Chemistry lecture 10

• The electronegativity difference between two atoms determines the polarity of a bond.
• A polar covalent bond is formed when there is an unequal sharing of electrons, resulting in a partial negative charge on one atom and a partial positive charge on another.
• Mole


  Unit of measurement used in chemistry to express amounts of a chemical substance, equal to about 6.02214×10^23 molecules of that substance
• The mole is one of the base units in the International System of Units, and has the unit symbol mol
• Dozen


  12 eggs
• A mole of hydrogen gas molecules is 6.02214×10^23 molecules, which occupy 22.416 dm^3 at 0°C and 1 atm pressure
• Gram molecular mass


  Mass of a mole of hydrogen gas molecules, 2.01565 grams
• Atomic Mass Scale


  Relative scale based on the mass of carbon-12 atom
• 1 amu = 1/12 mass of C-12 = 1.6604 x 10^-24 g
• 1 mole amu = 1 gram
• 1 mole of oxygen = 15.9996 amu = 15.9996 g
• Mole expressions


  1. For solids - mole (n) = mass measured (m) / molecular mass (Mm)

  2. For liquids - mole (n) = concentration x volume

  3. For gases - mole (n) = Pressure x Volume / RT
• At 0°C and 1 atmosphere of pressure, one mole of every gas occupies about 22.4 liters of volume
• Gases


  • Molecules are approximately 10 diameters apart
  • Occupy their containers uniformly and completely
  • Easier to compress than liquids or solids
• Doubling the pressure of a gas

  Reduces its volume to about half of its previous value
• Doubling the mass of gas in a closed container

  Doubles its pressure
• Increasing the temperature of a gas enclosed in a container

  Increases its pressure
• Ideal gas


  Obeys the Ideal Gas Law: PV = nRT
• Real gases do not obey the Ideal Gas Law, but at high temperature and low pressure they behave more like an ideal gas
• Boyle's Law


  1. The volume of a given amount of gas held at constant temperature varies inversely with the applied pressure

  2. V α 1/P (constant T & mass)

  3. PV = k1
• Charles' Law


  1. The volume of a fixed amount of gas is directly proportional to its Kelvin temperature when the gas pressure is held constant

  2. V α T (constant P & mass)

  3. V = k2T
• As temperature reaches absolute zero, the gas will not shrink down to zero volume, as all gases turn into liquids at low enough temperatures
• Gay-Lussac's Law


  1. The pressure of a fixed amount of gas held constant volume is directly proportional to the Kelvin temperature

  2. P α T (constant V & mass)

  3. P = k3T
• Combined Gas Law


  1. The ratio PV/T is a constant for a fixed amount of gas

  2. P1V1/T1 = constant = P2V2/T2
• Avogadro's Law


  1. The volume of a gas is directly proportional to the amount of gas (n) at a given temperature and pressure

  2. V α n (P and T constant)

  3. V = constant × n
• 1 mol of any gas at 0°C and 1 atm pressure occupies 22.4 × 10^-3 m^3 or 22.4 litre
• Ideal gas obeys Boyle's and Charles' law
• Ideal Gas Law


  1. PV/T = constant = nR

  2. PV = nRT
• The gas constant R = 0.0821 L·atm/mol·K
• Van der Waals Equation

  • An attempt to modify the Ideal Gas Law to fit experimental data for real gases
  • (P + n^2a/v^2)(V-nb) = nRT
  • Where a and b are van der Waals constants
• Density and molar mass of a gas


  d = PM / RT
• Diffusion


  The rate at which two gases mix
• Effusion


  The rate at which a gas escapes through a pinhole into a vacuum
• Graham's Law of Diffusion


  The rate at which gases diffuse is inversely proportional to the square root of their densities
• Graham's Law of Effusion

  The rate of effusion of a gas is inversely proportional to the square root of either the density or the molar mass of the gas
• The average kinetic energy of gas molecules depends only on temperature, so two different gases at the same temperature must have the same average kinetic energy
• Velocity (rate) of gas molecules

  Inversely proportional to the square root of their molar masses