# Gases & the kinetic molecular theory

## Gases & the kinetic molecular theory

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# Chemistry

Gases & the Kinetic Molecular Theory

Properties of Gases

• Matter exists in three states: solids, liquids, and gases.
• A vapor is a gas that has formed either by the evaporation of a liquid or the sublimation of a solid.
• The density of gases is significantly less than liquids and solids – this indicates that the gas molecules are far apart relative to their size and their interactions are weak
• Gases are easily compressed and fill their container
• The interactions of gases would be minimal b/c of the molecules are so far apart if were not for their rapid motion.

• Properties of Gases:
• Gases can be compressed into smaller volumes; their densities can be increased by increasing pressure
• Gases exert pressure on their surroundings; therefore, pressure must be exerted to be confined
• Gases expand w/out limits; so gases can uniformly and completely occupy the volume of any container
• Gases diffuse into one another; meaning that gases placed in the same container mix completely and cannot separate on standing
• The amounts and properties of gases are describe in terms of temperature, pressure, volume occupied and number of molecules

Basics

• Pressure = force/area
• What is a barometer?
• What is a manometer?
• Know the units of pressure: torr, mm Hg, and atm
• Be able to convert b/t all 3 units of pressure: 1 atm = 760 mm Hg at 0oC = 760 torr
• The SI unit of pressure is the pascal (Pa)
• 1 atm = 101.325 kPa

Boyle’s Law

• Know Boyle’s Law: expresses the relationship between volume and pressure

P1V1 = P2V2  (constant n, T)  Remember:  n = moles

Charles’s Law

• Know Charles’s Law: expresses the relationship between volume and temperature (in Kelvin)
•

(constant n, P)

STP

• Standard Temperature and Pressure (STP) = 0oC (273 K) and 1 atm

Combined Gas Law

• Know the Combined Gas Law

(at constant n)

(at constant T,P)

• Avogadro’s Law led to the Standard Molar Volume: 1 mole of a gas = 22.4 L at STP

Ideal Gas Law

• Know the Ideal Gas Law

• PV = nRT

• R is the Universal Gas Constant
• R = 0.0821 L atm/ mol K
• R = 8.314J/mol K
• You decide which R to use based on the units

• Be able to do solve for molecular weights of gases using the Ideal Gas Law
• By solving for n you can get moles; then using the provided mass solve for the molecular weight

Dalton’s Law

• Know Dalton’s Law of Partial Pressure – the total pressure exerted by a mixture of gases is the sum of the partial pressures of the each gas.

Ptotal = P1 + P2 + P3 + …

• For gases collected over water you must take into consideration that the gas is saturated with water vapor. (Patm is the atmospheric pressure)

Patm = Pgas + Pwater

• So if you are trying to solve for the pressure of the gas:

Pgas = Patm - Pwater

Gas Stoiciometry

• Be able to complete stoichiometric calculations for reactions with gases.
• Remember that for gases 22.4 L = 1 mol at STP
• If the conditions are not at STP you must use the Ideal Gas Law to solve for moles and then do your stoichiometry

KMT

• Know the Kinetic Molecular Theory
• #1.  Gas is composed of particles- usually molecules or atoms
• #2.  Particles in a gas move rapidly in constant random motion
• #3.  Collisions are perfectly elastic, no energy is lost or gained
• #4.  Between collisions, the molecules exert no attractive or repulsive forces on each other

Diffusion/Effusion

• Know what diffusion is.
• Know what effusion is.
• How does molecular size affect effusion/diffusion
• From a group of substances be able to determine which would have greatest rate of effusion/diffusion
• Know Graham’s Law

Real Gases

• Know how real gases deviate from ideal gases
• Non-ideal gas behavior is greatest when the gas is at high pressure and/or low temp. ( near liquefaction)

Source : http://teachers.greenville.k12.sc.us/sites/jhushen/AP%20Chemistry%20Resources/Chapter%205%20Gases/AP%20Chemistry%20Chapter5%20Study%20Guide.doc

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