GASESGASES::General Concepts

Sherrie ParkSherrie Park

Per. ¾Per. ¾

AP ChemistryAP Chemistry

General ConceptsGeneral Concepts::Charles, Boyles, Avogadro, Charles, Boyles, Avogadro,

& the Ideal Gas Law& the Ideal Gas Law•Charle’s LawCharle’s Law::

•states that if a given quantity of gas is held at a states that if a given quantity of gas is held at a constant pressure, its volume is directly constant pressure, its volume is directly proportional to the absolute temperature proportional to the absolute temperature •As the temperature of the gas increases, the gas As the temperature of the gas increases, the gas molecules will begin to move around more molecules will begin to move around more quickly and hit the walls of their container with quickly and hit the walls of their container with more force—thus the volume will increase more force—thus the volume will increase •V1/T1 = V2/T2V1/T1 = V2/T2

Boyle’s LawBoyle’s Law states that the volume of a confined gas at a fixed states that the volume of a confined gas at a fixed

temperature is inversely proportional to the pressure temperature is inversely proportional to the pressure

exerted on the gas.exerted on the gas. – This makes sense if you think of a balloon. When This makes sense if you think of a balloon. When

the pressure around a balloon increases, the the pressure around a balloon increases, the volume of the balloon decreases, and likewise, volume of the balloon decreases, and likewise, when you decrease the pressure around a balloon, when you decrease the pressure around a balloon, its volume will increase.its volume will increase.

Boyle’s law to can also be expressed in the Boyle’s law to can also be expressed in the following way, and this is the form of the law following way, and this is the form of the law that you should memorize:that you should memorize:

PP11VV1 = 1 = PP22VV22

Avogrado’s LawAvogrado’s Law The volume of a gas at a given temperature The volume of a gas at a given temperature

and pressure is directly proportional to the and pressure is directly proportional to the quantity of the gas. quantity of the gas.

Equal volumes of gases under the same Equal volumes of gases under the same conditions of temperature and pressure conditions of temperature and pressure contain equal numbers of moleculescontain equal numbers of molecules . .

VV = constant = constant nn (where (where nn is the number of is the number of moles of the gas)moles of the gas)

IDEAL GAS LAWIDEAL GAS LAW:: The ideal gas law is the most important gas law for you to The ideal gas law is the most important gas law for you to

know: it combines all of the laws previously.know: it combines all of the laws previously.The state of an amount of gas is determined by its The state of an amount of gas is determined by its

pressure, volume, and temperature according to the pressure, volume, and temperature according to the equation:equation: PV = nRT PV = nRT

– P = Pressure (units: Atm, Pa, Torr, etc..)P = Pressure (units: Atm, Pa, Torr, etc..)– V = Volume (units: L)V = Volume (units: L)– N = Moles (units: mols)N = Moles (units: mols)– R = Gas Constant (depends on the units used in the R = Gas Constant (depends on the units used in the

formula)formula)– T = Temperature (unit: Kelvin)T = Temperature (unit: Kelvin)

An Ideal Gas is modeled on the Kinetic Theory of An Ideal Gas is modeled on the Kinetic Theory of Gases which has 4 basic postulates Gases which has 4 basic postulates

– All gas particles are in constant, random motion. All gas particles are in constant, random motion. – All collisions between gas particles are perfectly All collisions between gas particles are perfectly

elastic (meaning that the kinetic energy of the elastic (meaning that the kinetic energy of the system is conserved). system is conserved).

– The volume of the gas molecules in a gas is The volume of the gas molecules in a gas is negligible. negligible.

– Gases have no intermolecular attractive or Gases have no intermolecular attractive or repulsive forces. repulsive forces.

– The average kinetic energy of the gas is directly The average kinetic energy of the gas is directly proportional to its Kelvin temperature and is the proportional to its Kelvin temperature and is the same for all gases at a specified temperature. same for all gases at a specified temperature.

Real Gases deviate from Ideal Gas Real Gases deviate from Ideal Gas Behaviour because Behaviour because

– at low temperatures the gas molecules have less at low temperatures the gas molecules have less kinetic energy (move around less) so they kinetic energy (move around less) so they dodo attract each other attract each other

– at high pressures the gas molecules are forced at high pressures the gas molecules are forced closer together so that the volume of the gas closer together so that the volume of the gas molecules becomes significant compared to the molecules becomes significant compared to the volume the gas occupiesvolume the gas occupies