Unit B - Gasses

• • Pressure is a measure of force per unit area
  • SI units for pressure are Pa(Pascal)= 1 N(Newton) on 1 m²
  • So 1 Pa= 1 N/m² and 1kPa= 1000 Pa= 1 kN/m²
• • Atmospheric pressure is the force per unit area exerted by air on all objects
  • Average atmospheric pressure at sea level is about 101 kPa
  • Also referred to as standard pressure
• • Standard temperature and pressure= 0℃ and 1 atm (101kPa)
  • SATP (standard ambient temperature and pressure)= 25℃ and 100 kPa  (closer to realistic lab values)
• • Gases vary in reactivity depending on the number of valence electrons
  • E.g. fluorine and chlorine are extremely reactive, while helium and argon are inert
• Physical Properties of gases

  • Gases have no definite shape or volume; as a result, they always fill their container
  • Gases are all highly compressible (volume can be decreased by increasing pressure)
  • Gases can readily diffuse through any available space
  • The volume and/or pressure of gases can be affected by temperature
• BOYLE’S LAW:

  • Pressure and volume are inversely related; that is, as the pressure applied to a gas increases, the volume of that gas decreases.
  • Boyle’s Law states that as the pressure on a gas increases, the volume of the gas decreases proportionally, provided that the temperature and amount of gas remain constant
  • P1V1 =P2V2
• Absolute zero- the lowest 

  possible temperature

   = -273℃ or 0 K
• • We therefore can convert from Celsius to Kelvins by simply adding 273 K.
• CHARLES’ LAW:

  • Temperature and volume are directly related; that is, as the temperature of a gas increases, the volume of that gas also increases.
  • Charles’ Law states that as the temperature of a gas increases, the volume increases proportionally, provided that the pressure and amount of gas remain constant
  • V1/T1 =V2/T2
• The Combined Gas Law 

  • Boyle’s Law only works by assuming that temperature remains constant. 
  • Charles’ Law only works by assuming that pressure remains constant.
  • By combining these two equations, we can eliminate any assumptions, thereby allowing us to calculate the pressure/volume/temperature of a system under changing conditions (more realistic)
  • P1V1/T1=P2V2/T2
• • When measured at the same temperature and pressure, the volume of gaseous reactants and products of any chemical reaction can be represented using simple whole-number ratios
• The relationship between the ratio of volumes in a chemical reaction and coefficient ratios is explained by Avogadro’s Theory, which states that equal volumes of gases at the same temperature and pressure contain equal numbers of molecules (n). 
• • Molar volume is the volume that one mole of a gas occupies at a specific temperature and pressure. 
• • Under STP conditions (273 K & 100 kPa), the molar volume of a gas is 22.4 L/mol (I memorized this by saying that "S" in STP looks like a 2 so theres more "2's"
• • Under SATP conditions (298 K & 100 kPa), the molar volume of a gas is 24.8 L/mol (I memorized this by saying that A looks like a 4 so its 24.8)
• • An ideal gas is a hypothetical gas that obeys all of the gas laws perfectly under all conditions.
  • An ideal gas, for example, does not condense into a liquid when cooled.
  • In reality, however, gases do not always behave the way we predict them to according to kinetic molecular theory.
•  

  IDEAL GASES

  Molecules are very far apart compared to their size; thus, the size of molecules is negligible

  Gas molecules are in constant, random motion 

  Gas molecules undergo perfectly elastic collisions in which no energy is lost
•  

  REAL GASES

  Under high pressures, molecules are forced much closer together than usually possible; thus, the size of molecules becomes significant

  As temperature decreases, molecules slow down, and intermolecular attractions may cause molecules to “stick” together

  Molecules are more like “soft” spheres which change shape during collisions, causing them to rebound more slowly
• The Ideal Gas Law

  • The universal gas constant (R) is 8.314 kPa৹L/mol৹K
  • The value of the universal gas constant depends on the units chosen to measure volume, pressure, and temperature
  • PV=NRT