Chapter 11: Thermochemistry

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Chapter 11 Thermochemistry

• A. Energy Transformations
• Thermochemistry heat changes that occur
  during chemical reactions
• Energy to supply heat
• - the capacity for doing work
• Potential energy (P.E.) stored energy
• Work converts P.E. into K.E.
• ex. Gasoline
• Heat (q) energy that is transferred from a
  warmer object to a cooler object until both
  objects are at the same temp.

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• System the object being focused on
• Surroundings everything around the system
• Universe the system and the surroundings
• Law of conservation of energy energy is neither
  created nor destroyed
• - energy can change its form
• - heat
• - light
• - sound
• - P.E. or K.E.

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• Enthalpy (?H) the change in heat (q ?H)
• Thermochemical equation a chemical equation or
  reaction that shows the heat
  change
• Endothermic heat absorbed from the surroundings
• - surroundings get colder (temp dec)
• - a positive heat change (?H)
• - in a rxn A B heat ? C
  D
• Exothermic heat released to the surroundings
• - surroundings get warmer (temp inc)
• - a negative heat change (?H)
• - in a rxn A B ? C D heat

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• B. Heat Calculations
• Unit of energy/heat
• Joule (J) or kiloJoule (kJ)
• Calorie (C), calorie (cal), kilocalorie (kcal)
• Conversion of units
• 1 C 1 kcal 1000 cal
• 1 kJ 1000 J
• 1 J 0.239 cal (will be given)
• 4.184 J 1 cal (will be given)

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• Calorimetry the measurement of heat changes
• Calorimeter an insulated device that can be
  used to measure the absorption or
  release of heat
• ex. Styrofoam cups
• Calorimetry calculations
• Formula q m C ?T
• q heat ?H (in J or cal)
• m mass (in grams)
• CH2O specific heat (4.184 J/g?C 1 cal/g?C)
• ?T change in temp (in ?C)
• Tfinal Tinitial
• (can be positive or negative)

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• ex. How much heat is released when 25 mL of
  water is cooled from 90 ?C to 30 ?C?
• q m C ?T
• q (25 g) x (4.184 J/g?C) x (-60 ?C)
• q -6276 J
• q (25 g) x (1 cal/g?C) x (-60 ?C)
• q -1500 cal
• Endothermic or Exothermic? Explain
• EXOTHERMIC! (heat released, ?H, q)

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• Heat calculations during phase changes
• q m ?Hfus
• q m ?Hvap
• ?Hfus heat of fusion ( will be given)
• ?Hvap heat of vaporization ( will be given)
• ex. How much heat (in kcal) is absorbed when
  24.8 g H2O(l) at 100 ?C is converted to H2O(g)
  at 100 ?C if ?Hvap is 539.4 cal/g?
• q m ?Hvap
• q (24.8 g) x (539.4 cal/g)
• q 13377.12 cal 13.37712 kcal

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• C. Heat and Changes of State of Matter
• Heat of fusion (?Hfus) heat absorbed when going
  from S ? L
• Heat of solidification (?Hsolid) heat released
  when going from L ? S
• ?Hfus ?Hsolid (same value)
• Heat of vaporization (?Hvap) heat absorbed when
  going from L ? G
• Heat of condensation (?Hcond) heat released
  when going from G ? L
• ?Hvap ?Hcond (same value)
• Heat of solution (?Hsoln) heat change when a
  solute is added to a solvent (can be
  endo or exo)

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• Heating curve
• ex.