the gas laws do now read pages 70-71. the gas laws what happens if the pressure and volume are...
TRANSCRIPT
The Gas Laws
Do Now read pages 70-71
The Gas Laws
• What happens if the Pressure and Volume are changed and constant temperature
Pressure – A reminder
Pressure is defined as the normal (perpendicular) force per unit area
P = F/A
It is measured in Pascals, Pa (N.m-2)
Pressure – A reminder
What is origin of the pressure of a gas?
Pressure – A reminder
Collisions of the gas particles with the side of a container give rise to a force, which averaged of billions of collisions per second macroscopically is measured as the pressure of the gas
Change of momentum
The behaviour of gases – Boyles Law
When we compress (reduce the volume) a gas at constant temperature, what happens to the pressure? Why?
Let’s do it!
The behaviour of gases
When we compress (reduce the volume) a gas at constant temperature, what happens to the pressure? Why?
pV = constant
The Boyle’s laws – copy
We have found experimentally that;
At constant temperature, the pressure of a fixed mass of gas is inversely proportional to its volume.
If the volume halves the pressure doubles
p α 1/V or pV = constant
This is known as Boyle’s law
Explaining the behaviour of gases
When we compress (reduce the volume) a gas at constant temperature, the pressure increases. Why?
Explaing the behaviour of gases
When we compress (reduce the volume) a gas at constant temperature, the pressure increases. Why?
A smaller volume increases the likelihood of a particle colliding with the container walls.
Boyle’s Law
The Gas Laws
Do Q1-3 page 71
The behaviour of gases- Pressure Law
http://phet.colorado.edu/sims/ideal-gas/gas-properties.jnlp
When we heat a gas at constant volume, what happens to the pressure? Why?
Let’s do it!
The behaviour of gaseshttp://phet.colorado.edu/sims/ideal-gas/gas-properties.jnlp
When we heat a gas at constant volume, what happens to the pressure? Why?
P α T (if T is in Kelvin)
Boyle’s Law
States that the pressure of a fixed mass of gas is inversely proportional to its volume at constant temperature
P 1/V or PV = constant
When the conditions are changed P1V1 = P2V2
What to do
• A column of trapped dry air in a sealed tube by the oil
• The pressure on this volume of air can be varied by pumping air in or out of the oil reservoir to obtain different pressures
• Wait to allow the temperature to return to room temperature
Charles’ Law
States that the volume of a fixed mass of gas is directly proportional to its absolute temperature at constant pressure
V T or V/T = constant
When the conditions are changed V1/T1 = V2/T2
The ExperimentTap 1
Tap 2 Tap 3
Water reservoir
Fixed massof gas
Mercury in U tube
What to do
Fill the mercury column with mercury using the right hand tube (tap 1 open, tap 2 closed)
With tap 1 open drain some mercury using tap 2, then close tap 1 and 2. To trap a fixed mass of gas
Fill the jacket with water (make sure tap 3 is closed)
and then
Change the temperature of the water by draining some water from tap 3 and adding hot water
Equalise the pressure by leveling the columns using tap 2
Read the volume from the scale
The Results
V
T K
V
T oCA value forabsolute zero
The Results
P
V
P
1/ V
PV
P
The Charles’ Law copy
At constant pressure, the volume of a fixed mass of gas is proportional to its temperature;
V α T or V/T = constant
This is known as Charles’ lawIf T is in Kelvin
Explaing the behaviour of gases
When we heat a gas a constant pressure, the volume increases. Why?
Explaining the behaviour of gases
When we heat a gas a constant pressure, the volume increases. Why?
Increasing the volume reduces the chance of particles colliding with the container walls, opposing the effect of the particles increased kinetic energy.
Charles Law
Explaing the behaviour of gases
When we heat a gas a constant pressure, the volume increases. Why?
Explaining the behaviour of gases
When we heat a gas a constant pressure, the volume increases. Why?
Increasing the volume reduces the chance of particles colliding with the container walls, opposing the effect of the particles increased kinetic energy.
Charles Law
The Pressure law
At constant volume, the pressure of a fixed mass of gas is
proportional to its temperature;
p α T or p/T = constant
This is known as the Pressure law
If T is in Kelvin
Explaining the behaviour of gaseshttp://phet.colorado.edu/sims/ideal-gas/gas-properties.jnlp
When we heat a gas at constant volume, the pressure increases. Why?
Explaining the behaviour of gases
When we heat a gas at constant volume, the pressure increases. Why?
Increased average kinetic energy of the particles means there are more collisions with the container walls in a period of time and the collisions involve a greater change in momentum.
Pressure Law
Absolute Zero and the Kelvin Scale Charles’ Law and the Pressure Law suggest
that there is a lowest possible temperature that substances can go
This is called Absolute Zero The Kelvin scale starts at this point and
increases at the same scale as the Celsius Scale
Therefore -273oC is equivalent to 0 K ∆1oC is the same as ∆1 K To change oC to K, add 273 To change K to oC, subtract 273
The equation of state
By combining these three laws
pV = constantV/T = constantp/T = constant
We get pV/T = constant
Or p1V1 = p2V2
T1 T2
Remember, T must be in Kelvin
An exampleAt the top of Mount Everest the temperature is around 250K, with atmospheric pressure around 3.3 x 104 Pa. At sea level these values are 300K and 1.0 x 105 Pa respectively. If the density of air at sea level is 1.2 kg.m-3, what is the density of the air on Mount Everest?
“Physics”, Patrick Fullick, Heinemann
An exampleAt the top of Mount Everest the temperature is around 250K, with atmospheric pressure around 3.3 x 104 Pa. At seas level these values are 300K and 1.0 x 105 Pa respectively. If the density of air at sea level is 1.2 kg.m-3, what is the density of the air on Mount Everest?
Take 1kg of air at sea level
Volume = mass/density = 1/1.2 = 0.83 m3.
Therefore at sea level
p1 = 1.0 x 105 Pa, V1 = 0.83 m3, T1 = 300K.
An exampleAt the top of Mount Everest the temperature is around 250K, with atmospheric pressure around 3.3 x 104 Pa. At seas level these values are 300K and 1.0 x 105 Pa respectively. If the density of air at sea level is 1.2 kg.m-3, what is the density of the air on Mount Everest?
Therefore at sea level
p1 = 1.0 x 105 Pa, V1 = 0.83 m3, T1 = 300K.
At the top of Mount Everest
p2 = 3.3 x 104 Pa, V2 = ? m3, T1 = 250K.
An exampleAt the top of Mount Everest the temperature is around 250K, with atmospheric pressure around 3.3 x 104 Pa. At seas level these values are 300K and 1.0 x 105 Pa respectively. If the density of air at sea level is 1.2 kg.m-3, what is the density of the air on Mount Everest?
Therefore at sea level p1 = 1.0 x 105 Pa, V1 = 0.83 m3, T1 = 300K.
At the top of Mount Everest p2 = 3.3 x 104 Pa, V2 = ? m3, T1 = 250K.
p1V1/T1 = p2V2/T2
(1.0 x 105 Pa x 0.83 m3)/300K = (3.3 x 104 Pa x V2)/250K
V2 = 2.1 m3,
This is the volume of 1kg of air on Everest
Density = mass/volume = 1/2.1 = 0.48 kg.m-3.
pV = constantT
The equation of state of an ideal gas
Experiment has shown us that
pV = nR T
• p - pressure (Pa)• V - volume (m3)• n - number of mols• R - molar gas constant ( 8.31 J mol-1 K-1) • T - Temperature (K)
Remember, T must be in Kelvin
Sample question
• A container of hydrogen of volume 0.1m3 and temperature 25°C contains 3.20 x 1023 molecules. What is the pressure in the container?
K.A.Tsokos “Physics for the IB Diploma” 5th Edition
Sample question
• A container of hydrogen of volume 0.1m3 and temperature 25°C contains 3.20 x 1023 molecules. What is the pressure in the container?
# moles = 3.20 x 1023/6.02 x 1023 = 0.53
K.A.Tsokos “Physics for the IB Diploma” 5th Edition
Sample question
• A container of hydrogen of volume 0.1m3 and temperature 25°C contains 3.20 x 1023 molecules. What is the pressure in the container?
# moles = 3.20 x 1023/6.02 x 1023 = 0.53
P = RnT/V = (8.31 x 0.53 x 298)/0.1 = 1.3 x 104 N.m-2
K.A.Tsokos “Physics for the IB Diploma” 5th Edition
An Ideal Gas
Is a theoretical gas that obeys the gas laws
And thus fit the ideal gas equation exactly
Real Gases
Real gases conform to the gas laws under certain limited conditions
But they condense to liquids and then solidify if the temperature is lowered
Furthermore, there are relatively small forces of attraction between particles of a real gas
This is not the case for an ideal gas
Questions!
Questions Lots of questions.
Homework questions due 24th
January