AP Chemistry Unit 5 - Gases
Common Gases at Room Temperature
Know these!
HCN
H2S
CO
CO2
CH4
C2H4
C3H8
N2 O
NO2
NH3
SO2
toxic
toxic
toxic
methane
ethylene
propane
nitrous oxide
toxic
ammonia
slight odor of almonds
odor of rotten eggs
odorless
odorless
odorless, flammable
ripens fruit
flammable
laughing gas
red, brown gas
pungent
irritating odor
Atmospheric Pressure
the amount of pressure created by the weight of the atmosphere on 1 m2
at sea level
P = F/A
Pressure Units
Atm = 1.01325 x 105 Pa (N/m2) = 760 mm Hg = 760 torr
Boyle’s Law
PV = constant
P1V1 = P2V2
closed container (n constant)
temperature constant
as volume decreases, pressure increases
Charles’ Law
V/T = constant
V1/T1 = V2/T2
pressure is held constant
as temperature increases, volume increases
Avogadro’s Law
V/n = constant
V1/n1 = V2/n2
temperature and pressure are held constant
as number of molecules increase, the volume increases
Gay-Lusaac’s Law
P/T = constant
P1/T1 = P2/T2
volume is constant
as temperature increases, pressure increases
Ideal Gas Law PV = nRT
a combination of all the gas laws
R = 0.0821 L atm / mol K
(R = 8.314 J/mol K)
Calculate the volume of an ideal gas at STP.
A 0.50 mol sample of oxygen gas is confined at 0.0 C in a cylinder with a
moveable piston and an initial pressure of 1.0 atm. The gas is compressed so
that the final volume is half the initial volume and the final pressure is 2.2 atm.
Gas Density and Molar Mass
Calculate the average molar mass of dry air if it has a density of 1.17 g/L
at 21 C and 740.00 torr. (29.0 g/mol)
The safety air bags in cars are inflated by nitrogen gas generated by the
rapid decomposition of sodium azide (NaN3, the other product is
sodium). If an air bag has a volume of 36 L and is filled at a pressure of
1.15 atm at 26.0 C, how much sodium azide must decompose? (72 g)
Dalton’s Law of Partial Pressures
Pt = P1 + P2 + P3 + …
Pt = n1RT/V + n2RT/V + n3RT/V
Pt = n1 + n2 + n3 (RT/V)
Pt = ntRT/V
A gaseous mixture made from 6.00 g of oxygen and 9.00 g of methane (CH4) is
placed in a 15.0 L vessel at 0.0 C. What is the total pressure in the vessel?
(1.122 atm)
Partial Pressure and Mole Fraction
P1 = X1Pt
A synthetic atmosphere of 1.50 mole % CO2, 18.0 mol % O2, and 80.5 mol %
Ar is created. What is the partial pressure of O2 if the total pressure is 745
torr? (134 torr)
If the atmosphere is to be held in a 120 L space at 295 K, how many moles
of O2 are needed? (0.872 mol O2)
Kinetic Molecular Theory
Gases consist of large numbers of molecules that are in continuous,
random motion.
The combined volume of all the molecules is negligible compared to the
volume of the container.
Attractive and repulsive forces are negligible.
Energy can be transferred between molecules during collisions, but the
average kinetic energy remains constant. (Perfectly elastic collisions –
no friction.)
The average kinetic energy is proportional to the absolute temperature.
(At any temp, any gas would have the same average KE.)
Boltzmann’s Equation
as molar mass increases, root-mean-square (rms) speed (u, m/s)
decreases
u
3RT
M
Effusion – the escape of a gas through a tiny hole
Diffusion – the spread of one substance throughout space or another
substance
Graham’s Law of Effusion
r1
r2
M2
M1
r = rate of effusion
M = molar mass
T = temperature (Kelvin)
An unknown composed of homonuclear diatomic molecules effuses at a
rate that is only 0.355 times that of O2 at the same temperature. Identify
the unknown gas. (I2)
Rates of Diffusion
depend on the mean free path of the molecules
Real Gases: Deviations from Ideal Behavior
Ideal gases in the real world only occur at high temperatures and low
pressure
Real gases take up space and have attractions and repulsions between
molecules
van der Waal’s Equation
takes into account real world deviations
P
n2a
V
V2
nb
nRT
b = actual volume of a mole of gas (L/mol)
a = accounts for decrease in pressure due to attractive forces (L2
atm/mol2)
see table, page 429.
Consider 1.000 mole of carbon dioxide gas confined to a volume of 3.00
L at 0.0 C. Calculate the ideal pressure and the pressure with van der
Waal’s equation. (7.473 atm, 7.182 atm)
Summary Problem!
Cyanogen, a highly toxic gas, is composed of 42.6% C and 53.8% N by
mass. At 25 C and 751 torr, 1.05 g of cyanogen occupies 0.500 L. What
is the molecular formula of cyanogen? (C2N2)
Predict its geometry and polarity. (linear, non-polar)