Class notes for Chapter 10: Gases in the class General Chemistry: Macroscopic Investigations and Reaction Principles (CHEM 130) at the University of Michigan. Topics covered include kinetic molecular theory, pressure, gas laws (Boyle's, Charles's, and Avogadro's), ideal gas law, gas mixtures, and d...
Kinetic Molecular Theory of Gases
●Pressure: force exerted per unit area
○In chemistry, force exerted by gas molecules as they strike surfaces around them
●Gaseous atoms exert a force when they collide with a surface
○Result of many of those collisions is a constant force on the surfaces exposed to any gas – pressure
●Total pressure exerted by a gas depends on concentration of gas particles in a sample
○Lower concentration = lower pressure
●Kinetic Molecular Theory: model of an ideal gas as a collection of point articles in constant motion undergoing completely elastic collisions
○Gas: collection of particles (either molecules or atoms) in constant motion
●Gases are composed of particles moving in straight lines until they collide with another particle or surface
●3 assumptions:
○Particle size is negligibly small
■The gas particles themselves take up no volume – the space between particles in gas is so large compared to the size of the particles themselves that it doesn’t really “take up space”
○Average kinetic energy is proportional to temperature (in Kelvin)
■Motion of particles in a gas is due to thermal energy which distributes itself among the particles in a gas
■Higher temperature → faster overall motion → greater average kinetic energy
○Particle collisions are elastic
■When 2 particles collide, they may exchange energy but there is no overall loss of energy
■Particles are not deformed by collisions
Pressure: The Result of Particle Collisions
●Pressure = Force exerted per unit area
○P = F/A
●More particles in a volume = higher pressure
○Number of gas particles generally decreases with increasing altitude → pressure decreases with altitude
Pressure Units
●Millimeter of Mercury (mmHg): common unit of pressure referring to the air pressure required to push a column of mercury to a height of 1mm in a barometer
○760 mmHg = 1atm
○Also called a torr ●Barometer: used to measure atmospheric pressure
○Traditionally used mercury
●Atmosphere (atm): unit of pressure based on the average pressure of air at sea level
○1atm = 101,325 Pa
●Pascal (Pa): SI unit of pressure
○1 newton (N) per square meter (1 N/m2)
●Other units of pressure include bar, inches of mercury, and pounds per square inch (psi)
●EX: A cyclist inflates her road bike tire to 132psi. What is the pressure in mmHg?
○Use atm as an intermediate step
○132psi×1atm
14.7psi×760mmHg
1atm=6.82×103mmHg
The Manometer
●Manometer: instrument used to determine pressure of a gaseous sample, usually using mercury
●If the pressure of the gas sample is equal to atmospheric pressure, then mercury levels on both sides of the tube are the same
●If sample pressure > atmospheric pressure, mercury level on the left is higher than the level on the right
●If sample pressure < atmospheric pressure, mercury level on the right is higher than the level on the left
●Diff in height (h) = difference between sample’s pressure and atmospheric pressure
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