1. 1.2.5 Hess’s Law- the equation

2. The equation
There is another way to calculate enthalpy changes based on the principal of Hess's Law.
If you are not given the intermediate reactions then: ΔH for a reaction may be calculated using ΔHf values and the equation:
ΔH = ∑ΔH (products) – ∑ΔH (reactants)

3. The Equation
You may not be familiar with the ∑ symbol. It stands for "summation" or "the sum of".
To find ΔHo for the reaction, add together all ΔHof of the products and subtract ΔHof of all of the reactants.

4. Use a Table of Thermochemical Data to locate ΔHf values for all reactants and products

5. Things to watch out for:
The physical state is important (s,l,g,aq)
The balancing coefficients in the equation, as you must multiply the ΔHf values by the coefficients.
be very careful with + and - values.
you should begin by writing all the ΔHf values directly below all participants in the equation

6. Example:
Using a Table of Thermochemical Data, calculate ΔH for the combustion of benzene, C6H6, as shown by the following reaction:
C6H6 (l) + 15/2 O2 (g) → 6 CO2 (g) + 3 H2O (l)

7. Common Sources of Error
Forgetting to multiply ΔH values by the appropriate coefficient.
Using the wrong value of ΔH for water: ΔHf° for H2O(l) = kJ/mol; ΔHf° for H2O(g) = kJ/mol
Solving for ΔH as "Reactants - Products" instead of "Products – Reactants".
Accidentally changing the sign for ΔH.

8. Practice Problem
What is the standard heat of reaction for the reaction of gaseous carbon monoxide with oxygen to form gaseous carbon dioxide?
2CO(g) + O CO2 (g)
Hint- use your table to find the heat of formation values for CO and CO2