Properties of Gases PDF

R. Gallagher www.theconicalflask.ie Properties of Gases Temperature - We must use Kelvin in our calculations. Celsius to Kelvin conversion: - e.g. Convert 50°C to Kelvin = 50 + 273 = 323 K - Both s...

R. Gallagher www.theconicalflask.ie Properties of Gases Temperature - We must use Kelvin in our calculations. Celsius to Kelvin conversion: - e.g. Convert 50°C to Kelvin = 50 + 273 = 323 K - Both scales difference in size of degree are the same. E.g. a rise in temperature of 10°C is the same as a rise in 10 K. Pressure - We must use Pascal (Pa) in our calculations. kPa to Pa conversion: - e.g. Convert 20 kPa to Pa: 20 x 1000 = 20,000 Pa Volume - We must use m3 in our calculations. cm3 to m3 conversion: - e.g. Convert 20cm3 to m3: 20 x 10-6 m3 Standard Temperature and Pressure (s.t.p.) Standard temperature = 273 K Standard pressure = 1 x 105 Pa (100 kPa) - Volume of gas varies with temperature and pressure. - So when comparing volumes of gases, it is important to keep the temperature and pressure constant. 20 R. Gallagher www.theconicalflask.ie Boyles Law: - P ∝ 1/V for a fixed mass of gas at constant temperature. Charles Law: - V ∝ T (Kelvin) for a fixed mass of gas at constant pressure. - It is consistent with PV = nRT due to constant. Avagadro's Law: - Under the same conditions of temperature and pressure, equal volumes have the same number of molecules/atoms. Gay-Lussac's Law of combining volumes: - The volume of reacting gases and their gas products are in small whole number ratios when at same temperature and pressure. Molar volume calculations: - The gas occupied by one mole of any gas is called the molar volume. - Standard temperature and pressure (STP) or Room temperature and pressure (RTP) are two units we often use in chemistry. Volume (l) = No. moles x STP Volume (l) = No. moles x RTP The Kinetic Theory of Gases 1. There are no attractive or repulsive forces between molecules of gas. 2. Gas molecules are so small and so widely separated that the actual volume of all the molecules is negligible compared with the space that they occupy. 3. Collisions between particles are perfectly elastic, i.e. there is no loss of kinetic energy in these collisions although there may be a transfer of energy. Ideal gas: - A gas that perfectly obeys all the assumptions of kinetic theory under all conditions of temperature and pressure. 21 R. Gallagher www.theconicalflask.ie Limitations to the Kinetic Theory of Gases - There are forces of attraction between gas molecules e.g. Van Der Waals, dipole- dipole - The volume is not negligible under high pressure. Real gases come close to an ideal gas under two conditions: 1. Low pressure. 2. High temperatures. Molecules are moving rapidly, preventing the forces between molecules from operating. The Equation for an Ideal Gas PV = nRT P = pressure (Pa) V = Volume (m3) n = number of moles R = Universal gas constant = 8.31 T = Temperature (K) 22