Holt Chemistry Chapter 12: Gases 

12.1 Characteristics of Gases

• Gases are unique because the distance between particles is greater than the distances in liquids and solids
• Gases are considered fluids because they can flow
  • Because gas particles are far apart they are able to move past one another 
• Gases have lower densities the liquids and solids 
  • Because most of the volume of gas is empty space gases have low densities 
• Gases are very compressible because the total space the particles take up is much smaller than the total volume 
• Gases completely fill their containers 
• Air pressure is the collision of gas molecules 
• Pressure is forced divided by area, meaning the amount of force exerted per surface area unit 
• The unit of force is called a Newton 
  • 1 Newton = 1 kg x 1 m/s² = 1 N
• The unit of pressure is called a pascal 
  • 1 Pa = 1 N / 1m²
• There are also other units of pressure 
  • Atmosphere/atm: 1 atm = 101,325 Pa
  • Bar: 1 bar = 100,025 Pa
  • Millimeter of mercury/mm Hg: 1 mm Hg = 133.322 Pa
  • Pounds per square inch/psi: 1 psi = 6.89286 x 10³ Pa
  • Torr: 1 torr = 133.322 Pa
• Standard temperature and pressure, STP, is equal to 0°C and 1 atm
• The kinetic molecular theory is used to predict gas behavior and states 
  • Gas particles are in constant rapid, random motion 
  • Particles are far apart relative to their size 
  • Pressure exerted is a result of collisions of molecules against the container 
• The average kinetic energy is proportional to the temperature in kelvin

12.2 The Gas Laws 

• P = pressure exerted by the gas 
• T = temperature of the gas in kelvin 
• V = total volume occupied by the gas 
• n = number of moles of the gas 
• k = constant 
• Robert Boyle discovered the gas pressure of a gas increased, the volume decreased but the product remained constant 
• His law states that there is an inverse relationship between pressure and volume. As one increases the other must decrease 
  • PV = k
  • P₁V₁ = P₂V₂
• There is a direct relationship between temperature and volume. This is called Charle’s Law 
• It states that as the temperature of a gas increases, the volume also increases. And I’d the temperature decreases, the volume will also decrease 
  • V/T = k 
  • V₁/T₁ = V₂/T₂
• There is a direct relationship between pressure and temperature known as Gay-Lussac’s law 
• It states that the pressure of a gas is proportional to the absolute temperature 
  • P = kT
  • P/T = k 
  • P₁/T₁ = P₂/T₂
• Avogadro’s law states equal volumes of gases at the same temperature and pressure have an equal number of molecules 
  • V =kn 

12.3 Molecular Composition of Gases 

• No gas perfectly follows the four laws from section 2. The model of a gas that would follow these laws is called an ideal gas 
• The ideal gas is imaginary and has particles that are infinitely small and do not interact with one another 
• The relationship between all four variables(P, V, T, and n) can be combined into one equation. This equation is called the ideal gas law 
  • PV = nRT where R is constant 
  • R = 8.314 L x kPa / mol K 
  • R = 0.0821 L atm / mol K 
• Diffusion is the movement of particles from regions of high density to areas of lower density 
  • For example the smell of cookies spreading from the kitchen to the living room 
• Gases diffuse at rapid rates 
• Diffusion causes an increase in entropy 
• Effusion is the passage of gas through a small opening 
  • For example a hole in a tire 
• Thomas Graham discovered that the rate of effusion is inversely proportional to the square root of the gas’s molar mass(M)
  • This is known as Graham’s law of Diffusion 
• v_a and v_b are the molecular speeds where v stands for velocity 
• v_a / v_b = the square root of M_b / M_a
• Particles of lower molar masses travel much faster than heavier particles 
• Gay-Lussac’s law of combining volumes states that the volume of gases involved in chemical changes can be represented by a ratio of small whole numbers 
• In a mixture of gases, each individual gas will exert a certain amount of pressure 
• The pressure of each had in a mixture of gases is called the partial pressure 
• The total pressure of the mixture can be found by adding the partial pressures. This is known as Dalton’s laws of partial pressure 
  • P_total = P_A + P_B + P_C
• The ratio of gas volumes is the same as the ratio of moles in a balanced equation