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Holt Chemistry Chapter 12: Gases 

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 
  • vand vb are the molecular speeds where v stands for velocity 
  • va / vb = the square root of Mb / Ma
  • 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 
    • Ptotal = P+ P+ PC
  • The ratio of gas volumes is the same as the ratio of moles in a balanced equation 

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 
  • vand vb are the molecular speeds where v stands for velocity 
  • va / vb = the square root of Mb / Ma
  • 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 
    • Ptotal = P+ P+ PC
  • The ratio of gas volumes is the same as the ratio of moles in a balanced equation