a) Using heats of formation
The calculation of H using Hess's law and equations is time consuming and somewhat difficult. There is another way to find the H of a reaction using what are called heats of formation. Examine the diagram of an energy change over the course of a reaction shown below.




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Key
Idea
Calculating H using Heats of formation
The H of the reaction is the difference between the total heat of the products and that of the reactants.
H = Heat of Products - Heat of Reactants
Chemists can calculate the total heat of reactant and products using what are termed Heats of formations. The heat of formation of a compound is the amount of heat required to form the compound from it's elements.
To calculate the heat of the reactant or product substances in an equation we multiply the balance of each substance ( in the chemical equation ) by the heat of formation of that substance.
Heats of formations of various substances are listed in a tables of heats of formation, Heats of formation are given the symbol H f.
Note : The heats of formation of elements are zero and they do not often appear in these tables.
b) Restating Hess's Law formula, using heats of formation.
Using the idea above we can restate Hess's Law to calculate H of a reaction.
The heat of the overall reaction is the sum of the heats of products subtract the sum of the heats of the reactants.
H overall
=
H of products
-
H of reactants
means sum of


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Important
Procedure
Calculations using formula.
To calculate H reaction with this formula we need to;
1) Balance the equation.
2) Locate the heats of formation from the table.
3) Plug heats of formation and balance into summation formula and solve.


Examples of Calculations
Example : Calculate the H of the following reaction using heats of formations
___C3H8 (l) + ___O2(g) ___CO2 (g) + ___H2O (g)
Answer:
Step 1 : Balance equation
1 C3H8 (l) + 5 O2 (g) 3 CO2 (g) + 4 H2O (g)
Step 2 : Locate heats of formation using table.
C3H8 (l) = - 103 .8 kJ/mol
O2 (g) = 0 kJ/mol
CO2 (g) = -393.5 kJ/mol
H2O (g) = - 242.0 kJ/mol

Step 3 : Plug values into formula and calculate answer.
H overall = H products - H reactants
H = [ (3 * -393.5 kJ/mol) + (4 * -242.0 kJ/mol ) ] - [ ( 1 * -103.8 kJ/mol) + ( 5 * 0 kJ/mol ) ]

H = - 2044.7 kJ
 
Activities on calculating Heats of Reactions using Heats of formation
a) Check your understanding by completing the following question, and checking the answer
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Calculate the heat of the following reaction using the listed heats of formation.
__ Fe (s) + __CO2 (g) __ Fe2O3 (s) + __CO (g) (unbalanced )
Heats of Formations :
Fe = 0 kJ/mol
CO2 (g) = -393.5 kJ/mol
Fe2O3 (s) = -824.2 kJ/mol
CO (g) = - 110.5 kJ/mol
Click the check mark to view answer
b) Complete the following assignment on Heats of formation.
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Assignment : Calculating H using heats of formation.
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