Presentation on theme: "Mole concept applied to gases"— Presentation transcript:
1 Mole concept applied to gases 1.4.4 Apply Avogadro’s law to calculate reacting volumes of gasApply the concept of molar volume at standard temperature and pressure in calculations1.4.6 Solve problems between temperature, pressure and volume for a fixed mass of an ideal gas.Solve problems relating to the ideal gas equation, PV=nRTAnalyse graphs relating to the ideal gas equation.
2 Avogadro’s Hypothesis At a constant temperature and pressure, a given volume of gas always has the same number of particles.The coefficients of a balanced reaction is the same ratio as the volumes of reactants and products
3 2CO (g) + O2 (g) 2CO2(g)For the above example, it is understood that half the volume of oxygen is needed to react with a given volume of carbon monoxide.This can be used to carry out calculations about volume of gaseous product and the volume of any excess reagents.
4 Example10cm3 of ethyne (C2H2) is reacted with 50cm3 of hydrogen to produce ethane (C2H6), calculate the total volume and composition of the remaining gas mixture, assuming constant T and P.1st get balanced equation: C2H2(g) + 2H2(g) C2H6(g)2nd look at the volume ratios: 1 mol ethyne to 2 mol ofhydrogen, therefore 1 vol to 2 vol3rd analyse: If all 10cm3 of ethyne is used, it needsonly 20cm3 of hydrogen, therefore hydrogen is in excessby 50cm3-20cm3 = 30 cm3. In the end you’ll have 10 cm3Ethane and the leftover 30 cm3 hydrogen
5 Molar volumeThe temperature and pressure are specified and used to calculate the volume of one mole of gas.Standard temperature and pressure (STP) is at sea level 1 atm = kPa and 0oC = 273 K this volume is 22.4 dm3 (or 22.4 L)Molar gas volume, Vm.It contains 6.02 x 1023 molecules of gas
6 ExampleCalculate how many moles of oxygen molecules are there in 5.00 dm3 at STPn= VSTP = = mol22.4 dm dm3
7 Boyle’s Law (1659)Boyle noticed that the product of the volume of air times the pressure exerted on it was very nearly a constant, or PV=constant.If V increases, P decreases proportionately and vice versa. (Inverse proportions)Temperature must be constant.Example: A balloon under normal pressure is blown up (1 atm), if we put it under water and exert more pressure on it (2 atm), the volume of the balloon will be smaller (1/2 its original size)P1V1=P2V2
8 Charles’ Law (1787)Gas expands (volume increases) when heated and contracts (volume decreases) when cooled.The volume of a fixed mass of gas varies directly with the Kelvin temperature provided the pressure is constant. V= constant x TV1 = V2T1 T2
9 Gay-Lussac’s LawThe pressure of a gas increases as its temperature increases.As a gas is heated, its molecules move more quickly, hitting up against the walls of the container more often, causing increased pressure.P1 = P2T1 T2
10 Laws combined… P1V1 = P2V2 T1 T2 T must be in Kelvins, but P and V can be any proper unit provided they are consistently used throughout the calculation
11 PracticeIf a given mass of gas occupies a volume of 8.50 L at a pressure of 95.0 kPa and 35 oC, what volume will it occupy at a pressure of 75.0 kPa and a temperature of 150 oC?1st convert oC to K:= 308 K = 423 K2nd rearrange equation and solve problem:V2 = V1 x P1 x T2 = x 95.0 x 423 = 14.8 LP2 x T x 308
12 TemperatureKelvin temperature is proportional to the average kinetic energy of the gas molecules.It is a measure of random motion of the gas moleculesMore motion = higher temperature
13 Ideal gas behaviourIdeal behaviour is when a gas obeys Boyle’s, Charle’s and Gay-Lussac’s laws wellAt ordinary temperature and pressures, but there is deviation at low temperature and high pressures
14 Ideal gaswhere all collisions between molecules are perfectly elastic and in which there are no intermolecular attractive forces.Its like hard spheres bouncing around, but NO interaction.
15 Ideal gas law PV = nRT P= pressure (kPa) Volume = (dm3) n= number of molesR=universal gas constant = J mol-1 K-1T= temperature (K)
16 PV= nRT rearrange equation for n Example3.376 g of a gas occupies dm3 at 17.6 oC and a pressure of kPa, determine its molar mass.PV= nRT rearrange equation for nn= PV/RT= (96.73 x 2.368) / (8.314 x 290.6)= molMolar mass = mass/ mole= g / mol= g/mol