The Gas Laws Flashcards
According to the molecular theory of matter, describe a solid, liquid and gas.
Solid - atoms arranged in lattice. Each molecule exerts a force upon its neighbour and is continuously in motion, oscillating about a mean position
Add heat to a solid –> increase vibrations between molecules –> move further apart –> melt
Liquid - Molecules move more freely in relation to each other and the forces of attraction between the molecules are called van der Waals forces
Ad heat to a liquid –> increase kinetic energy of molecules which will eventually be able to overcome the van der waals forces exerted by their neighbours and move about freely in space = a gas or vapour
Define vapour pressure
The pressure exerted by a vapour above the surface of a liquid
What is saturated vapour pressure
The pressure exerted by a vapour in equilibrium with liquid of the same substance. It is influenced by temperature and pressure
Define boiling point
The boiling point temperature is the temperature above which vapour pressure equals atmospheric pressure. A lower atmospheric pressure will result in a lower boiling point temperature.
What is critical temperature
the temperature above which it is not possible to liquify a gas by increasing its pressure
When is a substance a gas and when is it a vapour
A substance is a gas when it is above its critical temperature
A substance is a vapour when it remains in a gaseous phase below its critical temperature
What is Boyles law
This is the 1st of the ideal gas laws
Boyles law states that at constant temperature, the volume of a given mass of gas varies inversely with the absolute pressure (think of halving the volume of a sealed 20 ml syringe at constant temperature)
V ~ 1/P
or VP = k
What is Charles’ Law
This is the 2nd of the ideal gas laws
Charles’ Law states that at constant pressure, the volume of a given mass of gas varies in direct proportion to the absolute temperature (think of heating a syringe at constant pressure)
V ~ T
or
V/T = k
What is Gay-Lussac’s law
This is the 3rd of the ideal gas laws
Gay-Lussac’s law states that at constant volume, the absolute pressure of a given mass of gas varies in direct proportion to the absolute temperature (think of heating a syringe at constant volume)
What is an application of the 3rd ideal gas law
The hydrogen thermometer
A constant volume of hydrogen is heated, the rise in pressure may be accurately recorded and it gives a measure of the absolute temperature increase
Define Standard Temperature and Pressure
STP
T = 273.15 K (0 deg C)
P = 101.325 kPa (760 mmHg)
What is an adiabatic change. Give an example
The ideal gas laws describe the behaviour of gas when one of three variables is constant (temp, pressure, volume). In order for these conditions to apply, het is required to be added or taken from a gas as the change occurs.
The state of a gas can also be changed without allowing the gas to exchange heat energy with its surroundings
–> this is called an adiabatic change.
Example:
Connect and open gas cylinder to anaesthetic machine –> Gas compressed rapidly –> pressure rises rapidly –> temperature of gas rises rapidly = adiabatic change.
Cryoprobe: Compressed gas expands rapidly –> pressure decreases rapidly –> gas expands adiabatically –> cooling occurs.
Explain how a cryoprobe works
Cooling from a cryoprobe is the result of an adiabatic process.Gas is allowed to expand rapidly out of a capillary tube and the cooling is as result of rapid expansion.
The cooling effect results from the fact the energy is required as the gas expands to overcome the van der Waals forces of attraction between the molecules of a gas. Heat exchange does not take place between the gas and its surrounding, the energy required is taken from the kinetic energy of the gas molecules themselves which results in the gas cooling as it expands.
What is Dalton’s law of partial pressures
States that in a mixture of gases the pressure exerted by each gas is the same as that which it would exert if it alone occupied the container
A cylinder filled with entonox has an absolute pressure of 100 kPa. What would is the partial pressure of both O2 and N2O within this mixture and why
Entonox = 50% O2 with 50% N@O.
According to daltons law the partial pressure a gas within a mixture of gases is the same as what it would be if it were alone in the container. Therefore as the absolute pressure is 100 kPa the partial pressure of both is 50 kPa
How do manufacturers fill a cylinder with 90% O2 and 10% CO2 to an absolute pressure of 138 bar
First 13.8 bar CO2. (10% of absolute pressure desired)
Then fill the tank to an absolute pressure of 138 bar
What is Avagadro’s hypothesis
Avagadro’s hypothesis states that equal volumes of gases at the same temperature and pressure contain the equal numbers of molecules
Why is it more convenient to use a concept related to the number of molecules when referring to quantities of gas? What is this concept?
Two syringes: One filled with O2 and one filled with H2 are at equal volume, pressure and temperature. These gases have different masses (molecular masses) but each syringe has the same number of molecules. Therefore the concept of the mole was created.
What is a mole
The quantity of a substance containing the same number of particles as there are atoms in 0.012 kg of carbon 12. this is 6.022 x 10^23 atoms and is known as Avagadro’s number
What is the point of Avagadros hypothesis
It states that equal volumes of gas at equal pressures contain equal numbers of molecules. Therefore, rather than express a quantity of gas in terms of its mass, it is more convenient to use a concept related to the number of molecules. This is a mole.
What volume goes 1 mole of gas occupy
At STP one mole of gas occupies 22.4 L
E.g.
2g of hydrogen –> 1 mole —-> 22.4 Litres
32g of Oxygen —> 1 mole —> 22.4 L
44 g of CO2 ——-> 1 mole —> 22.4 L
n = m/M
What important role does Avogadros hypothesis play in the calibration of vaporizers. Provide an example
A steady flow of O2 through a vaporizer converts 18.45 g of isoflurane into 224 L.
MW of isoflurane is 184.5 g/mol
n = m/M = 18.45/184.5 = 0.1 mol
So from above:
0.1 mol of isoflurane has produced 224 L
But: we know that 1 mol isoflurane = 22.4L. therefore 0.1 mol isoflurane should produce 2.24 L so the concentration of isoflurane produeced:
= 2.24 L/ 224 = 1% —-> calibration.
What is the tare weight
This is the weight of an empty cylinder. The weight of Nitrous Oxide within the cylinder can be calculated by subtracting the tare weight from the measured weight of the cylinder
Combine the gas laws with Avogadro’s hypothesis to explain why the pressure gauge acts as a contents gauge for a gas cylinder provided the cylinder contains gas
- Boyles: V ~ 1/P or PV = k1
- Charles: V ~ T or V/T = k2
- Guy-Lussac: P ~ T or P/T = k3
So PV/T = constant
For 1 mol of gas PV/T equals a unique gas constant called the universal gas constant, R.
Universal gas law: PV = nRT.
If there is only gas in a cylinder:
V is constant
R is constant
T is kept constant
Therefore P ~ n –> so the pressure is proportional to the number of moles left in the cylinder.
