Chapter 9 - Gases

Gases

• have similar physical properties even when chemically different

• tend to be nonmetallic, low MW substances

• expand spontaneously and are highly compressible (unlike solids & liquids)

• form homogeneous mixtures regardless of identities

• behavior is due to the very small volume take up by gas molecules

Pressure

the amount of force applied to an area
P = F/A

Atmospheric pressure

the weight of air per unit of area

Barometer

Barometer measures atmospheric pressure

Manometer

used tomeasure the difference inpressure betweenatmospheric pressure andthat of a gas in a vessel

Amontons's law or Gay-Lussac's law

• the pressure of a given amount of gas is directly proportional to its temperature on the kelvin scale when the volume is held constant
• 𝑃/𝑇 = constant
• when temperature increases, gas particles move faster and hit the container more often → pressure increases

Charles's Law

• the volume of a given amount of gas is directly proportional to its temperature on the kelvin scale when the pressure is held constant
• 𝑉/𝑇 = constant
• when temperature increases, particles spread out → volume increases

Boyle's Law

• the volume of a given amount of gas held at constant temperature is inversely proportional to the pressure under which it is measured
• 𝑃⁢𝑉 = constant
• if volume decreases → particles collide more → pressure increases
• if volume increases → fewer collisions → pressure decreases

Avogadro's Law

• for a confined gas, the volume (V) and number of moles (n) are directly proportional if the pressure and temperature both remain constant
  -𝑉/𝑛 = constant
• more gas particles = more space needed → volume increases

Ideal Gas Law

• 𝑃⁢𝑉 = 𝑛⁢𝑅⁢𝑇
• ideal gas is a model that assumes gas particles don't interact and behave perfectly
• real gases act like this best at low pressure and high temperature.

Combined Gas Law

• used when moles stay constant, but P, V, and T all change

STP

• 273.15 K and 1 atm (101.325 kPa).
• at STP, one mole of an ideal gas has a volume of about 22.4 L

Assumptions of Ideal Gas Law

1. There are no interactions between molecules
2. Molecules take up NO space in the container (actually take up about 0.1% by volume)
3. Corrections for non-ideal gas made with the Van der Waals Equation

Partial Pressure

The pressure exerted by each individual gas in a mixture

Dalton's law of partial pressures

• the total pressure of a mixture of ideal gases is equal to the sum of the partial pressures of the component gases

Diffusion

the process by which gas particles spread out from an area of high concentration to low concentration due to their constant random motion

Mean Free Path

average distance travelled by a molecule between collisions

Effusion

the escape of gas molecules through a tiny hole

Key Idea of Diffusion and Effusion

lighter gases diffuse or effuse faster than heavier gases

The Kinetic-Molecular Theory

model that explains gas behavior by describing how gas particles move and interact

1st Postulate of KMT

Gas particles are in constant, random motion, moving in straight lines until they collide with something

2nd Postulate of KMT

The volume of gas particles is extremely small compared to the space between them, so most of a gas is empty space

3rd Postulate of KMT

Gas pressure is caused by collisions of particles with the walls of the container

4th postulate of KMT

Gas particles do not attract or repel each other, and their collisions are elastic (no energy is lost)

5th Postulate of KMT

The average kinetic energy (KE) of gas particles depends only on temperature (in Kelvin)