3CompressibilityCompressibility – a measure of how much the volume of matter decreases under pressure.Gases are easily compressed because of the space between the particles.
4Factors Affecting Gas Pressure The amount of gas, volume, and temperature are factors that affect gas pressure.Pressure (P) in kilopascals (kPa)Volume (V) in liters (L)Temperature (T) in Kelvin (K)Number of moles (n) in mole (mol)
5Amount of GasYou can use kinetic theory to predict and explain how gases will respond to a change of conditions.As you add more gas particles the pressure increases.
6Amount of GasOnce the pressure exceeds the strength of the container the container will burst.
7Aerosol cans depend on the movement of gas from a region of high pressure to a region of low pressure.Pushing the spray button creates an opening between the inside of the can and the outside.
8The gas flows through the opening to the lower pressure region outside. The movement of the gas propels the paint out of the can until the gas can no longer propel paint out.
9VolumeYou can raise the pressure exerted by a contained gas by reducing its volume.The more a gas is compressed the greater the pressure.
10TemperatureAs a gas is heated, the temperature increases and the average kinetic energy also increases.
11When the volume of a container is held constant and the temperature increases and the pressure increases.
27Ideal Gas LawThe combined gas law is good when the amount of gas does not change – this does not always stay constant though.
28To calculate the number of moles of a contained gas requires an expression that contains the variable n.The number of moles is directly proportional to the number of particles and can be introduced into the combined gas law by dividing each side by n.
29Ideal gas constant – (R) has the value of 8.31 (L•kPa)/(K•mol). Ideal gas law – includes the variables of P, V, T, and n.P is the pressure (units of kPa)V is the volume (units of L)T is the temperature (units of K)n is the number of moles (units of mol)
31Ideal Gases and Real Gases An ideal gas is one that follows the gas laws under all conditions of temperature and pressure.Real gases differ most from an ideal gas at low temperatures and high pressures.
37Dalton’s law of partial pressures – states that at constant volume and temperature, the total pressure exerted by a mixture of gases is equal to the sum of the partial pressures of the component gases.
38Example:Determine the total pressure of a gas mixture that contains oxygen, nitrogen, and helium. The partial pressures are: PO2 = 20kpa, PN2 = 46.7kPa; and PHe = 26.7kPa.
39Graham’s LawDiffusion – tendency of molecules to move toward areas of lower concentration until the concentration is uniform throughout.
40Bromine gas is put in a cylinder and after several hours you can see how the gas has diffused.
41Effusion – a gas escapes through a tiny hole in its container. Gases of lower molar mass diffuse and effuse faster than gases of higher molar mass.
42Thomas Graham’s Contribution Scottish chemist Thomas Graham studied rates of effusion in the 1840’sRelates to KE = ½ mv2.Kinetic energy of the particles (KE) is related to the mass (m) and their velocity (v).
43Graham’s law of effusion – states that the rate of effusion of a gas is inversely proportional to the square root of the gas’s molar mass.
44Example:Determine the rate of effusion for helium compared to nitrogen.This result tells me that the helium effuses/diffuses faster than the nitrogen at the same temperature.