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Gases. Chapter 12. Characteristics of Gases. Expand to fill a volume (expandability) Compressible Readily forms homogeneous mixtures with other gases. Pressure. Pressure - force acting on an object per unit area. Pressure. Conversion Factors 1 atm (atmosphere) = 760 mmHg

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gases

Gases

Chapter 12

Chapter 12

slide2

Characteristics of Gases

  • Expand to fill a volume (expandability)
  • Compressible
  • Readily forms homogeneous mixtures with other gases

Chapter 12

slide3

Pressure

Pressure - force acting on an object per unit area.

Chapter 12

slide4

Pressure

  • Conversion Factors
    • 1 atm (atmosphere) = 760 mmHg
    • 1 atm (atmosphere) = 760 torr
    • 1 atm (atmosphere) = 1.01325  105 Pa (Pascal)
    • 1 atm (atmosphere) = 101.325 kPa (Kilopascal)

Chapter 12

slide5

The Gas Laws

  • There are four variables required to describe a gas:
    • Amount of substance: moles
    • Volume of substance: liters
    • Pressures of substance: atmospheres (atm)
    • Temperature of substance: kelvin
  • The gas laws will hold two of the quantities constant and see how the other two vary.

Chapter 12

slide6

The Gas Laws

The Pressures-Volume Relationship: Boyle’s Law

Boyle’s Law - The volume of a fixed quantity of gas is inversely proportional to its pressure at constant temperature.

Chapter 12

slide7

The Gas Laws

The Pressures-Volume Relationship: Boyle’s Law

Boyle’s Law - The volume of a fixed quantity of gas is inversely proportional to its pressure at constant temperature.

Chapter 12

slide8

The Gas Laws

The Temperature-Volume Relationship: Charles’s Law

Charles’s Law - The volume of a fixed quantity of gas at constant pressure is directly proportional to the substances temperature in Kelvin.

Chapter 12

slide9

The Gas Laws

The Temperature-Volume Relationship: Charles’s Law

Charles’s Law - The volume of a fixed quantity of gas at constant pressure is directly proportional to the substances temperature in Kelvin.

Chapter 12

slide10

The Gas Laws

The General Gas Law

- This is a combination of Boyle’s and Charles’s gas law.

Chapter 12

slide11

The Gas Laws

The Quantity-Volume Relationship: Avogadro’s Law

Avogadro’s Law - The volume of gas at a given temperature and pressure is directly proportional to the number of moles of gas.

Chapter 12

slide12

The Ideal Gas Equation

  • Combine the gas laws (Boyle, Charles, Avogadro) yields a new law or equation.

Ideal gas equation:

PV = nRT

R = gas constant = 0.08206 L(atm)/mol(K)

P = pressure (atm) V = volume (L)

n = moles T = temperature (K)

Chapter 12

slide13

The Ideal Gas Equation

  • We define STP (standard temperature and pressure) as 0C (273.15 K), 1 atm.
  • Volume of 1 mol of gas at STP is 22.4 L (molar vol.).

Chapter 12

slide14

Applications of The Ideal-Gas Equation

Gas Densities and Molar Mass

  • Rearranging the ideal-gas equation with M as molar mass yields

Chapter 12

slide15

Gas Mixtures and Partial Pressures

Dalton’s Law - In a gas mixture the total pressure is given by the sum of partial pressures of each component:

Ptotal = P1 + P2 + P3 + …

- The pressure due to an individual gas is called a partial pressure.

Chapter 12

slide16

Gas Mixtures and Partial Pressures

Partial Pressures and Mole Fractions

  • The partial pressure of a gas can determined if you know the mole fraction of the gas of interest and the total pressure of the system.
  • i is the mole fraction of gas i (ni/ntotal).

Pi = iPtotal

Chapter 12

slide17

Kinetic-Molecular Theory

  • Theory developed to explain gas behavior
  • To describe the behavior of a gas, we must first describe what a gas is:
    • Gases consist of a large number of molecules in constant random motion.
    • Volume of individual molecules negligible compared to volume of container.
    • Intermolecular forces (forces between gas molecules) negligible.
    • Energy can be transferred between molecules, but total kinetic energy is constant at constant temperature.
    • Average kinetic energy of molecules is proportional to temperature.

Chapter 12

slide18

Molecular Effusion and Diffusion

Graham’s Law of Effusion

Graham’s Law of Effusion - The rate of effusion of a gas is inversely proportional to the square root of its molecular weight.

  • Effusion is the escape of a gas through a tiny hole (a balloon will deflate over time due to effusion).

Chapter 12

slide19

Real Gases: Deviations from Ideal Behavior

  • The assumptions in kinetic molecular theory show where ideal gas behavior breaks down
    • When the volume of the gas becomes very small (the volume of the gas molecules become significant)
    • When the pressure become very large (gas molecules start to attract each other).

Chapter 12

slide21

Real Gases: Deviations from Ideal Behavior

The van der Waals Equation

  • We add two terms to the ideal gas equation one to correct for volume of molecules and the other to correct for intermolecular attractions
  • The correction terms generate the van der Waals equation:

where a and b are empirical constants.

Chapter 12

slide22

Homework

2, 14, 20, 28, 36, 38, 48, 56

Chapter 12