Ch 8 - The Gas Phase

Question 1

How does the density of gases compare to the other phases?

Answer 1

gases are the least dense phase of matter

Question 2

What are the characteristics of gases?

Answer 2

• they are fluids and therefore conform to the shapes of their containers
• easily compressible

Question 3

Which variables describe the gas systems?

Answer 3

• temperature (T)
• pressure (P)
• volume (V)
• number of moles (n)

Question 4

What is 1 atm equivalent to for pressure?

Answer 4

1 atm = 760 mmHg = 760 torr = 101.325 kPa

Question 5

What does a simple mercury barometer measure?

Answer 5

• incident (usually atmospheric) pressure
• as pressure increases, more mercury is forced into the column, increasing its height
• as pressure decreases, mercury flows out of the column under its own weight, decreasing its height

Question 6

What is the difference between STP and standard state?

Answer 6

• STP: 273 K (0 C) and 1 atm

- standard: 298 K (25 C), 1 atm, 1 M

Question 7

What is unique of ideal gases?

Answer 7

• ideal gases have no attractive forces between molecules

- though mass and volume are negligible, they are measurable

Question 8

How much volume does 1 mole of an ideal gas occupy at STP?

Answer 8

• 22.4 L

- regardless of the identity of the gas, equimolar amounts of 2 gases will occupy the same volume and same temperature

Question 9

What is the ideal gas law?

Answer 9

describes the relationship between the 4 variables of the gas state for an ideal gas
PV = nRT

Question 10

What is Avogaadro’s principle?

Answer 10

• a special case of the ideal gas law for which the pressure and temperature are held constant
• it shows a direct relationship between the number of moles of gas and volume
  n/V = k or n/V = n/V

Question 11

What is Boyle’s law?

Answer 11

• a special case of the ideal gas law for which temperature an number of moles are held constant
• it shows an inverse relationship between temperature and volume
  PV = k or PV = PV

Question 12

What is Charle’s law?

Answer 12

• a special case of ideal gas law for which pressure and number of moles are held constant
• shows a direct relationship between temperature and volume
  V/T = k or V/T = V/T

Question 13

What is Gay-Lussac’s law?

Answer 13

• a special case of the ideal gas law for which volume and number of moles are held constant
• shows a direct relationship between temperature and pressure
  P/T = k or P/T = P/T

Question 14

What is the combined gas law?

Answer 14

• a combination of Boyle, Charles, and Gay-Lussac’s laws
• shows an inverse relationship between pressure and volume along with direct relationship between pressure and volume with temperature
  PV/T = PV/T

Question 15

What is Dalton’s law of partial pressures?

Answer 15

• states that individual gas components of a mixture of gases will exert individual pressures in proportion to their mole fractions
• the total pressure of a mixture of gases is equal to the sum of the partial pressures of the component gases
  Pt = Pa + Pb + Pc…
  Pa = XaPt –> Xa = moles of A/total gas moles

Question 16

What is Henry’s law?

Answer 16

states that the amount of gas dissolved in solution is directly proportional to the partial pressure of that gas at the surface of a solution
[A] = kh x Pa or [A]/P = [A]/P = kh

Question 17

What is the kinetic molecular theory?

Answer 17

attempts to explain the behavior of gas particles

- it makes a number of assumptions about the gas particles

Question 18

What assumptions does the kinetic molecular theory make?

Answer 18

• gas particles have negligible volume
• gas particles do not have intermolecular attractions or repulsions
• gas particles undergo random collisions with each other and the walls of the container
• collisions between gas particles (and with the walls of the container) are elastic
• the average kinetic energy of the gas particles is directly proportional to temperature

Question 19

What is Graham’s law?

Answer 19

describes the behavior of gas diffusion and effusion, stating that gases with lower molar masses will diffuse or effuse faster than gases with higher molar masses at the same temperature
r1/r2 = square root (M2/M1)

Question 20

What is the difference between effusion and diffusion?

Answer 20

• diffusion: the spreading out of particles from high to low concentration
• effusion: the movement of gas from one compartment to another through a small opening under pressure

Question 21

What are real gasses deviated from?

Answer 21

ideal behavior under high pressure (low volume) and low temperature conditions

Question 22

What do real gases do at moderately high pressures, low volumes, or low tempeartures?

Answer 22

occupy less volume than predicted by the ideal gas law because they particles have intermolecular attractions

Question 23

What do real gases do at extremely high pressures, low volumes, or low temperatures?

Answer 23

occupy more volume than predicted by the ideal gas law because the particles occupy physical space

Question 24

What is van der Waals equation used for?

Answer 24

to correct the ideal gas law for intermolecular attractions (a) and molecular volume (b)
(P + n^2a/V^2)(V - nb) = nRT
- if a and b are both 0, the equation reduces to the ideal gas law
- a = attractive forces
- bb = ig particles

Question 25

How does partial pressure affect solubility?

Answer 25

the solubility of a gas will increase with increasing partial pressure of the gas

Question 26

How is density calculated with pressures?

Answer 26

density = mass/volume = PM/RT

Question 27

How does temperature and size affect molecular speed?

Answer 27

• the higher the temperature, the faster the molecules
• the larger the molecule, the slower they move
  KE = 1/2 mv^2 = 3/2kT

Question 28

How is root mean square speed calcualted?

Answer 28

Urms = square root (3RT/M)