Chapter 4

• Kinetic Molecular Theory


  Collective hypotheses about the particulate nature of matter and the surrounding space
• Greeks had the earliest written ideas on atoms
• Matter


  • Comprises microscopic particles - atoms
  • Atoms combine to form molecules
  • Many macroscopic phenomena can be traced to interactions on this level
• Postulates of Kinetic Molecular Theory

  • Gases are composed of a large number of particles that behave like hard, spherical objects in a state of constant, random motion
  • These particles move in a straight line until they collide with another particle or the walls of the container
  • These particles are much smaller than the distance between particles. Most of the volume of a gas is therefore empty space
  • There is no force of attraction between gas particles or between the particles and the walls of the container
  • Collisions between gas particles or collisions with the walls of the container are perfectly elastic. None of the energy of a gas particle is lost when it collides with another particle or with the walls of the container
  • The average kinetic energy of a collection of gas particles depends on the temperature of the gas and nothing else
• Atom


  Tiny unit of structure that makes up matter
• Element


  Pure substance made up of only one type of atom
• Compound


  Made up of two or more types of atoms chemically combined, with more complex structures
• Molecule


  Smallest particle of a compound in which all of the atoms maintain their identity, and which maintains all of the chemical and physical properties of the compound
• Silicon atoms imaged with a scanning tunneling microscope show a characteristic structure of alternating hexagons
• Phases of Matter


  • Solids
  • Liquids
  • Gases
• Solids


  • Definite shape
  • Definite volume
  • Molecules are not free to move and have strong cohesive forces
• Liquids


  • Molecules are in clusters
  • Cohesive forces not as strong as in a solid
  • Definite volume, but not a definite shape
• Gases


  • Weak cohesive forces
  • High kinetic energy
  • Molecules far apart and move randomly
  • No fixed shape or volume
• Vapor


  A gas that is above a liquid phase
• Molecular Interactions


  • Attractive, causing materials to cling together
  • Repulsive
• Cohesion


  Attractive forces between like molecules
• Adhesion


  Attractive forces between unlike molecules
• Adhesion


  • Water wetting skin
  • Glue mechanism; adhesives
• Repulsive Interactions


  • Water beading on wax
• Solids


  • Definite shape and volume
  • Rigid 3-D structure
  • Atoms/molecules bonded in place
  • Allowed motions restricted to vibration in place only
• Liquids


  • Definite volume, indefinite shape
  • Only weak cohesive bonds between component molecules
  • Constituent molecules mostly in contact
  • Allowed motions: Vibration, Rotation, Limited translation
• Gases


  • Indefinite volume and shape
  • Molecules mostly not in contact
  • Allowed motions: Vibration and rotation (molecules with more than one atom), Translation on random, mostly free paths
• Temperature


  Measure of average kinetic energy of the molecules making up a substance, proportional to average kinetic energy
• Gases diffuse quicker at higher temperatures
• Expansion/contraction with increasing/decreasing temperature
• Thermometer


  Measures the average kinetic energy of an object
• Mercury is used in thermometers
• The lowest temperature possible is 0K, called absolute zero of temperature
• Specific heat


  The amount of heat needed to raise the temperature of 1 g of a substance by 1 degree Celsius
• Conversion formulas


  1. Fahrenheit to Celsius

  2. Celsius to Fahrenheit

  3. Celsius to Kelvin
• At what temperature is the value on Centigrade and Fahrenheit scales equal? 40
• Heat energy


  Q = mcΔT, where c is specific heat capacity, m is mass, and ΔT is change in temperature
• Heat


  A form of energy transfer between two objects, based on total internal energy of molecules
• Temperature


  A measure of hotness or coldness of an object, based on average molecular kinetic energy
• Heating Methods


  • Temperature difference: Energy always moves from higher temperature regions to lower temperature regions
  • Energy-form conversion: Transfer of heat by doing work
• Calorie


  Energy needed to raise the temperature of 1 g of water by 1 degree Celsius
• Kilocalorie


  Energy needed to raise the temperature of 1 kg of water by 1 degree Celsius
• Mechanisms of Heat Transfer


  • Conduction
  • Convection
  • Radiation
• Conduction


  • Heat flowing through matter
  • Hotter atoms collide with cooler ones, transferring some of their energy
  • Direct physical contact required; cannot occur in a vacuum
• Sample Conductivities


  • Silver
  • Iron
  • Water
  • Styrofoam
  • Air
  • Vacuum