Gases - PDF

							                                               Gases
Kinetic-Molecular Theory
•   Gases are made up of tiny particles in
•   Gas means “chaos” in
•   Gas molecules are so small that they are called
•   They have no volume – but they take-
•   Gas molecules are far apart –
•   No real attractive or repulsive forces between
•   Gas molecules move in
•   Their motion is random, only change direction when they collide with the container wall or
    other
•   This random motion allows gas molecules to
•   Elastic Collisions - When they collide – they do not lose
•   The kinetic energy of the gas particles can be calculated by
•   m = mass     v=
•   Temperature – Average kinetic energy of the particles in a
•   For the particles to have about the same K.E. – the light particles move fast and the heavy
    particles
•   Gases have low densities – small mass in a large
•   Gases can be compressed (squeezed into a smaller volume) because they are mostly empty
    space. Their density increases when
•   More than one gas can occupy the same volume – air is a
•   Diffusion – the spreading out of gas particles to fill their container because of the random
    motion from an area of high concentration to an area of
•   Effusion – gases escape from a container with a
•   The rate of diffusion and effusion depends on the mass of the
•   Graham’s Law – rate of effusion or diffusion for a gas is inversely proportional to the square
    root of its

•

• Used to compare the diffusion rate between different gases at the same temperature

• Calculate the ratio of effusion rates for methane (CH4) and nitrogen.




• Calculate the molar mass of butane. Butane's rate of diffusion is 3.8 times slower than that of
  helium.




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    Pressure
•    Tiny gas molecules hitting
•    Force per unit
•    Those tiny gas particles hit an area, they
•    The more collisions – greater the
•    Symbol for Pressure is

• P=           (force/area)

•    Pascal =
•    kPa =
•    Atmospheric Pressure – pressure of all the gas molecules hitting against
•    Standard Pressure is
•    1 atm =
•    An experiment at Sandia National Labs in New Mexico is performed at an atmospheric
     pressure of 758.7 mm Hg. What is the pressure in kPa and in Atm?




Measuring Pressure
• Barometer – used to measure
  o The atmospheric pressure pushes down on a surface of mercury that causes it to flow up a
    closed
  o At standard atmospheric pressure the height of the mercury is
  o The height of mercury depends on the density of mercury and the
  o Most laboratories have a barometer so the atmospheric pressure can be
  o Also used to detect changes in atmospheric pressure caused by changes in
• Manometer – U tube that contains fluid – usually mercury (sometimes alcohol or
  o Open-Ended – measures the difference between the pressure of a gas in a container and the
    atmospheric
  o Closed-Ended – measures the absolute pressure of the gas in a container. The atmospheric
    pressure does not affect the

Dalton’s Law of Partial Pressures
• The total pressure of a mixture of gases is equal to the sum of the pressures of all the gases
  in the
• Each gas contributes a partial pressure (a part of the
• Ptotal =

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• Air is 29.9% O2 and 70.0% N2
  o So if the atmospheric pressure is 101.3 kPa
  o 0.299 x 101.3 kPa =
  o 0.700 x 101.3 kPa =
• Calculate the partial pressure of neon in a flask that has a total pressure of 1.87 atm. The
  flask contains krypton at a partial pressure of 0.77 atm and helium at a partial pressure of
  0.62 atm.




• A sample of gas has a pressure of 175 kPa and contains 66.0% oxygen, 12.0% hydrogen, and
  22.0% carbon dioxide. What pressure does each individual gas exert?




• What is the total pressure in a canister that contains oxygen gas at a partial pressure of 804
  mm Hg, nitrogen at a partial pressure of 220 mm Hg, and hydrogen at a partial pressure of 445
  mm Hg?




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                                       Liquids and Solids
Liquids
•   Least abundant state of matter, but biologically the
•   Particles slip past one another, but are very
•   Definite volume, takes the shape of the
•   Low kinetic energy, but more than
•   Strong attraction between the liquid particles (
•   Density – the density of a liquid is greater than gases. The volume of a liquid is much smaller
    than the volume of the same
•   Fluidity – ability to flow. Liquids diffuse slower than gases because of the intermolecular
    forces and the mass of the
•   Viscosity – resistance to
    o The stronger the intermolecular forces, the more viscous the
    o More polar – higher the
    o Viscosity decreases as temperature
    o Motor Oil additives – Low temperature – they are spheres, high temperature – they change
      into long strands and increase the viscosity of
•   Cohesion – attraction between the same type of
    o Surface Tension – particles at the surface have stronger attraction to each other since
      there are no molecules above them. They pull together tighter to form
•   Adhesion – attraction between different types of
•   Capillary action – liquid is attracted to the walls of a
    o The greater the surface area (smaller the tube) the greater the
    o Allows water to be drawn upward in small capillary tubes, such as the
    o Creates a meniscus in
•   Water – a very special
    o It is a very
    o Structure is bent with a 105° angle between the
    o The oxygen forms a
    o Liquid water molecules are linked together by
    o This helps keep water in the liquid state over a large range of
    o When water freezes, the molecules line up in a hexagons leaving empty space
    o This empty space gives ice a
    o The less dense ice floats in water and keeps lakes and rivers from
    o The ice forms an insulating barrier and allows living organisms to
    o Water is the most dense at 4°C because the molecules are closest together at

Solids
• Definite shape,
• Particles vibrate, but
•   Very low
•   Extremely strong
•   Very ordered – Low
•   Particles are closely packed together, very

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• Density – very high because the mass of the solid is closely
• Two types of solids : amorphous and
• Amorphous – particles are not orderly – no geometric pattern to the molecules. They take the
  shape they are forced to take. Examples include
• Crystalline Solids – contains crystals – arranged in orderly, geometric,
  o Unit Cell – smallest arrangement of connected particles that can be repeated to form the
    crystal lattice. There are many types of
  o Crystal Lattice – groups of unit cells put
• Four types of crystals :
• The stronger the attraction in the crystal lattice, the higher the
• Ionic Crystals
  o Cations and anions (can be
  o Usually elements from groups 1 & 2 bonded to elements in groups
  o Strong bonding in
  o Hard,
  o Good
  o Salts –
  o Examples –
• Covalent Network Crystals
  o Lattice is made up single atoms that are covalently bonded to
  o Very hard,
  o High
  oS
  oM
  o Examples :
• Metallic Crystals
  o Nuclei (cations) surrounded by a
  o Electrons are free to move in the crystal
  o Melting points
  o Ductile,
  o Good conductors of
  o Transition
  o Examples :
• Covalent molecular crystals
  o Molecules are held together by weak intermolecular forces (

  o Low
  o Easily
  o Good
  o Examples :




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                                       Phase Changes




Changes that Require Energy
• Phase – another name for state but for a single type of substance in a mixture. Water : ice,
  liquid water,
• Vapor – the gaseous state of a compound that is normally a liquid or a solid at room

• Heat – transfer of thermal energy from higher temperatures to
• Melting Point – temperature that the crystal lattice of crystalline solid is broken and it
  changes into a
• Vaporization – liquid changes from a liquid to a gas or vapor, at the
• Evaporation – vaporization that occurs only at the surface of the liquid. The particles at the
  surface gain enough energy to break lose and turn into a gas. The temperature of the particles
  left
• Vapor pressure – Pressure exerted by the vapor
• Boiling Point – the vapor pressure of the liquid is equal to the
  o The molecules throughout the liquid can change because they have enough
  o Bubbles of vapor collect below the surface, rise to the surface and
  o If the surface of the container is smooth, these bubbles have trouble forming, and the liquid
    can be heated above the boiling point. When bumped, the liquid can “explode” and splatter!
    Avoid this in lab by using
  o Water will boil below 100ºC if the atmospheric pressure is less than
  o That is why you have to boil potatoes longer at
• Sublimation – Solid changes directly to a gas, skipping the
  o Examples : Carbon dioxide (dry ice), moth balls (p-dichlorobenzene), iodine, and

  o Ice cubes sublime in the freezer, that is why

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  o Freeze Drying – ice sublimes quickly at low pressures, so it is removed from the food. They
    freeze the food, and then use a vacuum pump to sublime the

Changes that Release Energy
• Condensation – gas or vapor becomes a liquid. Examples include
• Deposition – changing from a gas or vapor straight to a solid, skipping the
  o Snowflakes form from vapor, high in the atmosphere where the temperature and

• Freezing point – temperature at which a liquid is changed into a

Phase Diagram
• The phase of a substance depends on the temperature and
• An increase in pressure can cause a vapor to condense but an increase in temperature can cause
  the liquid to
• Phase Diagram – shows which phase a substance exists under different conditions of
  temperature and
• Triple point – temperature and pressure that a substance exists in all three phases.
• Six phase changes can occur : freezing/melting, vaporation/condensation, sublimation/

• Critical Point – the critical temperature and critical pressure above which the substance
  cannot exist as a
• Each substance has a different phase diagram. Depends on the
• Draw the Phase Diagram:




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                                             Gas Laws
Kinetic Molecular Theory
• Ideal Gases :
  o Gas particles do not attract or
  o Gas particles are much smaller than the distances between them. They are mostly

    o Gas particles are in constant,
    o No kinetic energy is lost when gas particles collide with each other or with the walls of

    o All gases have the same average kinetic energy at a given temperature. (note not the same

• Real gases behave like ideal gases except at high pressures and
• At high pressures or low temperatures the particles are closer together and are effected by
  the
• Temperature – average kinetic energy of the
  o Gas laws require temperature to be in units of
  o No negative
  o Does not have zero as a temperature except at
  o Kelvin =
  o 0ºC =
  o 100ºC =
  o 26ºC =
  o 379 K =
  o 318 K =
• STP –
  o 0ºC,
  o 0ºC,
  o 273 K,
  o 273 K,
• Gases are described by the :
  o Number
  o
  o
  o


Boyle’s Law
• The volume of a given amount of gas, held at constant temperature varies inversely with the

•   Constant temperature, constant
•   Pressure
•   Pressure
•   PV =



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•   When collecting gases in the lab, we use the technique of
•   The product gases bubble into a container
•   The collected gas is “wet” because it contains
•   The water vapor exerts a
•   To determine the pressure exerted by the gas, you subtract the pressure of the
•   The vapor pressure exerted by water depends on the
•   Pdry gas =




Charles’s Law
• The volume of a given amount of gas, held at constant pressure, varies directly with the

•   Constant pressure, constant
•   Temperature
•   Temperature
•   Constant =

• Temperature MUST be in

Gay-Lussac’s Law
• The pressure of a given amount of gas, held at constant volume, varies directly with the

•   Constant volume,
•   Pressure
•   Pressure
•   Constant =

• Temperature MUST be in

Combined Gas Law
• States the relationship among pressure, volume, and Kelvin temperature of a

•

• 1 = before 2 = after
• Can determine the pressure, volume, or temperature should the
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• Always convert temperature to
  Always onver emperat
• Constant Temperature, T1 = T2
  o So
• Constant Pressure, P1 = P2
  o So

• Constant Volume, V1 = V2
  o So

Example Problems
• A gas occupies a volume of 2.45 liters at a pressure of 104.3 kPa and a temperature of 20°C.
  What volume will the gas occupy if the pressure changes to 99.3 kPa and the temperature
  remains at 20°C?




• A 525 ml sample of hydrogen gas is collected over water at 80.0°C. The pressure of the wet
  hydrogen gas is 95.0 kPa. What volume will the dry hydrogen gas occupy at 101.3 kPa pressure?
  The vapor pressure of water at 80.0°C is 47.3 kPa.




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• A 325 ml sample of gas is collected over water at 27°C. The pressure in the container is 68
  kPa. What would the pressure be if the sample was dried and placed into a 500.0 ml container
  at 27°C? The vapor pressure of water at 27°C is 3.6 kPa.




• A tank of compressed carbon dioxide has a temperature of 23.6°C and a volume of 31.4 liters.
  The carbon dioxide is completely transferred into a smaller tank that has a volume of 25.0
  liter. Assuming none of the carbon dioxide escapes during the transfer, what is the
  temperature, in °C, of the carbon dioxide in the smaller tank if the temperature is lowered to
  achieve the same pressure as in the large tank?




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• The pressure in a bicycle tire is 1.34 atm at 33.0°C. At what temperature will the pressure
  inside the tire be 1.60 atm? Assume the volume is constant.




• What volume will a 8.6 liter sample of gas at 105.5 kPa, 25°C occupy at 101.3 kPa and 0°C?




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• A gas balloon is sitting in the freezer at -5°C, 3.25 liters, and a pressure of 102.1 kPa. The
  balloon is moved to the counter where the temperature is 28°C and the pressure is 100.8 kPa.
  What is the volume of the balloon after sitting on the counter several hours?




• Hydrogen gas at a temperature of 22.0°C that is confined in a 5.00 L cylinder exerts a
  pressure of 4.20 atm. What is the volume of the gas at STP?




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