Gases

Question 1

What is Boyle’s law:

Answer 1

at constant temperature, the volume of a fixed amount of gas is reduced in proportion as the pressure increases.

e.g. if you double the pressure, the volume occupied by the gas halved.

Question 2

SI unit of Pressure?

Answer 2

SI unit of Pressure = Pa

Question 3

What is Charles’s law

Answer 3

that for a constant pressure, the volume of a fixed amount of gas increased linearly with rising temperature

the volume of a fixed amount of gas, at constant pressure, is proportional to the absolute temperature.

Question 4

How many Pa is 1 bar equivalent to?

Answer 4

1 bar = 1 x 10^5 Pa

Question 5

What is: Avogadro’s law:

Answer 5

equal volumes of gases at constant temperature and pressure contain equal numbers of molecules.

Question 6

What is the ideal gas equation?

Answer 6

pV = nRT

Question 7

What are the units for each component in the ideal gas equation?

Answer 7

p = Pa
V = m^3
n = mol
R = 8.314 J/ K/ mol
T = K

Question 8

Assumptions on the ideal gas law:

Answer 8

The molecules in the gas have negligible size-point masses.

Whole volume of the container is just the container.

Don’t need to take into account the volume of the molecules themselves.

All collisions between gas molecules and the container walls are elastic and all motion is frictionless.

No energy lost in collisions or motion.

No attractive or repulsive forces between molecules or the surroundings.

The distance between molecules on average is larger than the size of the molecules.

Gas molecules are constantly moving in all directions and at a range of speeds.

Question 9

1 atm is equivalent to how many Torr?

Answer 9

1 atm = 760 Torr

Question 10

What are the standard conditions set by IUPAC?

Answer 10

298.15 K and 1 bar.

Question 11

Root mean squared speed equation is:

Answer 11

root mean squared speed = sqrt( 3RT/ Mr )

Question 12

What does the Maxwell-Boltzmann Distribution show?

Answer 12

it shows a distribution of speeds in a sample of molecules - some faster than the average and some slower than the average.

Question 13

how are the distribution of speeds influenced by molar mass of the gas?

Answer 13

high molar mass = narrow spread of speeds distributed at around a relatively low speed

low molar mass = significant fraction of molecules moving at higher speeds.

Question 14

how are the distribution of speeds influenced by the temperature?

Answer 14

As T increases, the distribution of speeds becomes broader.

Question 15

What is the Boltzmann constant equation?

Answer 15

Gas constant = Avogadro’s number x Boltzmann constant

Question 16

diffusion: definition?

Answer 16

the movement of a substance from an area of high concentration to an area of low concentration.

Question 17

effusion: definition?

Answer 17

refers to the movement of gas particles through a small hole, such as a pore in a membrane.

Question 18

rate of effusion: definition:

Answer 18

number of molecules passing through the hole per second.

Question 19

Graham’s law states:

Answer 19

in 1833, Thomas Graham found: at a given temperature and gas pressure, the rate of effusion is inversely proportional to the square root of the molar mass.

Question 20

Min theory of The kinetic model of gases:

Answer 20

basis of kinetic model: gas molecules are in constant random motion, in all directions, within a container.

Question 21

kinetic energy equation:

Answer 21

E = 0.5ms^2

Question 22

3 major assumptions in the kinetic model of gases:

Answer 22

Gas molecules have negligible size compared with the container (treated as point masses).

Collisions between gas molecules and the container walls are elastic – no energy loss/gain.

Molecules do not interact with each other.

Question 23

What is a real gas?

Answer 23

describes gases under normal conditions.

deviates from the ideal model behaviour, even at atmospheric pressures

Question 24

What is an ideal gas?

Answer 24

obeys Boyle’s law under all conditions – if the volume of fixed amount of gas is measured as a function of pressure at constant temperature, a plot of p against 1/V, will give a straight line if the gas obeys Boyles Law.

Question 25

How does liquification affect the appearance of isotherms?

Answer 25

• at high pressure - the substance is liquid
• pressure does not change when gas condenses to a liquid = flat potion on graph.
• at low pressure - the substance is gaseous,

Question 26

Accounting for the behaviour of real gases: 2 assumptions:

Answer 26

first assumption: size of molecules is negligible – good for when there are large distances between molecules (low pressures) - at high pressures, size of molecules matters as they occupy a significant proportion of the volume.

second assumption: there are no intermolecular forces – good when molecules are far apart so interactions would be weak – as pressure increase, the pressure exerted by the gas becomes less than the ideal gas predicts due to interactions between molecules – when molecules are close, in the gas phase, the attractive interactions hold back molecules a bit so they collide with the walls with slightly less force.

Question 27

What are the 2 correction factors in the Van der Waals equation for gases?

Answer 27

first correction introduced: accounts for the attraction between molecules – in a real gas the pressure is less than the ideal case – all the surrounding molecules are attracted to the one that is about to hit the wall, holding it back, so it strikes with a slightly lower force – so there is now a correction factor added to “p” to account for this.

second correction introduced: concerns the finite size of molecules – molecules have a small volume, the volume where the molecules can move is less than the size of the whole container – so correction factor accounts for this reduction.

Question 28

What is Dalton’s law?

Answer 28

the total pressure exerted by a mixture of gases is the sum of the partial pressures of each individual gas.

Question 29

What is a partial pressure?

Answer 29

the pressure that would be exerted if the gas were alone in the container.

Question 30

What equipment can you use to measure the total pressure exerted by the mixture?

Answer 30

a barometer or pressure gauge.

Question 31

What is a mole fraction, xA?

Answer 31

gives the fraction of the total number of moles that is component A in a mixture of gases.

Question 32

sum of all mole fractions for a mixture =?

Answer 32

sum of all mole fractions for a mixture = 1.