Unit 11 Heat in Chemical Reactions

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Unit 11 Heat in Chemical Reactions

• Parts of Chapter 11 in Addison-Wesley Textbook

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Chemical Reactions

• All chemical reactions involve a change in
  energy, many of them involve heat
• Heat definition Energy that is transferred from
  one object or substance to another because of
  differences in temperature

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Two Kinds of Reactions

• Endothermic Reactions These absorb heat from
  the environment
• Exothermic reactions These release heat to the
  environment
• You, the beaker, and often the water are in the
  environment
• Demo - Baggy reactions

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Exothermic reactions

• All combustion reactions are exothermic
• Example
• C3H8 5O2 ? 3CO2 4H2O 2043 kJ
• Picture heat stored in the bonds of reactants,
  released to environment
• Therefore, products are lower in internal energy

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Endothermic Reactions

• Example
• C(s) H2O(g) 113 kJ ? CO2(g) H2(g)
• Heat is stored in bonds
• Products end up higher in heat energy than
  reactants

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Enthalpy

• Enthalpy is the term for the heat energy stored
  in the bonds of a substance, H
• ?H is the change in heat that occurs during a
  chemical reaction
• ?H(reaction) H(products) - H(reactants)
• Exothermic is negative ?H value
• Endothermic is positive ?H value
• Overheads

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Stoichiometry

• Heat is treated as any reactant or product
• Try
• C(s) H2O(g) 113 kJ ? CO2(g) H2(g)

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Specific Heat

• All substances absorb heat and change their
  temperature differently, depending upon their
  chemical structure
• Beach example.
• Called specific heat capacity and symbolized
  c
• c 1 cal/gC for water 4.184 J/g C
• This is the definition of 1 calorie

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Equation

• q mXcX?t
• mass, specific heat of a substance, change in
  its temperature
• Try some problems.

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Calorimetry

• Lab procedure which uses water to calculate ?H
  for a chemical reaction
• Often in a Styrofoam cup
• Uses principle of heat lost heat gained
• Demo and calculation