PressurePartial Pressure

Gas StoichiometryPressure = Force/AreaDalton’s Law of Partial

PressuresAvogadro’s Law

What do we already know?

• Kinetic Molecular Theory– 5 assumptions

• No definite shape, no definite volume

• Expansion, diffusion, compressibility

Question

• By the end of the lecture try to answer the question:

• At the same temperature and volume, why do larger gas molecules exert more pressure on their container?

VOlume

• Gas particles will take on the volume of the container they are in

• To measure the volume of the gas, measure the volume of the container it is in

Volume MEasurements

• Boxes = length x width x height• Cylinders = pi x diameter x height

Pressure• Pressure is the force exerted over an

area• Force is measured in Newtons (N)• The Force of gas particles is created

by the moving gas particles hitting the sides of their container

Measuring Pressure• Pressure is measured with a

barometer• The first barometer measured

pressure by measuring how high a gas could raise a column of mercury, thus the units of pressure were: – mm of Hg

Units of Pressure• Like there are different ways to measure weight or

length, there are different ways to measure pressure

• Torr is equal to 1 mm of Hg• The SI unit for pressure is Pascal (Pa)

– 1 pascal = 1 N/m2

– The kilopascal (kPa) is also used

• The most widely used unit is the atmosphere (atm) • it is the average atmospheric pressure at sea level and 0oC

Standard Temperature and Pressure

• STP

• Comparing conditions at 1 atm and 0oC

Unit conversions• 1 atm =

– 760 mm Hg = – 760 torr = – 1.013 x 105 Pa = – 101.3kPa

1atm 760 mm Hg 760 torr

1.013 x 105 Pa

101.3kPa

760 mm Hg101.3kPa

1.013 x 105 Pa

760 torr

760 torr

1atm 1atm

1.013 x 105 Pa

101.3kPa

Dalton’s Law of Partial Pressures• “The total pressure of a mixture of gases is equal

to the sum of the partial pressures of the component gases”

• What does this mean?• Remember that we assume that gas molecules

are not affected by each other• Therefore, each type of molecule will act

independently of any other type• To find the total pressure, we just add the

pressures of the individual types of molecules together

Dalton’s Law of Partial Pressures• Pressuretotal = PressureA + PressureB

Finding Partial Pressures• PressureA = Pressuretotal - PressureB

Partial Pressures Last Comments• Partial pressures must be in the same units to be

compared• The number of pressures in a mixture does not

affect the Dalton’s law of partial pressures equation

Stoichiometry of Gases• What do we already know about stoichiometry?• Using chemical equations to know the ratios between

different compounds

• N2H4 + 2H2O2 -> N2 + 4H2O• 1 mol N2H4 for 2 mol H2O2

• 1 mol N2H4 for 1 mol N2

• 1 mol N2H4 for 4 mol H2O• 2 mol H2O2 for 1 mol N2

• 2 mol H2O2 for 4 mol H2O• 1 mol N2 for 4 mol H2O

Avogadro’s Law• “equal volumes of gases at the same temperature

and pressure contain equal numbers of molecules”\• What does this mean?

• H2(g) + Cl2(g) -> 2HCl(g)– 1 mol H2 = 1 mol Cl2 = 2 mol

HCl

– 1 molecule H2 = 1 molecule Cl2 = 2 molecules HCl

– 1 Volume H2 = 1 volume Cl2 = 1 volumeHCl

Standard Molar Volume• “the volume occupied by 1 mol of a gas at STP”• What does this mean?

– At standard temperature and pressure (STP) 1 mol of gas will always occupy the same volume

• 22.4 L / 1 mol at STP• This is a conversion factor you have seen before

Review• Pressure is Force over area• The partial pressures of a mixture of gases are

added together to form the total pressure of the mixture

• Avogadro’s law compares mols, volumes, and molecules of gases in a balanced chemical equation

• At STP, 1 mol of gas occupies 22.4 L

• At the same temperature and volume, why do larger gas molecules exert more pressure on their container?