Gas Laws Flashcards
What is a gas?
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
What is critical temperature of a gas?
Threshold above which a substance exists as a gas despite pressure applied
What is a vapour?
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
The factor which determines gas or vapour is what?
Temperature
The movement of molecules in a gas is?
Random
Molecules in a solid move how?
Continuous motion but oscillate at a mean position and regular formation held
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
In the liquid phase more energy has been added to the molecules and the forces of attraction weaken
What is the liquid and vapour phase?
Evaporation occurs with condensation.
Molecules with enough kinetic energy can leave the liquid surface but still within the containing space.
Nitrous oxide has a critical temperature of what?
36.5C
Liquid and gas in room temp cylinders
Inhaled as a vapour and at body temp exhaled as a fixed gas
Curves of constant temperature on a graph are known as what?
Isotherms
What are the 3 main physical characteristics of a gas?
Pressure
Volume
Absolute temperature
The 3 gas laws combine to form what?
Combined gas law
What are the 3 gas laws?
Boyle’s (1662)
Gay - lussacs
Charles’ (1787)
P inversely ~ 1/v in a closed system with a fixed mass of gas
Boyle’s law
Draw Boyle’s law graph
Pressure y
Vol x
Explain P1V1 = P2V2
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
A 5l cylinder containing oxygen at a pressure of 100Atm will provide what vol at 1Atm
P1V1 = P2V2
100 x 5 = 1 x V2
500L = V2 at 1Atm
Charles law describes what?
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
Draw Charles law graph
X temperature
Y volume
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
Gay-lussacs law
Draw gay- lussacs graph
Y pressure
X temperature
If heat is applied to a gas what happens to the pressure?
Is it directly proportional to the rise in absolute temperature
PV/T = k is which law?
Combined gas law P=pressure V=volume k = constant T = absolute temperature Assumes mass of gas constant and in a closed system
The ideal gas law is what?
PV = nRT
n is number of miles
R is the universal gas constant
In a closed system
Can the ideal gas law be applied to NO2?
No because a cylinder contains liquid and vapour (36.5C). Therefore it has to be weighed.
Dalton’s law
Total pressure is the sum of the pressure of individual gases as if they were alone in the same volume
Ptotal = P1+P2+P3
Partial pressure is what?
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
What happens to the pressure above a vapour as a liquid evaporates into the space above it?
The pressure of the vapour increases
Equilibrium of vapour and liquid is reached when?
When the rate of condensation equals that of condensation
When does a space become saturated?
When equilibrium is reached
What is saturated vapour pressure?
The pressure at equilibrium, the rate of condensation and evaporation are equal, and the space is saturated
What is latent heat of vaporisation?
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!
What happens to heat when vapour is condensed?
The energy is returned as heat
What variable alters SVP?
Temperature
A rise in temperature increases SVP
What daily equipment uses latent heat of vaporisation?
Heat and moisture exchangers HME
What happens when SVP reaches atmospheric pressure?
The liquid boils
Reached its boiling point
A gas that moves in and out of a liquid exerts what?
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
When does the partial pressure of the substance in solution equal its partial pressure in the gaseous phase?
When equilibrium is reached and there is not net movement of gas in or out of a liquid
What is Henrys law?
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.
What happens if the partial pressure of an anaesthetic agent in alveoli is doubled?
Once equilibrium is reached, the partial pressure and quantity in the blood would also double
What is the partition coefficient?
Relative concentration in each phase when an agent has been distributed between them at equilibrium.
As temperature rises what happens to solubility if a gas?
Solubility decreases as the gas molecules have more energy to get out of the liquid.
Above a critical temperature vapour exists as what?
A gas
Larger gas molecules exert a higher or lower pressure than small ones?
This is related to number of gas molecules not their size PV=nRT
Critical pressure is?
Minimum pressure required to liquefy a gas at its critical temperature
What is latent heat?
Energy change assoc with a change in a substances state in either direction. No change in temperature
How is oxygen supplied in hospitals?
Vacuum insulated evaporator Network of high integrity copper pipes Ambient vaporiser Standby cylinders Changeover control unit Standby cylinders
What is a VIE
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
Draw the isotherm for N2O
See diagram
Curves of constant temperature
What is the critical pressure of N2O?
73bar at which liquefaction occurs
How do you measure N2O cylinder?
Gross weight minus tare weight/ empty cylinder weight
Cannot use bourdon gauge as liquid exists
Entonox is?
Oxygen and N20 50:50
Separate below pseudo critical temperature of -5.5C
Cylinders should be stored above -5.5C 24hrs before use
Medical gas cylinders are checked every ? Years
5
Indicated by coloured rings around cylinder neck
What are standard medical gas cylinders made of?
Ferromagnetic
Molybdenum steel
NOT MRI COMPATIBLE
