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PHYSICS FRCA > gas laws > Flashcards

gas laws Flashcards

1
Q

define a gaseous state…

A

A state of matter whereby particles are freely moving without a given volume and will move to fill any space.

the particles are not touching but may collide and the space between them is larger than the size of the molecules.

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2
Q

define a gas…

A

A substance in a gaseous state

And above its critical temperature

(the critical temperature must be below room temperature for the substance to exist as a gas)

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3
Q

what is a perfect / ‘ideal’ gas?

A

This is a theoretical gas whereby…
- molecules are infinitely small and behave as individual particles
- no force of attraction between the particles.
- move in a random manner independent of each other
- obeys all the gas laws

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4
Q

Are gas’s in reality ideal?

A

no gas is 100% ideal - but close enough that the model can help make predictions
H2 is the closest
at standard temp and pressure most anaesthetic gases behave like an ideal gas.
at extreme temperature and pressure there are deviations from this model.

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5
Q

what is charles law? 2nd gas law

A

At a constant pressure the volume of a gas is directly proportional to its temperature.

Therefore V∝T or V/T = constant (k2)

linear graph T on x axis (y=mx+c)

‘charles in charge is cool under pressure’ - pressure is constant

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6
Q

what is boyles law? 1st gas law

A

the absolute Pressure of a gas is inversely related to its volume at constant temperature.

P α 1/V
P1V1=P2V2

graph would be P on x, V on y and rectangular hyperbola

boyles is boiling - temp is constant

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7
Q

what is gay lussacs law? 3rd gas law

A

at a constant volume,
the absolute pressure is directly propotional to the temperature.

linear graph (y=mx +c)
Therefore P∝T or P/T = constant (k3)

gay lussac is a volumous name - volume constant

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8
Q

what are the gas laws?

A

The gas laws are a set of rules that govern the relationship between
thermodynamic temperature, volume and pressure of ideal gases. to help predict the behaviour of an ideal gas.

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9
Q

what is the ideal gas equation?

A

combining all 3 gas laws into one
PV/T=K

For 1 mole of gas K is given symbol R to represent universal gas constant
For n moles of gas PV/T = nR = ideal gas law (includes no, of moles)

usually written as
PV = nRT

temperature is in Kelvin
V and P can be in any unit as long as there is consistency

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10
Q

what is R in the ideal gas equation?

A

R = universal gas constant = 8.32144 J/K/mol

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11
Q

what are the assumptions of the ideal gas equation?

A

the gas behaves as an ideal gas

molecules do not interact with one another and there are no forces acting between them.
their volume is negligible compared to volume of the container.

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12
Q

what is Avogadro’s hypothesis?

A

he states that equal volumes of different gases at same temp/pressure have the same number of molecules.

from this it can be deduced that at a standard temperature and pressure, 22.4L of gas contains 1 mole of gas molecules.

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13
Q

define 1 mole

A

1 mole is the quantity of substance containing the same number of particles as there are atoms in 12g of Carbon-12

6.022 x 10 ^23 = avogadros number

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14
Q

what is standard temperature and pressure?

A

standard temp = 273.15 K
standard pressure = 1 atm

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15
Q

what is the equation relating moles to mass and conc/vol?

A

mole = mass / Mr (mass in grams)

mole = conc x vol

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16
Q

A N20 cylinder weighs 60kg when empty and 64.4kg when full. how many moles of N20 are in this cylinder?

how many molecules?
how many litres of N20 in the cylinder?

A

mole = mass/ Mr
Mr of N20 = 44 (16+14+14)

mole = 4.4kg/44
mole = 4400/44 = 100

100 moles

no. of molecules = 100 x 6.022x10^23
= 6.022x 10^21

no. of litres = moles x 22.4 = 2240L

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17
Q

how does a hot air balloon work?

A

As the gas inside the balloon is heated, the air expands and becomes less dense and hence rises. This displays Charles law.

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18
Q

If you have 10L of oxygen at a pressure of 137 bar, how much oxygen will it deliver in volume?

how much can be used for transfer?

A

PV/T = PV/T
temp is equal so
PV = PV

absolute pressure in cylinder = cylinder pressure + atmospheric
= 137 bar + 1bar
= 138 bar

into equation
10 x 138 = 1x V
1380 L

10L will remain in the cylinder when emptied (i.e. can’t create negative pressure) therefore only 1370L available for use.

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19
Q

A cylinder storing oxygen at 137 atmospheres pressure indicates that it provides 680 L of oxygen for use. What is the approximate internal volume of the cylinder?

A

PV = PV
137 x V = 1x 680

680/137 = 5L

20
Q

what happens to the temperature of a gas if it expands and no external heat source is provided? when is this principle used?

A

If heat energy is not added to or lost from a system…
a rapid compression of gas will result in a rise in its temperature
a sudden expansion of gas will result in a fall in its temp.

this is known as a Adiabatic process - i.e. no external heat source.

This principle is harnessed by the cryotherapy probe used to freeze
lesions in surgery where a sudden expansion of gas through the end
leaves its tip extremely cold.

21
Q

what is Daltons law?

A

The total pressure exerted by a gaseous mixture is equal to the sum of the partial pressures of each of the individual gases within that mixture

the pressure exerted by each gas is the same as that which it would exert if it alone occupied the container.

22
Q

what is henrys law?

A

the amount of gas dissolved in a liquid is proportional to its partial pressure above that liquid at a given temperature

(the warmer the liquid, the less gas that dissolves in it)

P = Hv x M
- Where P is partial pressure of the gas
- Hv is Henry’s proportionality constant
- M is the molar concentration of gas

23
Q

what is the kinetic gas theory?

A

This model is of an ideal gas
States that …
Gases consists of large numbers of particles v. far apart all constantly moving with kinetic energy randomly and rapidly

Their diameter is a lot smaller than the space between each molecule

Moving in random directions at different speeds – they travel in straight lines

They can collide and bounce of the walls of container and one another

No attraction / forces between the particles.

Real gases deviate slightly from these laws because they have a small finite volume and small amounts of attractive forces. These forces become important when particles are close together e.g. high pressures/low temp.

24
Q

what is the equation for kinetic energy? what does this tell you about the kinetic energy depending on mass of gass.

A

Kinetic energy = ½ m v^2

(m = mass, v= velocity) hence increasing kinetic energy will increase velocity but more so for gases with lower mass.

25
Q

what is the SI unit for pressure?

A

derived SI unit
force per unit area

N/m^2 = pascal

26
Q

how does 1atm relate to pascals, mmHg, cmH20 and bar?

A

1 atmosphere = 101.3 kPa = 760 mmHg = 1033 cmH20 = 1.013 bar

27
Q

what is the difference between the gauge pressure and absolute pressure?

A

Absolute pressure = Gauge pressure + atmospheric pressure

An ‘empty’ gas cylinder has a gauge pressure of zero, but actually contains air at atmospheric pressure, so the absolute pressure in the cylinder is 1.013 bar

28
Q

what is gas tension?

A

Gas tension is the partial pressure of a gas that is in solution

technically the dissolved gas cannot exert a pressure however the gas tension is the partial pressure of the gas in the gas above the liquid at equilibrium. this works because of henrys law - gas dissolved is proportional to partial pressure in gas state at equilibrium.

29
Q

what is grahams law?

A

the rate of diffusion of a gas is inversely proportional to square root of molecule weight.

Rate ∝ 1/ √MW

30
Q

define critical temperature…

A

The temperature above which a gas cannot be liquified despite how much pressure is applied. the gaseous state is now called gas rather than vapour.

31
Q

define critical pressure…

A

the minimum pressure required to cause liquefication of a gas at is critical temp (or just below critical temp, as just above critical temp can not liquefy)

32
Q

what is an isotherm graph?

A

google image

lines of constant pressure to show the relationship between volume and pressure. can be used to predict critical temperatures/pressures.

33
Q

what is the difference between a vapour and a gas?

A

Vapour – a substance in gaseous phase , below critical temp - can be liquified under pressure.
Gas – a substance in gaseous phase , above critical temp

34
Q

what is the critical temp and pressure for N20, O2 and CO2?

A

N20 temp 36.5 degree C, pressure 72 atm
O2 temp -119, 50 atm
CO2 temp 30 , pressure 73atm

34
Q

draw an isotherm for N20

A

draw 3 lines - one for 40 degrees, one for 36.5 degrees, one for 20 degrees.

40 degrees - top right corner - above its critical temperature and exists as a gas. graph as a rectangular hyperbola - Boyle’s Law.

Temperature 36.5°C:Nitrous oxide is at its critical temperature.
Along the left side of curve, where the gas’s exist as liquid, there is a very sharp rise in pressures because liquids are relatively incompressible.

Temperature 20°C:Nitrous oxide is below its critical temperature. At 20°C the vapour partly compresses to a liquid with only 52. Further reduction in volume causes more vapour to condense with no change in pressure. A horizontal line is seen

google image

35
Q

what is the triple point of water?

A

Thetriple pointis the temperature and pressure at which the solid, liquid and gaseous phases of a substance can exist in equilibrium.

Triple point of water = 0.01 degrees C or 273.16 K, SVP 4.6mmHg / 611.73 pascals / 0.006 atm

36
Q

define saturated vapour pressure

A

the pressure exerted by a vapor in thermodynamic equilibrium with its liquid (or solid) phase at a given temperature in a closed system

A liquid in a closed container has too phases – vapour and liquid. The molecules move in and out of the liquid and at equilibrium there is an equal rate of evaporation and condensation.

37
Q

how does SVP vary with temperature?

A

increase in temp, paritcles have more kinetic energy, move from liquid to gaseous states. hence more leave the liquid phase and SVP is increased.

the relationship is non-linear

38
Q

how does SVP vary with pressure?

A

as pressure increases more gaseous molecules are liquified under pressure

hence SVP reduces with increasing pressure.

39
Q

how does boiling temp relate to SVP?

A

when SVP is at room temperature, the substance will boil.
Hence at altitude and lower atm pressure, water boils at a lower temperature – cant make a good cup of tea because not as hot.

40
Q

what is ficks law of diffusion?

A

This describes the rate of diffusion of a gas across a membrane

proportional to the membrane area (A) and the concentration gradient (C1-C2) and inversely proportional to its thickness (D)

Rate of diffusion ∝ A (C1-C2) / D

(can remove proportional for = and include k as a constant).

41
Q

what is meant by Adiabatic expansion ?

A

gas expands and energy used to do so comes from internal source rather than external heat source.

the energy used to break van der vaals for the expansion, comes from the gas itself. the gas cools

usually happens with rapid expansion- no time to exchange heat with the environment

42
Q

what is meant by isothermal change of a gas?

A

slow chnages to state of matter, allow time for exchange of heat with the environment

temperature of the gas remains constant

43
Q

what is the boiling point of O2, N20 and CO2?

A

O2 = -183 degrees C
N20 = -88
CO2 = -79

44
Q

what is the SVP at 20 degrees for O2, N20 and CO2?

A

O2 - 1 atm - but this is at a much lower temp than 20degrees. at 20 degrees - above critical temp so no vapour form.

N20 - 58 atm
CO2 - 57 atm

45
Q

between what temp does N20 exist as a vapour?

A

N2O has a boiling point of -88.5°C but the critical temperature is 36.5°C, making it a vapour from -88.5°C to 36.5°C and a gas above that

PHYSICS FRCA (8 decks)

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