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Flashcards in gases Deck (14):
1

In reference to the KMT, why can matter in the gas phase be compressed?

The Kinetic Molecular Theory (KMT) postulates that gas phase molecules are in constant motion and compared to the size of the individual molecules, there is significant space between the particles.

A gas is compressed when gas phase molecules are pushed closer together. Matter in the liquid and solid phase is essentially not compressible because there is no space between the solid and liquid molecules - they cannot be pushed closer together.

2

In reference to the KMT, why is the volume of matter in the gaseous phase determined by the size of the container?

The Kinetic Molecular Theory (KMT) postulates that gas phase molecules are in constant motion and compared to the size of the individual molecules, there is significant space between the particles.

The volume of a gas is therefore detemined by the space occupied by these moving molecules and this space is dictated by the size of the container. If the size of the container is changed (e.g. a plunger of a syringe is pushed in or out) the molecules will adjust to fully occupy the given space. By contrast, the volume of a liquid and a solid is determined by the size and arrangement of the molecules only (essentially there is no additional space between the molecules).

3

Matter in the gas phase can be completely described by what four parameters?

Matter in the gas phase can be completely described by articulating the pressure, volume, temperature and amount (in moles).

4

Why are so many different units for pressure used?

The fact that multiple units for pressure are used (needed) is in part historical and connected to how measurements were made originally made.

For our purpose 1 atm = 101.3 kPa = 760 torr = 760 mm Hg. The reference to torr and mm Hg is related to the fact that pressure was originally measured using a Torricelli barameter, which involved measuring the height of a column of mercury in millimeters. These units are still useful in being able to detemine the pressure of a gas which has been collected by the displacement of water.

A Pascal is a useful unit in that it is a SI derived unit based on the definition of pressure (force/area). 1 Pa = 1 N/m2. Note that we will typically use atm as units in the CHEM 112 course (in relation to the ideal gas law constant R = 0.08206 L atm/mol K).

5

Define ideal gas.

An ideal gas is a gas whose behaviour is consistent with the assumption articulated by the KMT (gas phase molecules are in constant motion compared to the size of the individual molecules, there is significant space between the particles, and the average kinetic energy of the gas molecules is directly proportional to the temperature in kelvin).

One can also say that an ideal gas is one whose parameters (pressure, volume, temperature and amount) can be accurately described by the ideal gas law (PV=nRT). For gas problems in CHEM 112, we will assume that gases behave ideally.

Non-ideal behaviour occurs at relatively low temperatures and high pressures when the distance between the molecules becomes small compared the size of the individual molecules which comprise the gas.

6

State and explain (in term of KMT) the relationship of pressure and volume for an ideal gas at constant amount and temperature.

The pressure of a gas is the result of molecules colliding with the walls of the container. At constant temperature and amount, if the volume is decreased, this will cause the molecules to collide more frequently with the walls of the container. Pressure and volume are said to be inversely proportional.

7

State and explain (in term of KMT) the relationship of temperature and volume for an ideal gas at constant amount and pressure.

Given that the average KE is directly proportional to temperature for an ideal gas, if the temperature is increased, molecules will be moving faster and with more momentum. Constant pressure can only be maintained if the volume of the container increases proportionately. Temperature and volume are said to be directly proportional.

8

State and explain (in term of KMT) the relationship of amount and volume for an ideal gas at constant temperature and pressure.

Adding more molecules to a closed container will potentially result in greater collisions with the walls of the containter. Constant pressure can only be maintained, if the volume of the container increases proportionately. Amount and volume are said to be directly proportional.

9

Compare the densities for He, Ne and Ar all at STP.

At a given temperature and pressure, density is proportional to the molar mass of a gas. Therefore, in increasing order: He < Ne < Ar.

10

Why does the density (at constant pressure) of a gas depend on temperature?

Density is a ratio of mass to volume. If the mass (and pressure) are constant, and the temperature is increased, this means that that the volume must increase. In turn the density must decrease, given the same mass but a larger volume.

11

Two containers are at equal volume and temperature. Which one will have a greater partial pressure of the "grey" gas.

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Container b) will have the greater partial pressure of the "grey" gas because container b) has the most molecules of the "grey gas".

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12

Two containers are at equal volume and temperature. Which one will have a greater mol fraction of "grey" gas.

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Container a) will have the greater mol fraction of the "grey" gas because container a) has a mol fraction of 4/10, whereas container b) has a mol fraction of 6/26.

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13

Compare the average KE for He, Ne and Ar all at STP.

Given that all gases are at the same temperature (and average KE is proportional to temperature) all of these gases will have the same average kinetic energy.

14

Compare the relative speeds for He, Ne and Ar all at STP.

Speed depends both on temperature (reflection of KE) and mass. The larger the mass, the slower the molecule. Therefore the relative speed of these gases would be in increasing order Ar < Ne < He.