Calculating ?H using molar heats of formation

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Calculating ?H using molar heats of formation

• Chem 12
• Mrs. Kay

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• If 1 mol of compound is formed from its
  constituent elements, then the enthalpy change
  for the reaction is called the enthalpy of
  formation, ?Hof .
• Standard conditions (standard state) 1 atm and
  25 oC (298 K).
• Standard enthalpy of formation 1 mol of compound
  is formed from substances in their standard
  states.

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• Examples write formation reactions for each
• Remember the compounds are formed directly from
  their elements.
• H2SO4
• NH4Cl

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• You want only one mole of the product being
  formed.
• Look up the ?Hf on the table
• H2 S 2O2 ? H2SO4 ?H -814 kJ/ mol
• 2. ½ N2 2H2 ½ Cl2 ? NH4Cl
• ?H -314.4 kJ/ mol

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Enthalpies of Formation
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• Using Enthalpies of Formation for Calculating
  Enthalpies of Reaction
• For a reaction

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• By definition, the enthalpy of formation of an
  element in its standard state is zero.
• Example, oxygen (O2) and chlorine (Cl2) both
  have ?Hof of zero.

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Sample Problem 1

• Calculate ?H for the following reaction using
  standard molar heats of formation, ?Hf .
• 2NH3(g) 3Cl2(g) ? N2(g) 6HCl(g)
• ?H ?

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• ?Hf for NH3(g) -45.9 kJ/mol
• ?Hf for HCl(g) -92.3 kJ/mol
• ?Hf for Cl2(g) and N2(g) is 0
• ?H S n?Hf(product) - S n?Hf(reactant)
• ?H (0 6(-92.3 kJ)) - (2(-45.9 kJ) 0)
• (-553 kJ) - (-91.8 kJ)
• -461.2 kJ

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• Examples
• Use the tables in the back of your book to
  calculate ?H for the following reactions
• 4 CuO (s) ? 2 Cu2O (s) O2 (g)
• C3H8 (g) 5 O2 (g) ? 3 CO2 (g) 4 H2O (l)
• NH3 (g) HCl (g) ? NH4Cl (s)

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Practice

• Page 685 15
• Page 687 19-21
• Page 691 5