Gases and vapours

Question 1

What happens at a molecular level when evaporation occurs?

Answer 1

Some molecules have a kinetic energy that is greater than the force of attraction to surrounding molecules so will break free. Heat increases this process.

Question 2

What is saturated vapour pressure?

Answer 2

The pressure exerted by vapour in equilibrium with a liquid at a certain temperature. Occurs when liquid is stored in an enclosed place.

Question 3

Is the relationship between SVP and increasing temperature linear or non-linear?

Answer 3

Non linear, it is a slight upward curve

Question 4

What happens when SVP reaches atmospheric pressure?

Answer 4

The substance boils and 100% becomes vapour

Question 5

How is SVP related to ambient pressure?

Answer 5

Ambient pressure (pressure of surrounding medium) doesn’t effect SVP.

However, as boiling point occurs when SVP meets atmospheric pressure, more vapour will be present with decreased ambient pressure due to boiling. SVP is unaffected, but partial pressure of the gas vapour will increase.

Question 6

What happens if you try and make a cup of tea on Everest?

Answer 6

Decreased BP due to decreased pressure.

Therefore, it might not have a chance to brew the tea, prior to the water turning to vapour.

Question 7

What is the BP, SVP and MAP of desfluorane?

Answer 7

BP (degrees C at atm) = 23
SVP (at 20 deg c) = 88.5
MAC 6.0

Question 8

What is the BP, SVP and MAP of sevo?

Answer 8

BP (degrees C at atm) = 59
SVP (at 20 deg c) = 21.3
MAC = 2.0

Question 9

What is the BP, SVP and MAP of iso?

Answer 9

BP (degrees C at atm) = 48
SVP (at 20 deg c) = 32
MAC = 1.15

Question 10

What is the BP, SVP and MAP of enflurane?

Answer 10

BP (degrees C at atm) = 56.5
SVP (at 20 deg c) = 23
MAC = 1.68

Question 11

What is the BP, SVP and MAP of halothane?

Answer 11

BP (degrees C at atm) = 50.2
SVP (at 20 deg c) = 32.5
MAC = 0.75

Question 12

What is latent heat?

Answer 12

The heat energy used to cause a phase change

More vigorous molecules have enough energy to escape to gas phase and those left behind have a lower average energy. Therefore to vaporise, the liquid below is left colder.

Question 13

What is latent heat of vaporisation?

Answer 13

Heat energy required to convert a liquid into vapour whilst maintaining the same overall temperature.

Question 14

What is latent heat of fusion?

Answer 14

The heat energy required to change a solid into a liquid whilst maintaining the same overall temperature

Question 15

Why do the temperatures not change is latent heat of vaporisation/fusion even when a constant heat is being applied?

Answer 15

The energy is being used to break/make bonds between molecules so is not used for overall warming/increasing the average kinetic energy.

I.e. heat is used to change the phase of the substance

Question 16

What is the difference between heat and temperature?

Answer 16

Heat is energy whilst temperature is a measure of hot/coldness

Question 17

How does latent heat of vaporisation vary with liquid temperature?

Answer 17

Water closer to BP will require less latent heat to achieve vaporisation than water at room temperature

Question 18

What is specific latent heat?

Answer 18

The heat required to change one kg of a substance from one phase to another J/kg, must be quoted in reference to a temperature

Question 19

What is the specific latent heat of water at 100 deg c and at body temperature?

Answer 19

100 = 2.26 mJ/kg
Body temp = 2.43

Question 20

What is adiabatic change?

Answer 20

No heat is transferred to or from a system

And be achieved by using an insulated container or by a process being done quickly, so heat doesn’t have enough time to transfer.

Question 21

What happens is we decompress a gas with an adiabatic process?

Answer 21

Gas expands and requires energy to overcome the VDW’s between molecules. As adiabatic no energy is gained from the environment and all energy is taken from the molecules internal energy, so the overall gas cools.

Question 22

What happens is we compress a gas with an adiabatic process?

What is an example?

Answer 22

The temperature rises as pushing the molecules together increases the pressure and the kinetic energy (Joule Kelvin principle or joule Thomson effect)

Can occur if a cylinder attached to a machine is turned on too quickly. Can cause an explosion if combustibles around.

Question 23

What is isothermal change?

Answer 23

If compression or expansion happens slowly, then there is time for heat loss/transfer, to/from the environment and so a state change occurs with no temperature change.

Question 24

Why do we use vaporiser?

Answer 24

SVP’s of volatiles are higher than room temps and so we need a machine to mix fresh gas with some volatile in a reliable manner. The dial gives a splitting ratio e.g. splits the carrier gas between the volatile chamber and the gas outlet to deliver a controlled, predictable level of the anaesthetic agent, compared to the carrier gas.

Question 25

What are the two main types of vaporisers and the difference in them?

Answer 25

Plenum uses positive pressure to drive gas through the vaporiser (UK). Has high internal resistance, so used as a vaporiser out of circle. (Boyles bottle)

Draw-over - uses downstream negative pressure e.g. respiration/bellows (developing world). Vaporiser in circle (Goldman bottle).

Question 26

What is a TEC vaporiser?

Answer 26

UK standard - Temperature controlled vaporiser chamber

Network of channels and wicks gives a high surface areas for vaporisation, so SVP can be achieved without a change in vaporiser temp, meaning gas in chamber is always 100% saturated. Dial alters a gas splitting ratio, allowing some gas into chamber and some along gas outflow. The gas going through the chamber then becomes saturated with gas.

Latent heat of vaporisation would cause cooling, lowering the SVP and reducing concentration delivery. Increased flow would also do this.

Several different temp compensation mechanisms exist (different question) to stop this.

Question 27

Why must vaporisers be calibrated to individual gasses?

Answer 27

Higher SVP = more volatile and a higher concentration of the agent. Splitting ratio must be altered to ensure too much gas is not picked up. If more volatile agent is in a less volatile vaporiser, then a dangerous over delivery can occur.

Question 28

What temperature compensation mechanisms are used in vaporisers?

Answer 28

Lowering the flow rate to reduce LHV

Bimetallic strip
Bellows
Metal rod orifice that expands and contracts
Metal heat sink
Water bath

Question 29

How does a vaporiser bimetallic strip work?

Answer 29

Two metals with different thermal expansion coefficients bonded. Causes one of the strips to bend and open/close an orifice. Used on TEC machines (named 1-5 depending on where the strip is)

Question 30

How does a vaporiser bellows work?

Answer 30

Small and flexible and expand/contract to open/close a valve e.g. ohio

Question 31

How does a vaporiser metal rod orifice work?

Answer 31

Metal rod expands and contracts depending on temp to let more/less gas out,

Question 32

How does a vaporiser metal heat sink work?

Answer 32

Have a great heat capacity so buffer latent heat loss instead of the liquid (usually copper)

Question 33

How does a vaporiser water bath work?

Answer 33

Buffers loss through LHV.

Rarely used now