Thermochemistry

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Thermochemistry

• Heat and Chemical Change

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• Thermochemistry the study of heat changes that
  occur in a chemical reaction
• Heat (q) is energy that transfers from one object
  to another because of temperature difference

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• Energy the capacity to do work or supply heat
• Chemical potential energy energy stored within
  the structural units of chemical substances

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• System The part of the universe you are studying
• Everything else in the universe ? region in the
  immediate vicinity to the system

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Exothermic Reactions

• The system loses heat and the surroundings heat
  up
• Ex. Fire, sodium metal water, acid/base
  neutralization
• The D H lt 0

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Endothermic Reactions

• The system gains heat as the surroundings cool
  down
• Ex. Melting Ice, Boiling Water
• The DH is gt0

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Calories

• 1 calorie is the amount of energy required to
  raise 1 g H2O 1oC
• 1000cal 1kcal 1Cal
• 4.184 J 1 cal

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Specific Heat Capacity (C)

• Specific Heat the amount of energy required to
  raise 1 g of a specific substance 1oC

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Calorimetry
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Calorimetry

• The accurate and precise measurement of heat
  change for chemical and physical processes
• q m X C X DT

Heat Mass Specific Change
Heat in Temperature
DT Tf - Ti
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Enthalpy (DH)

• Enthalpy heat at constant pressure
• q DH so
• q m X C X DT DH
• Remember
• DH Endothermic reaction
• -DH Exothermic reaction

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Example 1

• How much energy (J) will it take to heat 100.0 mL
  of H2O, from room temperature 25oC to boiling?
  Hint The density of H2O 1g/mL

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Ex. 2

• A student mixed 50.0 mL of water containing 0.50
  mol. of HCl at 22.5oC with 50.0 mL of water
  containing 0.50 mol NaOH at 22.5oC in a foam
  calorimeter. The temperature of the resulting
  solution increased to 26.0oC. How much heat in
  kJ was released by this reaction? Assume the
  density of solution to be 1g/mL

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Ex. 3

• When 435 J of heat is added to 3.4g of olive oil
  at 21.0 oC, the temperature increased to 85oC.
  What is the specific heat of olive oil?

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Thermochemical Equations

• An equation that includes the heat of change
• CaO(s) H2O (l) ? Ca(OH)2 (s) 65kJ

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Heat of Reaction

• Heat change for the equation exactly as written
• Standard Temperature 25oC 278K
• Standard Pressure 1 atm 101.3 kPa
• Physical states MUST be given

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• The amount of heat releases depends on the number
  of moles involved.
• Decomposition of 2 mol NaHCO3 129 kJ
• Decomposition of 4 mol NaHCO3 258 kJ

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Example 1

• 2NaHCO3 (s) 129 kJ Na2CO3 H2O (g) CO2 (g)
  
• If 2.24 mol NaHCO3 decomposes ?kJ

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Example 2

• C(s) 2S (s) ? CS2 (l)
• If DH 89.3 kJ
• 5.66 g CS2 ? kJ

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• The amount of heat released also depends on the
  physical state
• H2O (l) ? H2 (g) ½ O2 (g) DH 258.8 kJ
• H2O (g) ? H2 (g) ½ O2 (g) DH 241.8 kJ

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Heat of Combustion (DHcom)

• The heat of reaction for the complete burning of
  1 mole of a substance

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Molar Heat of Fusion (DHfus)

• The heat absorbed by 1 mole of a substance in
  melting from a solid to liquid at constant
  temperature

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Molar Heat of Solidification (DHsolid)

• The heat lost when 1 mole of a liquid solidifies
  at constant temperature
• The quantity of heat absorbed by a melting solid
  is exactly the same as the heat lost when the
  liquid solidifies.
• Therefore - DHsolid DHfus

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Example 1

• How many grams of ice at 0oC and 101.3 kPa could
  be melted by the addition of 0.400 kJ of heat?
  DHfus 6.01 kJ/mol

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Heat of Vaporization (DHvap)

• The amount of heat necessary to vaporize 1 mole
  of a given liquid

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Molar heat of Condensation (DHcond)

• The amount of heat released when 1 mole of vapor
  condenses
• Therefore DHvap -DHcond

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Molar Heat of Solution (DHsoln)

• The heat change caused by dissolution of 1 mole
  of a substance

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Experiment

• Design an experiment to determine the DHsoln for
  4 compounds
• NaHCO3
• CaCl2
• NaCl
• NH4Cl