1.2.5 Hess’s Law- the equation

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Presentation transcript:

1.2.5 Hess’s Law- the equation

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)

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.

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

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

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)

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) = -285.8 kJ/mol; ΔHf° for H2O(g) = -241.8 kJ/mol Solving for ΔH as "Reactants - Products" instead of "Products – Reactants". Accidentally changing the sign for ΔH.

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) + O2 2 CO2 (g) Hint- use your table to find the heat of formation values for CO and CO2