Thermochemistry

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Thermochemistry
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Thermochemistry

• All changes in matter, both physical and
  chemical, involve changes in energy
• Thermochemistry is the study of energy changes is
  physical and chemical processes

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Energy

• All forms of energy can be classified as either
• Kinetic Energy - energy of motion
• Potential Energy - stored energy
• SI unit joule (J) kgm2/s2
• 1 J is approximately the amount of energy
  required to lift an apple 1 m (very small!) so
  often expressed as kJ (1000 J)
• 1 calorie 4.184 J

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System Surroundings

• System substances in which a change (physical or
  chemical) occurs e.g. reactants and products
• Surroundings Everything else in the universe!
  Usually only consider anything nearby that is
  capable of exchanging energy or matter

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Endothermic Exothermic

• Endothermic Processes
• Heat is absorbed by the system from the
  surroundings
• Feels cold
• Exothermic Processes
• Heat is released from the system into the
  surroundings
• Feels hot

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Energy, Temperature Heat

• Thermal Energy The sum of all kinetic energies
  of all the particles in a system. Cannot be
  measured.
• Temperature A measure of the average kinetic
  energy of all the particles in a system. Can be
  measured.
• Heat Kinetic energy transferred between
  substances. I.e. an object cannot possess heat.

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Heat Capacity

• Heat Capacity Amount of heat required to raise
  the temperature of a substance of any mass by
  1C.
• C J/C
• Specific Heat Capacity Amount of heat required
  to raise the temperature 1 g of a substance by
  1C.
• c J/gC
• You are provided with a glass of milk and a
  swimming pool of milk.
• Which will have the higher heat capacity?
• Which will have the higher specific heat
  capacity?
• Molar Heat Capacity Amount of heat required to
  raise the temperature 1 mole of a substance by
  1C. c J/molC

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Factors that Affect Heat

• Mass - the greater the number of particles, the
  more heat required
• Type of Substance - each substance has a
  different specific heat capacity
• Temperature Change - the greater the temperature
  change the more heat required

Q mc?T
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Calculating Heat

• When a 1.25 kg sample of water was heated in a
  kettle, its temperature increased from 16.4C to
  98.9C. How much heat did the water absorb?

Q mc?T Q ? m 1.25 kg c 4.18 J/gC ?T
T2 - T1 98.9C - 16.4 C 82.5C
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Practice

• p. 300 1-5
• p. 302 8-13
• p. 305 1-6
• Specific Heat Capacity Worksheet