enthalpies of formation. an enthalpy of formation, h f, is defined as the enthalpy change for the...
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Enthalpies of Formation
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Enthalpies of Formation
An enthalpy of formation, Hf, is defined as the enthalpy change for the reaction in which a compound is made from its constituent elements in their elemental forms.
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Standard Enthalpies of Formation
Standard enthalpies of formation, Hf°, are measured under standard conditions (25 °C and 1.00 atm pressure).
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Standard Enthalpies of Formation
• The law of summation of heats of formation states that the enthalpy of a reaction is equal to the total formation energy of the products minus that of the reactants.
is the mathematical symbol meaning “the sum of”, and m and n are the coefficients of the substances in the chemical equation..
)reactants(Hm)products(HnH of
of
o
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A Problem to Consider
• You record the values of Hfo under the
formulas in the equation, multiplying them by the coefficients in the equation.
- You can calculate Ho by subtracting the values for the reactants from the values for the products.
)9.45(4 )0(5 )3.90(4 )8.241(6
)g(OH6)g(NO4)g(O5)g(NH4 223
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Practice Problem 6.45
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Practice Problem 6.46
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Calculation of H
• Imagine this as occurringin three steps:
C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)
C3H8 (g) 3 C (graphite) + 4 H2 (g)
3 C (graphite) + 3 O2 (g) 3 CO2 (g)
4 H2 (g) + 2 O2 (g) 4 H2O (l)
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Calculation of H
• Imagine this as occurringin three steps:
C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)
C3H8 (g) 3 C (graphite) + 4 H2 (g)
3 C (graphite) + 3 O2 (g) 3 CO2 (g)
4 H2 (g) + 2 O2 (g) 4 H2O (l)
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Calculation of H
• Imagine this as occurringin three steps:
C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)
C3H8 (g) 3 C (graphite) + 4 H2 (g)
3 C (graphite) + 3 O2 (g) 3 CO2 (g)
4 H2 (g) + 2 O2 (g) 4 H2O (l)
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C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)
C3H8 (g) 3 C (graphite) + 4 H2 (g)
3 C (graphite) + 3 O2 (g) 3 CO2 (g)
4 H2 (g) + 2 O2 (g) 4 H2O (l)
C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)
Calculation of H
• The sum of these equations is: