Chapter 20
The Experimental Gas Laws
20.1
20.2
20.3
Pressure= Force per unit area that the gas exerts normally on a surface
Boyle's law
Charles' law
Pressure law
Isothermal- change at constant temperature
States that form a mixed mass of gas at a constant temperature pV=constant
Graph of pressure against 1/V = straight line through origin
Isobaric- change at constant pressure
Shows how volume varies with absolute temperature
Shows how pressure varies with volume
V/T = constant
Work done=pΔV
Constant volume
Shows how pressure varies with temperature
p/T=constant
Molecules in a gas
Molecules in a gas move randomly with different speeds
Every time a collision occurs a molecule bounces off without losing speed, causing tiny force- lots of collisions= measurable pressure
Brownian motion
All laws require constant mass
Evidence for particles and their random motion
Avogadro Constant
Number of atoms in exactly 12g of the carbon-12 isotope
6.02x10^23
Molar mass
1 Mole- the quantity containing 6.02x10^23 particles of the substance
Number of moles in a substance is its molarity
Molar mass- mass of 1mol of the substance
Therefore:
1) Number of moles in a substance, n=m/M
2) Number of molecules in a substance N=NAm/M where NA= Avagadro's constant
Ideal gas equation
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Putting more particles in a smaller volume increase number of collisions resulting in a higher pressure
1) Negligible intermolecular forces- no attraction or repulsion
2) Volume occupied by gas small compared to volume of container
3) Time of collision small compared to time between collisions
4) Uniform velocity between collisions
5) Collisions perfectly elastic
Observed with pollen grains in water- particles direction and magnitude change randomly due to it being bombarded unevenly and randomly by individual molecules- shows existence of molecules and atoms
1au= 1/12th mass of a carbon-12 atom
Gases
Point molecules
Move at random
Constantly colliding with container walls, each impact causes force, force from many impacts cause pressure
Higher temp.- Higher velocity of particles so harder and more frequent collisions
Molecular Speeds
Molecules of ideal gases have a continuous spread of speeds
Speed of individual molecules change when it collides with another gas molecule
Distribution constant if temp is constant
Increase temp. = increased root mean square speed
Assumptions about ideal gas
1) Molecules are point molecules
2) They don't attract each other
3) Move in continual random motion
4) Collisions are elastic
5) Collisions with container has shorter duration than time between collisions
Molecular and Kinetic Energy
Internal energy of ideal gas due to kinetic energy only
Higher temp.= greater mean kinetic energy of gas