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Chapter 6 - Gases. Physical Characteristics of Gases. Although gases have different chemical properties, gases have remarkably similar physical properties. Gases always fill their containers (recall solids and liquids). No definite shape and volume

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physical characteristics of gases
Physical Characteristics of Gases
  • Although gases have different chemical properties, gases have remarkably similar physical properties.
    • Gases always fill their containers (recall solids and liquids). No definite shape and volume
    • Gases are highly compressible: Volume decreases as pressure increases Volume increases as pressure decreases
    • Gases diffuse (move spontaneously throughout any available space).
    • Temperature affects either the volume or the pressure of a gas, or both.
definition of a gas
Definition of a Gas
  • Therefore a definition for gas is: a substance that fills and assumesthe shape of its container, diffuses rapidly, and mixes readily with other gases.
three gas laws
Three Gas Laws
  • Pressure
    • force of colliding particles per unit area
    • According to the KMT gases exert pressure due to the forces exerted by gas particles colliding with themselves and the sides of the container
    • SI unit for pressure is kilopascals - kPa
1 kPa = 1000 N/ 1 m2
  • Atmospheric pressure – pressure exerted by air particles colliding
  • SATP – 100 kPa at 25 °C
  • STP – 101.3 kPa at 0 °C
boyle s law
Boyle’s Law
  • As pressure on a gas increases the volume of the gas decreases proportionally as the temperature is held constant
  • P1V1 = P2V2
charles law
Charles Law
  • the volume of a gas increasesproportionally as the temperature of the gas increases, if the pressure is held Constant
  • V1 = V2T1 T2
kelvin temperature scale
Kelvin Temperature Scale
  • Temperature - the average kinetic energy of the particles making up a substance
  • Kelvin Temp Scale: based of absolute zero — all kinetic motion stops
  • 273°C= 0 K 0°C = 273 K 30°C =303 K -20°C = 253 K
  • Formulas °C = K - 273 K= °C+273
combined gas law
Combined Gas Law
  • This is when all variables (T,P, and V) are changing
    • P1V1 = P2V2

T1 T2

avogadro s theory and molar volume
Avogadro’s Theory and Molar volume
  • The kinetic molecular theory is strongly supported by experimental evidence.
  • The K M theory explains why gases, unlike solids and liquids, are compressible.
  • The K M theory explains the concept of gas pressure.
  • The K M theory explains Boyle’s Law — Increase volume \ decrease pressure
  • The KM theory explains Charles’ Law Increase volume \ increase temperature
history lesson
History Lesson
  • 1808 – Joseph Guy – Lussac
    • “Law of Combining Volumes”
      • When measuring at the same temp and pressure, volumes of gas reactants and products (in chemical reactions) are always in simple whole number ratios
  • 1810 – AmadeoAvogadro
    • “Avogadro’s Theory”
      • Equal volumes of gases at the same temp and pressure have equal number of molecules
molar volume of gases new conversion ratio
Molar Volume of Gases“new conversion ratio”
  • Avogadro says :
          • T1 = T2
          • P1 =P2
          • V1 = V2
          • Then # particles of gas 1 = # particles of gas 2
  • 1 mol = 6.03 x 10 23 particles
  • Lets put these two ideas together……
Therefore for all gases at a specific temp and pressure there must be a certain volume that contains exactly 1 mole of particles - molar volume
  • The two most standard temps and pressures are STP and SATP
molar volume
Molar Volume
  • When gases are at STP:
    • 1 mole of any gas = 22.4 L/mol
  • When gases are at SATP:
    • 1 mole of any gas = 24.8 L/mol
ideal gas equation
Ideal Gas Equation
  • Ideal Gas — is ahypothetical gas that obeys all the gas laws perfectly under all conditions. It is composed of particles with no attraction to each other. (Real gas particles do have atiny attraction)
  • The further apart the gas particles are, the faster they are moving the less attractive force they have and behave the most like ideal gases
  • The smaller the molecules the closer the gas resembles an ideal gas
  • We assume ideal gases always.


  • PV = nRT
  • P= pressure (kPa)
  • V = volume (L) n = moles (mol) R = universal gas constant (8.31 kPa*L ) Mol * K T = temperature (K)
  • Sometimes the n must be converted to mass after the equation is completed. If this is necessary, use a conversion