Princeton Ch 8 - Gases

Question 1

What makes gases unique from solids and liquids.

Answer 1

Unlike the condensed phases of matter (solids and liquids), gases have no fixed volume. A gas will fill all available space, and also unlike solids and liquids, molecules of gas are free to move over large distances.

Question 2

The most important macroscopic properties of a gas are three things:

Answer 2

Pressure, volume, and temperature. How these macroscopic properties are related to each other can be derived from basic assumptions about the behavrior of gas at the microscopic level.

Question 3

Kinetic molecule theory - a model for describing the behavior of gases are based on the following assumptions. This is an IDEAL GAS.

Answer 3

1) molecules of gas are so small they themselves take up essentially no volume
2) molecules of gas are in constant motion, moving in straight lines and random directions and colliding with one another and the wall; all collisions are said to be elastic (the total KE is the same after the collision)
3. Since molecules move at a constant speed b/t collisions and the collisions are elastic, the molecules of gas experience no IM forces.
4. the molecules of gas span a distribution of speeds, the average KE is directly proportional to the absolute temp

Question 4

Definition of pressure in terms of gas.

Answer 4

The collisions of molecules with the walls of the container defines the pressure of the gas.

Pressure = force per unit area; measured in Pascal (Pa)
1Pa = 1N/m^2; 

Atmospheric pressure = 100kPa = 1 atm = 760 Torr

Question 5

Units for volume.

Answer 5

cm^3 = 1cc = 1 mL; 1m^3 = 1000L

Question 6

All gas law equations uses kelvins to avoid negative numbers. How to convert K to

Answer 6

T (in K) = T (in C) + 273.15

Question 7

Absolute zero.

Answer 7

The temperature where molecule motion is at a minium.

Question 8

1 atm = ___ Pa = ___ Torr = ____ Hg

Answer 8

1 atm = 760 Torr = 760 mm Hg = 101.3 KPa

Question 9

Ideal gas law

Answer 9

PV = nRT; 
P = the pressure of the gas in atmosphere
V = volume of the container in liters
n = number of moles of the gas
R = the universal gas constant, 0.021atm/K-mol
T = absolute temperature of the gas (K = 273.15)

Question 10

Argon, at P = 2atm, fills a 100M=mL vial at a temp of 0C. What would the P of the argon be if we increase the volume to 500mL, and the temp to 100.

Answer 10

PV/T = P2V2/T2

moles not given but it doesn’t matters - it doesn’t change. R is a constant so that cancels out. nR thus remains constant and we can set the two scenarios equal to one another.

Question 11

Charles law for gases at a constant pressure. This assumption is made when “n” drops out.

Answer 11

If the P is constant, V/T = k (where k is a constant). Therefore volume is proportional to temperature. If the pressure is to remain constant, then a gas will expand when heated and contract when cooled. Temp increase means molecules will move faster, but the pressure is constant because the volume also expands.

Question 12

Boyle’s law for gases at a constant temperature.

Answer 12

If the temperature is constant, PV = k(where k is a constant). Therefore pressure is inversely proportional to the volume. If the volume decreases, the molecules have less space to move around, and they’ll collide more often with the container and the P increases.

Question 13

If the volume, moles, and R are constants, what’s the relationship between temperature and pressure?

Answer 13

If the volume is constant, P/T = k (where k is a constant). Pressure is proportional to temperature. As temperature increases the molecules move faster. The strike the walls of the container surface more often and with greater speed.

Question 14

We can use the following equations to compare properties of gas under two different conditions. In a system with constant n:
At a constant P:
At a constant V:
At a constant T:

Answer 14

At a constant P: V1/T1 = V2/T2
At a constant V: P1V1 = P2V2
At a constant T: P1/T1 = P2/T2

Question 15

Assuming a constant n, we can get the combined gas law:

Answer 15

P1V1/T1 = P1V2/T2

Question 16

What if n, P, V, and T are constants?

Answer 16

Avogadro’s law explains this case. If two equal volume containers how gas at the same P and T, then they contain the same number of particles (regardless of the identity of the gas). This law can be used to determine the standard molar volume of an ideal gas at STP. which is the V that one mole of gas– any ideal gas – would occupy at 0C, 1 atm = 22.4L

Question 17

Name the deviation from ideal gas that has to do with forces inherent to the atoms themselves.

Answer 17

1) Intermolecular forces - particles of a real gas have attractive IM forces but not the same attractive forces to the wall. Increased particle interactions lead to fewer collisions with the walls of the container, and the collisions that do occur will have a smaller transfer of momentum. So P(real) < P(ideal)

Question 18

How does a real gas affect the volume?

Answer 18

The volume is simply the free space the particles have in which to move. For an ideal gas, this volume is simply the volume of the container. However, at HIGHER pressures, the volume occupied by each gas becomes a greater proportion of the gas sample. The overall effect is a DECREASE in volume. V(real) < V(ideal)

Question 19

Give the Vander Waals equation and explain what a and b mean.

Answer 19

(P + an^2/V^2)(V - nb) =nRT;
Both “a” and “b” are van der Waals constants and are generally larger for gases that experience greater IM forces (a) and have larger molecular weights, and therefore volumes (b)