Enthalpies of Formation
An enthalpy of formation, H f, is defined as the enthalpy change for the reaction in which a compound is made from its constituent elements in their elemental forms.
Standard Enthalpies of Formation Standard enthalpies of formation, H f °, are measured under standard conditions (25 °C and 1.00 atm pressure).
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.
A Problem to Consider You record the values of H f o under the formulas in the equation, multiplying them by the coefficients in the equation. -You can calculate H o by subtracting the values for the reactants from the values for the products.
Practice Problem 6.45
Practice Problem 6.46
Calculation of H Imagine this as occurring in three steps: C 3 H 8 (g) + 5 O 2 (g) 3 CO 2 (g) + 4 H 2 O (l) C 3 H 8 (g) 3 C (graphite) + 4 H 2 (g) 3 C (graphite) + 3 O 2 (g) 3 CO 2 (g) 4 H 2 (g) + 2 O 2 (g) 4 H 2 O (l)
Calculation of H Imagine this as occurring in three steps: C 3 H 8 (g) + 5 O 2 (g) 3 CO 2 (g) + 4 H 2 O (l) C 3 H 8 (g) 3 C (graphite) + 4 H 2 (g) 3 C (graphite) + 3 O 2 (g) 3 CO 2 (g) 4 H 2 (g) + 2 O 2 (g) 4 H 2 O (l)
Calculation of H Imagine this as occurring in three steps: C 3 H 8 (g) + 5 O 2 (g) 3 CO 2 (g) + 4 H 2 O (l) C 3 H 8 (g) 3 C (graphite) + 4 H 2 (g) 3 C (graphite) + 3 O 2 (g) 3 CO 2 (g) 4 H 2 (g) + 2 O 2 (g) 4 H 2 O (l)
C 3 H 8 (g) + 5 O 2 (g) 3 CO 2 (g) + 4 H 2 O (l) C 3 H 8 (g) 3 C (graphite) + 4 H 2 (g) 3 C (graphite) + 3 O 2 (g) 3 CO 2 (g) 4 H 2 (g) + 2 O 2 (g) 4 H 2 O (l) C 3 H 8 (g) + 5 O 2 (g) 3 CO 2 (g) + 4 H 2 O (l) Calculation of H The sum of these equations is: