A/Prof Fazlul Huq Discipline of Biomedical Science Faculty of Medicine The University of Sydney
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PROPERTIES OF GASES
Highly compressible No fixed shape and volume Molecules in constant random motion Average molecular kinetic energy proportional to temperature in Kelvin
KINETIC THEORY OF GASES
A gas consists of very small particles called molecules that are in constant random motion The molecules collide with each other and with the walls of the containing vessel The collisions are considered to be elastic so that there is no kinetic loss due to collision The average molecular kinetic energy is proportional to temperature (in Kelvin) of the gas The size of the molecules is negligibly small compared with the space they occupy
Emprical Laws of Gases
Boyle’s Law Charles’ Law Avogadro’s Law
At a given temperature, the volume occupied by a certain amount of a gas is inversely proportional to its pressure.
1/V P . . .. . .. . ..
V V = constant at a given T and n (n = no. of moles)
Breathing and Boyle’s Law
We breathe about 12 times a minute Inhaling and exhaling about 0.50 L of air each time In breathing in, volume of chest cavity is increased by lowering the diaphragm or raising the chest cavity In breathing out, volume of chest cavity decreased by raising the diaphragm or lowering the rib case
At given pressure, for a certain amount of a gas, volume increases linearly with the increase in temperature.
Why Absolute Zero?
If the extrapolation of V versus T graph is correct,
of a gas would become zero at 273.15oC and negative below the temperature
Negative volume and for that matter zero volume is not possible for material objects.
-273.15oC is considered to be the lowest temperature that can ever be reached.
Absolute or Kelvin Scale Defined
T(K) = ToC + 273.15
= 25 + 273.15 = 298.15 K
Charles’ Law Restated
At a given pressure, the volume of a given amount of gas is proportional to its temperature in Kelvin:
At constant P & n
V1 = V2 T2 = constant T
Note: T must be in Kelvin
To what Celsius temperature must a gas sample at 20oC be heated if its volume is to double while its pressure remains unchanged?
T1 = 20 + 273.15 K = 293.15 K V2 = 2V1 T2 = ?
T2 = 2 x 293.15 = 586.3 K 13 = 313.15oC
Gay Lussac’s Law
At a given volume, the pressure of a given amount of gas is proportional to its temperature in Kelvin:
At constant V & n
P1 = P2 T2 = constant T
Note: T must be in Kelvin
At a given temperature and pressure, the volume of a gas is proportional to the number of moles of the gas.
at constant T and P
Constant at constant T and P
Combined Gas Law Equation
PV = nRT
Temperature in kelvin
Molar gas constant
Number of moles
The constant has a value of 8.314 J K-1 mol-1
What is an ideal gas?
A gas that obeys the equation: PV = nRT under all conditions of temperature and pressure. Real gases only approximate ideal behaviour
do better than others (eg oxygen better than carbon dioxide, helium better nitrogen) Better at high temperatures and low