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MBBCh 1- CHEM1048A > Gas Laws > Flashcards

Gas Laws Flashcards

1
Q

What are the characteristics of gases? (5)

A
  1. Expand to fill their containers.
  2. Are highly compressible.
  3. Have extremely low densities.
  4. Form homogeneous mixtures with other gases.
  5. Only occupy about 0.1% of the volume of their container.
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2
Q

What is pressure?

A

Pressure is the amount of force applied to an area:
• Atmospheric pressure is the. weight of air per unit of area.
P= F/A

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3
Q

What is atmospheric pressure?

A

Atmospheric pressure is the weight of air per unit of area.

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4
Q

How to calculate the magnitude of the atmospheric pressure? (2)

A

F=ma (Newtons 2nd Law of Motion)
F = Force; m = mass; a = acceleration
• When applied to the atmosphere, the force is the gravitational force (9.8 m/s2), which is also commonly called weight.
• Gravitational force of the atmosphere on the Earth’s surface F=mg g = 9.8 m/s2

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5
Q

How do you determine the pressure exerted on a column of air? (2)

A
  • To determine the pressure exerted by a column of air on an area of 1m^2 on the surface of the earth we need to calculate the force exerted by all the gas molecules with in that area.
  • The mass of gas with in that column based on the density of air is approximately 10 000 kg or 104 kg.
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6
Q 
Units:
• Pascals – 1 Pa = 
• Bar – 1 bar = 
• 1 psi   = 
• 1 atm = 1.01325 x 105 Pa = 101.325 kPa
= 
=
A 
1 N/m2 = 1kg.m/s2
105 Pa = 100 kPa
6.89 kPa 
760 torr (mmHg) 
14.70 psi (lb/in2)
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7
Q

• mmHg or torr – These units are literally the difference in the _____ measured in mm (h) of
two connected columns of _____.
• Atmosphere– 1.00 atm = 760 torr

A

heights

mercury

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8
Q

What is the manometer used for?

A

• The manometer is used to measure the difference in pressure between atmospheric pressure and that of a gas in a vessel.

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9
Q
  • Normal atmospheric pressure at sea level is referred to as standard pressure.
  • It is equal to : (3)
A

– 1.00 atm
– 760 torr (760 mmHg)
– 101.325 kPa

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10
Q

What is Boyle’s Law?

A

The volume of a fixed quantity of gas at constant temperature is inversely proportional to the pressure.

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11
Q

What are the curves for Boyle’s law?

A

A plot of P versus V displays a hyperbola. Graphs with curved lines are difficult to read accurately at low or high values of the variables, and they are more difficult to use in fitting theoretical equations and parameters to experimental data. For those reasons, scientists often try to find a way to “linearize” their data. Graphically, the relationship between pressure and volume is shown by plotting the inverse of the pressure versus the volume, or the inverse of volume versus the pressure.

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12
Q

What are the applications of Boyle’s Law in terms of the Heimlich Maneuver? (2)

A

> Place your arms around the choking person, and push inwards and upwards on the abdomen with your fists.
There is a rapid decrease in the vol. of the chest which increases P in the lungs,menabling the obstruction in the trachea to be expelled.

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13
Q

Decreasing the volume of a contained gas will increase its pressure, and increasing its volume will decrease its pressure. If the volume increases by a certain factor, the pressure decreases by the same factor, and vice versa. Therefore, pressure and volume exhibit inverse proportionality: Increasing the pressure results in a decrease in the volume of the gas. Mathematically this can be written:

A

where k is a constant.

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14
Q

What is Charles’ law?

A

The volume of a fixed amount of gas at constant pressure is directly proportional to its absolute temperature.

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15
Q

The relationship between the volume and temperature of a given amount of gas at constant pressure is known as Charles’s law. The law states that the volume of a given amount of gas is directly proportional to its temperature on the kelvin scale when the pressure is held constant.

Mathematically, this can be written as:

A

where k is a proportionality constant that depends on the amount and pressure of the gas. For a confined gas at constant pressure, the ratio V/T is constant.

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16
Q

What are the applications of Charles’s Law in terms of Hot Air balloons?

A

A torch is used to heat the air molecules inside the balloon. The molecules move faster and disperse within the space. The gas inside the balloon takes up more space, becoming less dense than the air surrounding it. As such, the hot air inside the balloon rises because of its decreased density and causes the balloon to float.

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17
Q

What are the applications of Charles’s Law in terms of regular balloons?

A

If a balloon is filled with air and sealed, then the balloon contains a specific amount of air at atmospheric pressure (1 atm). If the balloon is placed in a refrigerator, the gas inside gets cold, and the balloon shrinks (although both the amount of gas and its pressure remain constant). If the balloon is made very cold, it will shrink a great deal. When it is warmed up, the balloon will expand again.

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18
Q

What is Avogadro’s law?

A
  • The volume of a gas at constant temperature and pressure is directly proportional to the number of moles of the gas.
  • Mathematically, this means V = kn
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19
Q

For a confined gas, the volume (V) and the number of moles (n) are directly proportional if the pressure and temperature both remain constant.

In equation form, this is written as:

A
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20
Q

What is the application of Avogadro’s law in terms of lungs?

A

Lungs expand as they fill with air. Exhaling decreases the volume of the lungs.

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21
Q

What is the application of Avogadro’s Law in terms of balloons?

A

A balloon filled with helium weighs much less than an identical balloon filled with air. Both balloons contain the same number of molecules. Helium atoms have lower mass than either oxygen molecules or nitrogen molecules in air, so the helium balloon is lighter.

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22
Q

What is Gay-Lussac’s Law?

A
  • The pressure of a fixed quantity of gas at constant volume is directly proportional to its absolute temperature.
  • If the temperature of a gas increases, then so does its pressure if the quantity and volume of the gas are held constant.
  • So, 𝑃/𝑇 =𝑘
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23
Q

Example of Gay-Lussac’s Law:

A
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24
Q

Temperature and pressure are _____ related, and this relationship is observed for any sample of gas confined to a constant volume. If the temperature is on the kelvin scale, then P and T are _____ proportional (again, when volume and moles of gas are held constant); if the temperature on the kelvin scale increases by a certain factor, the gas pressure increases by the same factor.

A

linearly

directly

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25
Q

What are the applications of Gay-Lussac’s law?

A

Imagine filling a rigid container attached to a pressure gauge with gas and then sealing the container so that no gas may escape. If the container is cooled, the gas inside likewise gets colder, and its pressure is observed to decrease. Since the container is rigid and tightly sealed, both the volume and number of moles of gas remain constant. If the sphere is heated, the gas inside gets hotter, and the pressure increases.

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26
Q

This pressure-temperature relationship for gases is known as Gay-Lussac’s law. The law states that the pressure of a given amount of gas is directly proportional to its temperature on the kelvin scale when the volume is held constant. Mathematically, this can be written as:

A

where k is a proportionality constant that depends on the identity, amount, and volume of the gas.

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27
Q

For a confined, constant volume of gas, the ratio P/T is therefore constant (i.e., P/T = k). If the gas is initially at ‘Condition 1’ (with P = P1 and T = T1), and changes to ‘Condition 2’ (with P = P2 and T = T2), then

A
28
Q

Combining these four laws yields the ideal gas law, a relation between the pressure, volume, temperature, and number of moles of a gas:

A
29
Q

Here, R is a constant called the ideal gas constant or the universal gas constant. The units used to express pressure, volume, and temperature determine the proper form of the gas constant as required by dimensional analysis. The most commonly encountered values of R are,

A

0.08206 L⋅atm mol–1⋅K–1 and 8.314 kPa⋅L mol–1⋅K–1.

30
Q

If the number of moles of an ideal gas is kept constant under two different sets of conditions, a useful mathematical relationship called the combined gas law (using units of atm, L, and K) is obtained:

A

Both sets of conditions are equal to the product of n × R (where n = the number of moles of the gas and R is the ideal gas law constant).

31
Q
  • We define STP (standard temperature and pressure) = 0C (or 273.15 K), 1 atm.
  • Volume of 1 mol of gas at STP is:
A
32
Q

The ideal gas law is universal, relating the pressure, volume, number of moles, and temperature of a gas regardless of the chemical identity of the gas:

A
33
Q

What does the density gas equation tell us?

A

This equation tells us that gas density is directly proportional to the pressure and molar mass, and inversely proportional to the temperature.

34
Q

We can manipulate the density equation to enable us to find the molecular mass of a gas:

A
35
Q

What is Dalton’s law of Partial Pressures?

A
  • The total pressure of a mixture of gases equals the sum of the pressures that each would exert if it were present alone.
  • In other words, Ptotal = P1 + P2 + P3 + …
36
Q

For the gas mixture shown: What is the total pressure?

A
37
Q

What is the mole fraction?

A

• The mole fraction, X is a dimensionless number that expresses the ratio of the number of moles of one component in a mixture to the total number of moles in the mixture.

38
Q

Thus for component i in a mixture:
𝑿𝒊 = 𝒏𝒊/𝒏𝒕
therefore:

A
39
Q

How do you find the pressure of the desired gas?

A
  • When one collects a gas over water, there is water vapor mixed in with the gas.
  • To find only the pressure of the desired gas, one must subtract the vapor pressure of water from the total pressure.
40
Q

How were Gas Laws developed?

A

Gas Laws were developed from observations.

41
Q

What do the Gas Laws describe?

A

Gas Laws describes how gases behave but not why gases behave the way they do.

42
Q

What does Kinetic Molecular Theory provide a model for?

A

Kinetic Molecular Theory provides a model which explains the physical properties of a gas at a molecular level in terms of the kinetic energy of the gas molecules.

43
Q

While the gas laws summarize the relationships between different properties of ideal gases, the kinetic molecular theory explains why gases follow the laws. The theory is based on a few assumptions or postulates. (5)

A
  1. Gases consist of large numbers of molecules that are in continuous, random motion.
  2. The combined volume of all the molecules of the gas is negligible relative to the total volume in which the gas is contained.
  3. Attractive and repulsive forces between gas molecules are negligible.
  4. Energy can be transferred between molecules duringcollisions, but the average kinetic energy of the moleculesdoes not change with time, as long as the temperature of thegas remains constant.
  5. The average kinetic energy of the molecules is proportional to the absolute temperature.
44
Q

What does the Kinetic Molecular Theory state is the reason for the pressure of a gas? (2)

A
  • The pressure of a gas is caused by collisions of the molecules with the walls of the container.
  • Magnitude of pressure is determined by how often and how forcefully the molecules strike the walls.
45
Q

What does the Kinetic Molecular Theory state about the temperature of a gas? (2)

A
  • The absolute temperature of a gas is a measure of the average kinetic energy of the gas molecules.
  • If the average kinetic energy of the molecules increases, the absolute temperature of the gas increases. – Therefore, molecular motion increases with increasing temperature.
46
Q

Gas molecules have average ____ _____ and thus average speed, but the individual molecules are moving at different speed.

A

kinetic energy

47
Q

What does the peak curve represent?

A

• The peak curve represents the most probable molecular speed.

48
Q

What is rms?

A

• Root-mean-square speed (rms) is the speed of molecules which have kinetic energy identical to
the average kinetic energy of the sample.

49
Q

Using KMT how do we explain:

A. An increase in volume at constant temperature causes pressure to decrease (Boyle’s Law). (3)

A
  • A constant temperature means that the average kinetic energy of the gas molecules remains unchanged. [RMS remains unchanged].
  • When the volume is increased, the molecules must move a longer distance between collisions.
  • Consequently, there are fewer collisions within the container walls, resulting in pressure decrease.
50
Q

Using KMT how do we explain:

B. A temperature increase at constant volume causes pressure to increase (Gay-Lussac’s Law). (3)

A
  • An increase in temperature means an increase in the average kinetic energy of the molecules and RMS. [Due to constant volume, the pressure increases causes more collisions with the walls within the container].
  • This also results in the molecules striking the walls more forcefully.
  • A greater number of more forceful collisions means the pressure increases, and the KMT explains this increase.
51
Q

• How is the RMS speed of N2 molecules in a gassample changed by:

a) An increase in temperature
b) An increase in volume
c) Mixing with a sample of Ar at the same temperature?

A

a) Increases
b) No effect
c) No effect

52
Q

Using KMT how do we explain: Charles’ Law

A

Charles’s law: If the temperature of a gas is increased, constant pressure may be maintained only if the volume occupied by the gas increases. This will result in greater average distances traveled by the molecules to reach the container walls, as well as increased wall surface area. These conditions will decrease both the frequency of molecule-wall collisions and the number of collisions per unit area, the combined effects of which balance the effect of increased collision forces due to the greater kinetic energy at the higher temperature.

53
Q

Using KMT how do we explain: Avogadro’s law

A

Avogadro’s law: At constant pressure and temperature, the frequency and force of molecule-wall collisions are constant. Under such conditions, increasing the number of gaseous molecules will require a proportional increase in the container volume in order to yield a decrease in the number of collisions per unit area to compensate for the increased frequency of collisions.

54
Q

Using KMT how do we explain: Dalton’s law

A

Dalton’s law: Because of the large distances between them, the molecules of one gas in a mixture bombard the container walls with the same frequency, whether other gases are present or not, and the total pressure of a gas mixture equals the sum of the (partial) pressures of the individual gases.

55
Q

The kinetic energy (KE) of a particle of mass (m) and speed (u) is given by:

A
56
Q

urms, is defined as the square root of the average of the squares of the velocities with n = the number of particles:

A
57
Q

The average kinetic energy for a mole of particles, KEavg, is then equal to:

A

or KE = 3/2 RT

58
Q

These two separate equations for KEavg may be combined and rearranged to yield a relationship between molecular speed and temperature:

A
59
Q

There are several different equations that better approximate gas behavior than does the ideal gas law. The first, and simplest of these, was developed by the Dutch scientist Johannes van der Waals in 1879. The van der Waals equation improves upon the ideal gas law by adding two terms: one to account for the volume of the gas molecules and another for the attractive forces between them.

A
60
Q

What is effusion?

A

Effusion is the escape of gas molecules through a tiny hole into an evacuated space.

61
Q

What is Graham’s Law of effusion?

A

Graham’s Law states that –the rate of effusion of a gas is inversely proportional to the square root of the molar mass of the gas.

62
Q

The rate of effusion of two gases r1 and r2 with molar masses M1 and M2 is given by,

A
63
Q

What is diffusion?

A

Diffusion is the spread of one substance throughout a space or throughout a second substance.

64
Q

What is the rate of diffusion equation?

A
65
Q

The rate of effusion of a gas is inversely proportional to the square root of the mass of its particles:

This means that if two gases, A and B, are at the same temperature and pressure, the ratio of their effusion rates is inversely proportional to the ratio of the square roots of the masses of their particles:

A

MBBCh 1- CHEM1048A (8 decks)

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