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Gases, Liquids and Solids: • Swimmers (and all Canadians who have been rescued by the Coast Guard!) know that the human body is slightly less dense than water but rather more dense than air. • Liquids and solids (high density) have small molar volumes. Gases have much larger molar volumes at “normal” temperatures and pressures. Gases are “easily” compressed! Table 1 Substance ( Phase ) Experimental Molar Volumes (L.mol-1) ( 00 C and 1.00 bar) H2O(l) 0.01802 H2O(s) 0.01965 (less dense than H2O(l)) C2H5OH(l) 0.0584 NaCl(s) 0.0270 He(g) 22.72 H2(g) 22.72 N2(g) 22.70 CO2(g) 22.56 Table 2 Substance ( Phase ) Densities ( g.L-1) at 00 C and 1.00 bar H2O(l) 999.8 H2O(s) 917.0 Icebergs! C2H5OH(l) 789 George Street. NaCl(s) 2165 He(g) 0.176 H2(g) 0.0887 N2(g) 1.234 CO(g) 1.234 Life is too tough?? CO2(g) 1.951 Photosynthesis. Party Balloons! Condensed Phases and Gases at “Normal” Temperatures and Pressures • Condensed phases (solids and liquids) have relatively small molar volumes and “high” densities. • Gases have relatively high molar volumes and “low” densities • Simple Explanation – in condensed phases molecules are “touching” each other – no “empty” space. Gases at “Normal” Temperatures and Pressures • Gases are mostly empty space – and are thus easily compressed. This is not true at very high P and low T. (Demonstration with dry ice!) • Gases at low pressure can be condensed if subjected to a higher (external) pressure. Gases at high pressure will expand if the external pressure is reduced (propane barbecue). • There are many(!) pressure units. Pressure Units • • • • • • By definition: Pressure = Force/Area “Old” units for P: lb.in-2, mm Hg or torr Modern or SI pressure units P = Force/Area = N/m2 = kg.m s-2/m2 = Pascal Standard atmospheric pressure = 101.325 kPa 101.325 kPa = 1.01325 x 105 Pa (usual metric abbreviations) • We often measure atmospheric pressure using a barometer containing Hg or another liquid. Figure 6-1 The gaseous state of three halogens (group 17) Copyright © 2011 Pearson Canada Inc. General Chemistry: Chapter 6 Slide 7 of 19 P (Pa) = F = W = A A gxm = gxVxd = gxhxAxd A A A = gxhxd liquid pressure is directly proportional to the liquid density and the height of the liquid column Figure 6-3 Liquid Pressure Copyright © 2011 Pearson Canada Inc. General Chemistry: Chapter 6 Slide 8 of 19 Standard Atmospheric Pressure 1.00 atm, 101.325 kPa, 1.01325 bar, 760 torr, ~760 mm Hg Figure 6-4 Measurement of atmospheric pressure with a mercury barometer Copyright © 2011 Pearson Canada Inc. General Chemistry: Chapter 6 Slide 9 of 19 Figure 6-5 Measurement of gas pressure with an open-end manometer Copyright © 2011 Pearson Canada Inc. General Chemistry: Chapter 6 Slide 10 of 19 6-2 Simple Gas Laws 1 Pa V PV = constant Figure 6-6 Relationship between gas volume and pressure – Boyle’s Law Copyright © 2011 Pearson Canada Inc. General Chemistry: Chapter 6 Slide 11 of 19 Boyle’s Law: • The equation PV = constant is valid for a fixed amount of a particular gas at a fixed temperature. One could take two points on the previous graph say (V1,P1) and (V2,P2) and write • P1V1 = P2V2 = constant or just P1V1 = P2V2 • This expression can be used to predict, for example, how the volume of a gas will change when the pressure is altered or….? We call this an initial state → final state problem. Class Example – Boyle’s Law: • At a particular temperature and a pressure of 242 kPa a sample of argon gas Ar(g) has a volume of 3.87 L. What will be the gas volume if the pressure is reduced to 88.6 kPa? (Mention the trichotomy axiom?)