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Total number of electron pairs

Recap VSEPR Theory. Total number of electron pairs. Gases. Gases assume the volume and shape of their containers. Gases are the most compressible state of matter . Gases will mix evenly and completely when confined to the same container .

josiah-ward
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Total number of electron pairs

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  1. Recap VSEPR Theory Total number of electron pairs

  2. Gases • Gases assume the volume and shape of their containers. • Gases are the most compressible state of matter. • Gases will mix evenly and completely when confined to the same container. • Gases have much lower densities than liquids and solids

  3. Empirical Gas Laws • Boyle’s Law - the volume of a gas is inversely proportional to the pressure it exerts: V = kB(1/P) or PV = kB • Charles’ Law - the volume of a gas is directly proportional to its absolute temperature • V = kCT • Avogadro’s Law – equal volumes of gas at the same pressure and temperature contain equal number of molecules • V = kA n

  4. Charles’ Law and Absolute Temperature • V = k2T • Charles’ Law predicts that the volume of a gas keeps decreasing as the temperature is lowered • The temperature at which the V is zero and the gas occupies no space is called absolute zero: it is the lowest possible temperature • On the Kelvin scale, this temperature is set as O K: • 0 K is the same as -273 oC • 100. K is the same as (-273 + 100.) = -173 oC • 0 oC is the same as 273 K

  5. absolute temperature gas constant number of moles volume Ideal Gas Equation PV = kB • V = kCT • V = kA n PV = nRT pressure

  6. Ideal Gas Equation PV=nRT if nand T are fixedthen V= nRT / P (Boyle’s Law) PV=nRT if nand P are fixed then • V = (nR/P) × T (Charles’ Law) PV=nRT if Tand P are fixed then • V = (RT/P) × n (Avogadro’s Law)

  7. Common Units of Pressure • SI unit is the Pascal (Pa) • Most convenient unit is the atmosphere (atm)

  8. The Gas Constant R PV = nRT R = 8.314 J mol–1 K–1 or Pa m3 mol–1 K–1 Use if you are using SI units e.g. pressure in Pa R = 0.08206 atm L mol–1 K–1 Use if you are using atmospheres and litres

  9. Example: Lung Capacity PV = nRT (1.00 atm) x (6 L) PV = n = RT (0.08206atm L mol–1 K–1x 298 K) • Q:The lung capacity of an average human born & living at sea level is ~6 L. How many moles of air will be present in full lungs at 25 C and 1.00 atm? • A:Volume and pressure has been provided in L and atm so use R = 0.08206 atm L mol–1 K–1 = 0.3 mol

  10. Effect of Changing Volume PV = nRT • If the size of a sealed container is changed then n, R and T do not change • If the initial pressure and volume are Pi and Vi and the new pressure and volume are Pf and Vf PiVi= PfVf

  11. Gas Mixtures: Partial Pressures • Dalton’s Law: the total pressure (PT) is the sum of the pressures due to the individual gases, Pi: PT = PA + PB + PC + …. • Air is a mixture of gases with ~79% N2 and ~21% O2: • At atmospheric pressure, PT = 1.0 atm so: • PN2 = 0.79 x 1.0 atm = 0.79 atm • PO2 = 0.21 x 1.0 atm = 0.21 atm • At a depth of 50 m, PT = 6.0 atm so: • PN2 = 0.79 x 6.0 atm = 4.7 atm • PO2 = 0.21 x 6.0 atm = 1.3 atm

  12. Gas Stoichiometry • V = (RT/P) × n 2H2(g) + O2(g)  2H2O(l) • V is proportional to the number of moles: • 2molof H2(g) requires 1molof O2(g) • 2volumesH2(g) requires 1volumes of O2(g) • If we react 2 L of H2 with 1 L of O2, then all of the reactants will react • If we react 1 L of H2 with 1 L of O2, then H2 is the limiting reagent and O2 will be left over at the end of the reaction • If we react 2 L of H2 with 0.5 L of O2, then O2 is the limiting reagent and H2 will be left over at the end of the reaction

  13. Gas Stoichiometry C6H12O6(s) + 6O2(g) 6CO2(g) + 6H2O(l) Q:What is the volume of CO2 produced at body temperature (37 oC) and 1.00 atm when 5.60 g of glucose is burnt in air? A: (i) Convert mass into moles: number of moles = m / M = 5.60 g / 180.2 g mol-1 = 0.0311 mol of glucose (ii) Use chemical equation for stoichometry: number of moles of CO2(g) = 6 x 0.0311 = 0.187 mol (iii) Use ideal gas law to calculate volume: V = nRT / P = (0.187)(0.082606)(37 + 273)/(1.00) = 4.76 L don’t forget to convert to Kelvin!

  14. Learning Outcomes: By the end of this lecture, you should: be able to describe how the volume of a gas varies with pressure, temperature and the number of molecules present be able to use the ideal gas equation and choose the appropriate value of R to use be able to work out and use partial pressures for gas mixtures be able to complete the worksheet (if you haven’t already done so…)

  15. Questions to complete for next lecture: • One mole of gas occupies 22.414 L at STP (1 atm and 0 oC). What is the volume in m3 occupied by one mole of gas at STP? (Hint : 1 m3 = 1000 L) • Use the ideal gas law and your answer to Q1 to work out the value of the gas constant, R, and its units when volume and pressure are given in S.I. units (m3 and Pa respectively). (Hint: from the data sheet, 1 atm = 1.013  105 Pa.)

  16. Questions to complete for next lecture: • A 2.0 L pressure cooker is sealed at atmospheric pressure and 25 oC. What will the pressure in the cooker be at 120 oC? • A 12 L air cylinder is filled to a pressure of 200. atm in an air conditioned diving shop at 22 °C. What will be the pressure inside the tank once it has been left in the sun at 35 °C?

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