CHEM 110 Chapter Notes - Chapter 5: Gas Constant, John Torrey, Scuba Diving
10 May 2018
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Chapter 5
- Pressure is the force exerted per unit area by gas molecules as they strike the surface
around it
o The total pressured exerted by a gas depends on several factors such as the
concentration of the gas molecules
▪ Higher the concentration, the greater the pressure
o Expanded volume results in a lower concentration of gas molecules
▪ Concentration goes down and there are fewer collisions amongst
molecules
• Opposite happens with a decreased volume
Pressure: The result of Molecular collisions
- Pressure is the result of constant collisions between the atoms or molecules in a gas
o Pressure of gas depends on several factors
▪ Number of gas particles in a given volume
• Fewer the gas particles, the lower the pressure
o Pressure decreases with increasing
▪ Low density of gas prarticles results in low pressure
▪ High density of gas particles results in high pressure
Pressure Units
- A common unit of pressure, mmHg (Millimetre of mercury)
o Originated from how pressure is measured with a barometer
▪ Barometer is an evacuated tube, where liquid is forced upwards by gas
pressure on the liquid surface
• When the atmospheric pressure rises, heigh of mercury rises as
well
• When atmospheric pressure falls, the height of the column falls
o Millimetre of mercury is also called a Torr
▪ 1 mmHg = 1 Torr
o The SI unit pressure is the pascal.
▪ Average sea pressures are 101 325 Pa
▪ Amount of pressure pushes a column of mercury to a height of 760 mmHg
o Atmosphere is another unit of pressure
▪ Equals 760 mmHg and 101 325 Pa
• 101 325 Pa = 101.325 kPa = 760 Torr = 1atm
o Bar is a unit of pressure that is equal to 100 000 Pa (1 bar is standard pressure for
gaseous components)
o Example: Converting between Pressure and Units
▪ Aircraft tire operates at high pressures: 2.00 x 102 psi. What is this
pressure in kPa and in bar
• Given: 2.00 x 102 psi
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• Find: Value in kPa & Bar
o 1 bar = 14.504 psi
o 100 kPa = 1 bar
• Plan: psi BarkPa
o 1 bar/14.504 psi 100 kPa/1 Bar
• Solution: 2.00 x 102 psi x 1 bar/14.504 psi = 13.8 bar
o 13.8 bar x 100 kPa/1 Bar = 1.38 x 103 kPa
- The Manometer: Way to measure Pressure in Laboratory
o A manometer is a U-shaped tube containing a dense liquid
▪ One end of the tube is open to atmospheric pressure and other attached to
a flask containing a gas sample
• If the pressure of the gas sample is equal to atmospheric pressure,
the mercury levels on both sides are the same
• If the pressure of the sample is greater than atmospheric pressure,
the mercury level on the left side is higher than on the right side
• If the pressure of the sample is less than atmospheric pressure, the
mercury level on the left side will be lower than the level on the
right side
• The difference in height between the two levels is equal to the
difference between the samples pressure and atmospheric pressure
Simple Gas Laws: Boyle’s law, Charles Law, Avogadro Law
- A relationship between Volume and pressI ure indicates that an increase in one results in
a decrease in the other (Boyle’s Law)
- Boyles Law assumes a constant temperature and amount of gas
o Follows the idea that pressure results from collisions of gas particles with the
walls of hteir container
▪ If volume of gas is decreased, same number of gas particles is crowded
into a smaller volume, resulting in more collisions, thus increasing
pressure
- Boyles Law and Scuba diving
o When a diver is 20m below the surface, the regulator delivers air at a pressure of
3 bar to match 3 bar pressure around the diver.
▪ 1 bar due to atmospheric pressure and 2 bar due to the weight of the water
at 20m
o If a diver inhaled a lungful of air at a pressure of 3 bar and swam to the surface (1
bar), the volume increase in the divers lung would be so great that the air would
force itself out of her mouth, damaging her lungs.
o Boyle’s law can be used to calculate the volume of a gas following a pressure
change or pressure of a gas following a volume change
▪ The temperature and amount
of gas must remain constant
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o If we multiply both sides by P we get: PV = Contant
▪ For two different sets of conditions:
• P1V1 = Constant = P2V2
o P1 = Initital pressure
o V1 = Initial volume
o P2 = final pressure
o V2 = Final volume
o Example: A cylinder is equipped with a piston that has a volume of 7.25L under a
pressure of 657 mbar. What is the applied pressure, in bar, required to decrease
volume of the cylinder to 2L?
▪ Given: V1 = 7.25L
• P1 = 657 mbar
• V2 = 2L
• P2 = ?
▪ Solution: P1V1 = P2V2
• P2 = P1V1/V2
o 657 mbar * 7.25L/2 L
o 2389 Mbar x 1bar/1000 mbar
▪ 2.38 bar
- Charles Law: Volumes and Temperature
o Volume and Temperature are linearly related
▪ If two variables are linearly related, plotting one against the other,
produces a straight line.
▪ If we extend or extrapolate the line in the plot backwards, the line shows
that gas should occupy zero volume at -273.15 celcius
▪ As temperature increases, volume increases
▪ As temperature decreases, volume decreases
▪ Assumes a constant pressure and amount of gas
• When temperature of a gas Is increased, particles move faster and
collisions with the walls are frequent
• When air is heated, its volume increases, resulting in lower density
▪ Kalvin = C + 273
▪ Can use Charles law to calculate the volume of a gas folwing a
temperature change or the temperature of a gas following a volume change
• Since V ∝ T, then V = constant × T
o When you divide both sides by T V/T = constant
▪ If temperature increases, the volume increases in direct proportion to the
quotient
• Temperature must always be expressed in Kelvins
o Volume of gas is proportional to temperature
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Document Summary
If the pressure of the gas sample is equal to atmospheric pressure, the mercury levels on both sides are the same. If the pressure of the sample is greater than atmospheric pressure, the mercury level on the left side is higher than on the right side. Simple gas laws: boyle"s law, charles law, avogadro law. A relationship between volume and pressi ure indicates that an increase in one results in a decrease in the other (boyle"s law) Boyles law assumes a constant temperature and amount of gas: follows the idea that pressure results from collisions of gas particles with the walls of hteir container. If volume of gas is decreased, same number of gas particles is crowded into a smaller volume, resulting in more collisions, thus increasing pressure. Boyles law and scuba diving: when a diver is 20m below the surface, the regulator delivers air at a pressure of. Charles law: volumes and temperature: volume and temperature are linearly related.
