6. Gases

Question 1

A gas the directly follows the kinetic-molecular theory is said to be an ideal gas, what are the 4 tenets of this theory?

Answer 1

1. Gas takes up essentially no volume
2. Gas molecules are in constant motion. Their collisions with the container wall define pressure.
3. All collisions are elastic, thus total KE is conserved. Therefore, gases do not experience IMFs
4. the average kinetic energy of a gas is proportional to its temperature (K)

Question 2

What is the conversion from 1 cm^3 to liters?

Answer 2

1000 cm^3 = 1L

1000L = 1m^3

Question 3

How do you find Kelvin from Celsius?

Answer 3

C + 273

Room temperature = 298K

Question 4

t or f,

1 atm = 760 torr = 760 mmHg = 100 kPa

Answer 4

True, these are the units for pressure

pressure = force per unit area

Question 5

What is standard temperature and pressure (STP)?

Answer 5

T = 273K, P = 1 atm

standard conditions…
T = 298K, P = 1atm, [] = 1M

Question 6

What is the ideal gas law equation? Explain each variable.

Answer 6

PV = nRT

P = pressure in atm
V = volume in liters 
n = moles
T = Temperature in K

R =0.082 L atm / mol K

Question 7

In many cases, the moles of a gas remain constant through a process. What is Charles’s law?

Answer 7

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.

Question 8

In many cases, the moles of a gas remain constant through a process. What is Boyle’s law?

Answer 8

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)

Question 9

In many cases, the moles of a gas remain constant through a process. What happens if you keep volume constant?

Answer 9

PV = nRT

If volume is held constant, pressure is proportional to temperature

my basketballs get more pressure when its hot outside.

Question 10

What is isobaric, isothermal, and isochoric?

Answer 10

isobaric = pressure remains constant (Charles's law)
isothermal = temperature remains constant (Boyle's law)
isochoric = volume remains constant

Question 11

What is the combined gas law?

Answer 11

If we know that moles (n) remains constant, we can combine Charles’s and Boyle’s’ laws.
capitalized = initial

PV / T = pv / t

Question 12

What is Avogadros law of gases?

Answer 12

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.

Question 13

What is standard molar volume of any gas at STP?

Answer 13

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.

Question 14

t or f, whenever moles are constant, use the combined gas law.

Answer 14

True

remember that temperature is always on the bottom

Question 15

Real gases: Why is the pressure of real gases typically less than the pressure of ideal gases?

Answer 15

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

Question 16

Real gases: Why is the volume of real gases typically less than ideal gases?

Answer 16

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

Question 17

When do gases behave like real gases?

Answer 17

At high pressures (decrease volume) and low temperatures (decreases pressure)

Question 18

What is Van der Waals equation?

Answer 18

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.

Question 19

How would the α in Van der Waals equation for H2O(g) compares to hexane gas?

Answer 19

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)

Question 20

What kind of gas behaves most ideally?

Answer 20

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

Question 21

What is Daltons law of partial pressures?

Answer 21

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

Question 22

What behaves most ideally, O2(g), Ar(g), CH4(g)?

Answer 22

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

Question 23

What is effusion?

Answer 23

Effusion is the escape of gas molecules in a pressurized space through a tiny valve / hole.

Question 24

true or false, if two gases in a container are at the same temperature, they have the same kinetic energy.

Answer 24

True, according to KE = 1/2mv^2, the smaller molecules must be moving faster.

Question 25

t or f, the faster molecules move around and collide, the faster they effuse.

Answer 25

true

Question 26

What is grahams law of effusion?

Answer 26

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.

Question 27

t or f, if only one kind of gas occupies a container, all molecules move at the same speed.

Answer 27

FALSE. They have an average kinetic energy due to an average speed. Some molecules may travel slightly faster, some slightly slower.

Question 28

What effuses slowest, F2, Cl2, CO2, or N2?

Answer 28

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.

Question 29

If temperature of gas A increased from 10C to 160C, how does the KE of gas A change?

Answer 29

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.