.

• If gas molecules collide, their energy remains constant (elastic collisions).

• Gases do NOT attract one another.  (They also don’t repel each other.

KMT and Temperature

• Absolute temperature (measured in Kelvin) is directly proportional to particles’ kinetic energy

  • At the same temperature, all gases have the same energy.

  • At 0 K particles would be motionless

  • At 50 K particles have ½ the kinetic energy as at 100 K

  • 0 K = -273 ^oC

Ideal vs. Real Gases: KMT Volume

1. Ideal gas particles are so small that the volume of the individual particles if they were at rest is essentially zero when compared to the total volume of the gas

• Ideal Gas Particle

  • Have no volume

• Real Gas Particles

  • Have volume

Ideal vs. Real Gases: KMT Motion

2. Ideal gas particles are in constant, rapid motion, moving in straight lines in all directions until they collide with other particles or the walls of their container.

• Ideal Gas Particles

  • Move constantly in straight lines until collisions occur

• Real Gas Particles

  • May move in curved paths

Ideal vs. Real Gases: KMT Collision

There are no attractive or repulsive forces between particles, and collisions between particles are elastic.

• Ideal Gas Particles

  • No attraction (polarity), do not lose energy during collisions

• Real Gas Particles

  • Have attractions (polarity), can lose energy during collisions

Ideal vs. Real Gases: KMT Condense

3. There are no attractive or repulsive forces between particles, and collisions between particles are elastic.

• Ideal Gas Particles

  • Never condense

• Real Gas Particles

  • As attractions increase, condensation occurs

What conditions will cause a gas to condense?

• Forces of attraction

  • When gases are about to condense they become less like an ideal gas because:

    • Volume of the particles is more significant

    • Attractive forces between particles increase

Four Variables that Describe a Gas

1. Temperature

2. Volume

3. Pressure

4. Amount

Temperature

• Average kinetic energy of gas particles

• Must be in Kelvin

  • Absolute temperature scale

• 0 Kelvin = 0 mL = 0 Pa = 0 Energy

  • All atomic motion stops!

• K = 273 + ºC

• Note no degree symbol-absolute scale!

Volume

• Length x width x height= m x m x m= m³

• 1 dm³ = 1 L

• 1 cm³ = 1 cc (cubic cm) = 1 mL

• Volume can be in any unit but the units must cancel!

Pressure

Pressure is caused by collisions between gas particles and the container walls

Pressure Units

• Force/unit area

• 101325 N/m² =1 atm

• 100 kPa is STP pressure

Amount

• Amount of gas in number of moles

• Number of particles

• STP 

  • 1 mol of any gas = 22.7 dm³

STP

• Standard temperature and pressure

• A standard set of conditions that we use to talk about gases

• Must be easy to reproduce anywhere in the world

• Temperature

  • 0 ^oC

  • 273 K (kelvin)

• Pressure

  • 100 kPa

Instruments

• Manometer 

  • Measures the pressure of an enclosed gas.

• Barometer 

  • Measures atmospheric pressure 

  • A type of manometer

Diffusion of Gases

• Diffusion describes how quickly gases mix with each other.

• Gases tend to move from higher concentrations to lower concentrations.

Effusion of Gases

• Effusion describes the movement of gases from an area of higher concentration into a vacuum

• Gases with lower mass effuse more quickly than gases with higher mass.

• KE = ½ mV²

Graham’s Law of Effusion

• Effusion 

The motion of gas through an opening into an evacuated chamber

The Gas with less mols move faster through a container.

Boyle’s Law - Pressure and Volume

• When the temperature is constant, the pressure and volume of a gas are inversely related.

• As one goes up the other goes down.

P₁V₁ = P₂V₂

Charles’ Law - Volume and Temperature

• When the pressure is constant, the volume and temperature of a gas are directly related.

• As one increases the other increases.

Kelvin Conversion

Gay - Lussac’s Law - Pressure and Temperature

• When the volume is constant, the pressure and temperature (in kelvin) of a gas are directly related.

Combined Gas Law

• Individual gas laws are combined

• The number of molecules remains constant; everything else changes.

• Pressure, volume & temperature may change.

• This law allows us to figure out one variable when two of the others change.

Ideal Gases

• To use the gas laws, we assume that gases behave ideally:

  • In reality, gases do not behave this way.

    • We make assumptions to make the math easier.

Characteristics of ideal gases:

• Particles have no individual volume

• No attractive forces between particles

• Real gases behave this way ONLY at high temperatures and low pressures

Ideal Vs. Real Gases

Ideal Gas Law Equation

R value in Ideal Gas Law Equation

Avogadro’s Law

• At constant temperature & pressure, given volumes of gas always contains the same number of particles

  • This means that the volumes of reactants/products are in the same ratio as the coefficients in the balanced equation

  • At STP

    • 1 mol = 22.7 L

  • At room temperature

    • 1 mol = 24.3 L