6. Gases Flashcards
A gas the directly follows the kinetic-molecular theory is said to be an ideal gas, what are the 4 tenets of this theory?
- Gas takes up essentially no volume
- Gas molecules are in constant motion. Their collisions with the container wall define pressure.
- All collisions are elastic, thus total KE is conserved. Therefore, gases do not experience IMFs
- the average kinetic energy of a gas is proportional to its temperature (K)
What is the conversion from 1 cm^3 to liters?
1000 cm^3 = 1L
1000L = 1m^3
How do you find Kelvin from Celsius?
C + 273
Room temperature = 298K
t or f,
1 atm = 760 torr = 760 mmHg = 100 kPa
True, these are the units for pressure
pressure = force per unit area
What is standard temperature and pressure (STP)?
T = 273K, P = 1 atm
standard conditions…
T = 298K, P = 1atm, [] = 1M
What is the ideal gas law equation? Explain each variable.
PV = nRT
P = pressure in atm V = volume in liters n = moles T = Temperature in K
R =0.082 L atm / mol K
In many cases, the moles of a gas remain constant through a process. What is Charles’s law?
PV = nRT
Charles’s law, pressure is held constant. In this case, volume is proportional with temperature
If temperature increases, volume expands to keep pressure constant.
In many cases, the moles of a gas remain constant through a process. What is Boyle’s law?
PV = nRT
Boyle’s law, temperature is held constant. In this case, pressure is inversely proportional to Volume.
If volume increases, pressure must decrease (T is constant)
In many cases, the moles of a gas remain constant through a process. What happens if you keep volume constant?
PV = nRT
If volume is held constant, pressure is proportional to temperature
my basketballs get more pressure when its hot outside.
What is isobaric, isothermal, and isochoric?
isobaric = pressure remains constant (Charles's law) isothermal = temperature remains constant (Boyle's law) isochoric = volume remains constant
What is the combined gas law?
If we know that moles (n) remains constant, we can combine Charles’s and Boyle’s’ laws.
capitalized = initial
PV / T = pv / t
What is Avogadros law of gases?
If pressure, volume, and temperature are constant in two separate containers holding a gas, then the two gases, regardless of their identities, must have an equal number of particles.
What is standard molar volume of any gas at STP?
STP = 1atm, 273K R = 0.082 moles = 1
plug all this into PV = nRT we find that any gas at STP has a standard molar volume of 22.4 L.
t or f, whenever moles are constant, use the combined gas law.
True
remember that temperature is always on the bottom
Real gases: Why is the pressure of real gases typically less than the pressure of ideal gases?
Real gases can have intermolecular bonding. This bonding pulls the gas particles together and reduces collisions with the container wall, effectively reducing pressure slightly.
P-ideal > P-real
Real gases: Why is the volume of real gases typically less than ideal gases?
At higher pressures, the volume occupied by each particle becomes a greater proportion of the gas sample. Eventually volume is no longer negligible. The high pressure reduces the free space of gas movement which reduces volume.
V-ideal > V-real
When do gases behave like real gases?
At high pressures (decrease volume) and low temperatures (decreases pressure)
What is Van der Waals equation?
This equation tries to account for the deviations of the ideal gas law.
(P + α)(V - β) = nRT
P + α accounts for intermolecular forces
V + β accounts for the fact the gases are not volumeless.
How would the α in Van der Waals equation for H2O(g) compares to hexane gas?
the alpha for vaporized water will be larger because water can hydrogen bond (stronger IMF’s).
the beta will be larger for hexane because it is a larger gas (takes up more volume)
What kind of gas behaves most ideally?
Low molecular weight (and volume), weak IMF gases
Remember periodic trends (atomic radii gets larger down and to the left)
while keeping pressure low, T high
What is Daltons law of partial pressures?
The partial pressure of a gas is equal to its mole fraction multiplied by total pressure.
pressure = 10 atm
X mole fraction = 0.8
x partial pressure = 8 atm
What behaves most ideally, O2(g), Ar(g), CH4(g)?
Ar(g) is the smallest according to atomic radius and has no IMFs, so this behaves most ideally.
Oxygen is lighter than argon, but it is diatomic (increased V) and larger radius.
CH4 takes up a lot of space (poly-atomic)
structure must be considered, not just MW
What is effusion?
Effusion is the escape of gas molecules in a pressurized space through a tiny valve / hole.
true or false, if two gases in a container are at the same temperature, they have the same kinetic energy.
True, according to KE = 1/2mv^2, the smaller molecules must be moving faster.
t or f, the faster molecules move around and collide, the faster they effuse.
true
What is grahams law of effusion?
If two gases in a container have the same KE, then KE1 = KE2
(1/2mv^2)1 = (1/2mv^2)2
if we rearrange, we see that the ratio of the gases speeds equals the root of the reciprocal ratio of their masses.
The ratio of their speeds also equals the ratio of their effusion rate. This is grahams law.
t or f, if only one kind of gas occupies a container, all molecules move at the same speed.
FALSE. They have an average kinetic energy due to an average speed. Some molecules may travel slightly faster, some slightly slower.
What effuses slowest, F2, Cl2, CO2, or N2?
When dealing with the ideal gas law, we want to consider molecular size (volume).
With effusion, we are only concerned with MW (granted the effusion hole is large enough) and velocity. Therefore Cl2 MW = 70 grams will diffuse the slowest.
If temperature of gas A increased from 10C to 160C, how does the KE of gas A change?
firstly, when dealing with gases we use kelvins.
283K –> 443K
We know that KE is proportional to T (even though there is no equation for it).
283 / 443 = 7/11
Gas A KE increaed by less than a factor of 2.
