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Anaesthetics > Gas Laws > Flashcards

Gas Laws Flashcards

1
Q

What is a gas?

A

Compressible fluid phase in which no liquid can be formed at that temperature. The molecules have enough kinetic energy that they no longer exist as liquid and can only do so after cooling.
Intermolecular spacing so large that forces are negligible
A fixed gas exists above its critical temperature

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2
Q

What is critical temperature of a gas?

A

Threshold above which a substance exists as a gas despite pressure applied

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3
Q

What is a vapour?

A

Gaseous phase below its critical temperature which is in a state of equilibrium with the same substance in liquid form
A vapour can only exist below its critical temperature. Compressible liquid phase

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4
Q

The factor which determines gas or vapour is what?

A

Temperature

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5
Q

The movement of molecules in a gas is?

A

Random

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6
Q

Molecules in a solid move how?

A

Continuous motion but oscillate at a mean position and regular formation held

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7
Q

In the liquid phase …… energy has been added to the molecules and the forces of attraction …….
The molecules are more free to move but constrained within a surface or boundary

A

In the liquid phase more energy has been added to the molecules and the forces of attraction weaken

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8
Q

What is the liquid and vapour phase?

A

Evaporation occurs with condensation.

Molecules with enough kinetic energy can leave the liquid surface but still within the containing space.

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9
Q

Nitrous oxide has a critical temperature of what?

A

36.5C
Liquid and gas in room temp cylinders
Inhaled as a vapour and at body temp exhaled as a fixed gas

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10
Q

Curves of constant temperature on a graph are known as what?

A

Isotherms

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11
Q

What are the 3 main physical characteristics of a gas?

A

Pressure
Volume
Absolute temperature

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12
Q

The 3 gas laws combine to form what?

A

Combined gas law

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13
Q

What are the 3 gas laws?

A

Boyle’s (1662)
Gay - lussacs
Charles’ (1787)

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14
Q

P inversely ~ 1/v in a closed system with a fixed mass of gas

A

Boyle’s law

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15
Q

Draw Boyle’s law graph

A

Pressure y

Vol x

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16
Q

Explain P1V1 = P2V2

A

In a cylinder of gas if the piston is driven inwards to halve the volume then the pressure will double.
Above equation represents to different settings of a gas

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17
Q

A 5l cylinder containing oxygen at a pressure of 100Atm will provide what vol at 1Atm

A

P1V1 = P2V2
100 x 5 = 1 x V2
500L = V2 at 1Atm

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18
Q

Charles law describes what?

A

In a closed system with a fixed mass of gas the vol is directly proportional to the absolute temperature but the pressure must remain constant
V~T
Change in temperature causes volume increase as gas expands

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19
Q

Draw Charles law graph

A

X temperature

Y volume

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20
Q

If a fixed mass of gas is kept at a constant volume in a closed system, its pressure is directly proportional to the absolute temperature
P~T

A

Gay-lussacs law

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21
Q

Draw gay- lussacs graph

A

Y pressure

X temperature

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22
Q

If heat is applied to a gas what happens to the pressure?

A

Is it directly proportional to the rise in absolute temperature

23
Q

PV/T = k is which law?

A 
Combined gas law 
P=pressure
V=volume 
k = constant 
T = absolute temperature
Assumes mass of gas constant and in a closed system
24
Q

The ideal gas law is what?

A

PV = nRT

n is number of miles
R is the universal gas constant

In a closed system

25
Q

Can the ideal gas law be applied to NO2?

A

No because a cylinder contains liquid and vapour (36.5C). Therefore it has to be weighed.

26
Q

Dalton’s law

A

Total pressure is the sum of the pressure of individual gases as if they were alone in the same volume

Ptotal = P1+P2+P3

27
Q

Partial pressure is what?

A

Pressure exerted by each gas at fixed volume at fixed temperature
Assuming gases don’t react with each other
Oxygen pp 0.21 x 101kpa = 21kpa

28
Q

What happens to the pressure above a vapour as a liquid evaporates into the space above it?

A

The pressure of the vapour increases

29
Q

Equilibrium of vapour and liquid is reached when?

A

When the rate of condensation equals that of condensation

30
Q

When does a space become saturated?

A

When equilibrium is reached

31
Q

What is saturated vapour pressure?

A

The pressure at equilibrium, the rate of condensation and evaporation are equal, and the space is saturated

32
Q

What is latent heat of vaporisation?

A

Liquid molecules require energy to become a vapour. If energy cannot be obtained from external sources then it is drawn as heat from the liquid therefore the liquid cools. This occurs in a vaporiser!

33
Q

What happens to heat when vapour is condensed?

A

The energy is returned as heat

34
Q

What variable alters SVP?

A

Temperature

A rise in temperature increases SVP

35
Q

What daily equipment uses latent heat of vaporisation?

A

Heat and moisture exchangers HME

36
Q

What happens when SVP reaches atmospheric pressure?

A

The liquid boils

Reached its boiling point

37
Q

A gas that moves in and out of a liquid exerts what?

A

Increases partial pressure within the liquid as gas dissolves but an equilibrium is reached when there is no net movement in or out.
This is Henry’s Law

38
Q

When does the partial pressure of the substance in solution equal its partial pressure in the gaseous phase?

A

When equilibrium is reached and there is not net movement of gas in or out of a liquid

39
Q

What is Henrys law?

A

At a constant temperature, more molecules are dissolved at a higher pressure.
The amount of a given has in a given type and vol of liquid is directly proportional to the partial pressure of that gas in equilibrium with that liquid.

40
Q

What happens if the partial pressure of an anaesthetic agent in alveoli is doubled?

A

Once equilibrium is reached, the partial pressure and quantity in the blood would also double

41
Q

What is the partition coefficient?

A

Relative concentration in each phase when an agent has been distributed between them at equilibrium.

42
Q

As temperature rises what happens to solubility if a gas?

A

Solubility decreases as the gas molecules have more energy to get out of the liquid.

43
Q

Above a critical temperature vapour exists as what?

A

A gas

44
Q

Larger gas molecules exert a higher or lower pressure than small ones?

A

This is related to number of gas molecules not their size PV=nRT

45
Q

Critical pressure is?

A

Minimum pressure required to liquefy a gas at its critical temperature

46
Q

What is latent heat?

A

Energy change assoc with a change in a substances state in either direction. No change in temperature

47
Q

How is oxygen supplied in hospitals?

A 
Vacuum insulated evaporator
Network of high integrity copper pipes
Ambient vaporiser 
Standby cylinders
Changeover control unit
Standby cylinders
48
Q

What is a VIE

A

Double skinned steel pressure vessel
0.16-0.3kPa between skins
-160-(-)180C liquid oxygen with SVP 10bar
Requires latent heat of evaporation to re-establish equilibrium, therefore no external cooling required
Escape valve to allow liquid to escape and vaporise at base of vessel
Weighing scales used as liquid exists
Safety valve at 15bar
System of pressure regulators exist to lower to 4 bar

Critical temp of oxygen = -118C

49
Q

Draw the isotherm for N2O

A

See diagram

Curves of constant temperature

50
Q

What is the critical pressure of N2O?

A

73bar at which liquefaction occurs

51
Q

How do you measure N2O cylinder?

A

Gross weight minus tare weight/ empty cylinder weight

Cannot use bourdon gauge as liquid exists

52
Q

Entonox is?

A

Oxygen and N20 50:50
Separate below pseudo critical temperature of -5.5C
Cylinders should be stored above -5.5C 24hrs before use

53
Q

Medical gas cylinders are checked every ? Years

A

5

Indicated by coloured rings around cylinder neck

54
Q

What are standard medical gas cylinders made of?

A

Ferromagnetic
Molybdenum steel
NOT MRI COMPATIBLE

Anaesthetics (12 decks)

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