Stochiometry

• Gay-Lussac's Law of Combining Volumes states that, in a reaction between gases, the volumes of the reacting gases and the volumes of any gaseous products are in the ratio of small whole numbers provided the volumes are measured at the same temperature and pressure
• Avogadro's Law states that equal volumes of gases contain equal numbers of molecules under the same conditions of temperature and pressure
• Avogadro realised that atoms combine together to form molecules, and that Gay-Lussac's observations could be explained in terms of molecules
• Avogadro assumed that (Avogadro's Theory): Equal volumes of all gases (under the same conditions of temperature and pressure) contain the same number of particles
• The Ideal Gas Equation: PV = nRT
• P


  Pressure in pascale (kPa)
• V


  Volume in m³ (not cm³ or L)
• n


  Number of moles in mol
• R


  Universal Gas Constant 8.31 J mol⁻¹ K⁻¹
• T


  Temperature in Kelvin
• The Particle Theory


  • Everything is made up of very small particles
  • In solids, the particles can only vibrate about fixed positions
  • In liquids, the particles possess more energy and can move around each other, but do not possess sufficient energy to overcome the intermolecular forces between them to break free from each other
  • In gases, the particles have sufficient energy to almost completely overcome the intermolecular forces of attraction between them and are therefore almost completely independent of each other
• Assumptions of the Kinetic Theory of Gases

  • Gases are made up of particles that are in constant rapid random motion, colliding with each other and the walls of the container
  • There are no attractive or repulsive forces between the particles of a gas
  • The gas particles are so small and so widely separated that the total volume of all of the particles is negligible compared to the space they occupy
  • The average kinetic energy of the particles in a sample of gas is proportional to the temperature measured on the Kelvin scale
  • Collisions between particles are perfectly elastic
• Contrary to assumption 2, there are forces of attraction between particles of a gas, van der waals forces or dipole/dipole forces
• Contrary to assumption 3, it is not valid to say that the total volume of the particles of a gas is always negligible compared to the space that they occupy
• Real Gases


  • Do not obey the assumptions of the Kinetic theory under all conditions of temperature and pressure
  • Forces of attraction and repulsion do exist between the molecules
  • The volume of the molecules is not negligible
• Real gases come closest to ideal behaviour at low pressures and high temperatures
• Real gases deviate most from ideal behaviour at high pressures and low temperatures
• Under the same conditions of temperature and pressure, non polar molecules come closer to ideal behaviour than polar molecules
• Low temperatures


  The molecules are moving slowly, allowing the intermolecular forces to become effective, contrary to assumption 2
• High pressures


  The molecules are forced close together and the volume of the molecules themselves is no longer negligible compared to the volume in which they are moving, contrary to assumption 3
• Under the same conditions of temperature and pressure, non polar molecules come closer to ideal behaviour than polar molecules since the attractive intermolecular forces are less in the nen polar molecule
• The most ideal gas molecule

  The smallest, least polar one
• Boyle's Law Explanation: (Kinetic Theory)

  1. As the volume of the container (gas) is decreased

  2. The particles are closer together

  3. More collisions with each other and the walls of the container

  4. Causing an increase in pressure
• Charles Law Explanation: (Kinetic Theory)

  1. As the temperature increases

  2. The particles move faster

  3. More collisions

  4. Pressure increases

  5. BUT pressure is constant

  6. Volume has to increase
• Relative atomic mass A


  The average of the mass numbers of the isotopes of the element as they occur naturally, taking their abundance into account, expressed on a scale in which the atoms of the carbon-12 isotope have a mass of exactly 12 units
• Relative molecular mass M


  The average mass of one molecule of that compound compared with one twelfth of the mass of one atom of the carbon-12 isotope
• To calculate the relative molecular mass M, of a substance


  Add up the relative atomic masses of each of the elements present, taking account of the number of atoms of each element in the molecule
• Mole


  The amount of a substance that contains the Avogadro Constant (6 x 10^23) number of particles of that substance
• The molar volume of any gas at standard temperature and pressure (STP) is 22.4 L
• The molar volume of any gas at room temperature is 24 L
• Boyle's Law


  At constant temperature, the volume of a fixed mass of gas is inversely proportional to its pressure
• Increase Pressure


  Decrease Volume
• Boyle's Law: V = k/P
• Charles' Law


  At constant pressure, the volume of a fixed mass of gas is directly proportional to its temperature measured on the Kelvin scale
• Charles' Law: V = k*T
• Combined Gas Law (General Gas Law)


  Used to convert the volume of a fixed mass of gas under one set of conditions of temperature and pressure to what the volume of the gas would be under a different set of temperature and pressure conditions
• According to Avogadro's Law, equal volumes of gases contain equal numbers of molecules
• Molar Volume


  The volume occupied by one mole of a gas
• The molar volume of any gas is the same at a given temperature and pressure