Download Lesson 6.5 – Hess`s Law

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Date:_______________ Per. #:_______
Chemistry – Unit 6
Lesson 6.5 – Hess’s Law
I. Hess’s Law
Base Reaction:
C(s) + 2 H2(g)  CH4 (g)
ΔHf =
Constituent Rxns:

C(s) + O2(g)  CO2(g)
ΔHc =

H2(g) + ½ O2(g)  H2O(l)
ΔHc =

CH4(g) + 2 O2(g)  CO2(g) + 2 H2O(g) ΔHc =
General Steps of Hess’s Law:
Summary:
II. Examples
Example #1: Calculate the heat of reaction for the combustion of nitrogen monoxide
gas (NO) to form nitrogen dioxide gas (NO2), as given in the following
thermochemical equation:
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Date:_______________ Per. #:_______
2 NO(g) + O2(g)  2 NO2(g)
Use the heat of formation data from the Heat of Formation Handout. Solve by
combining the known thermochemical equations.
Step 1: Write the constituent reactions for the formation of nitrogen monoxide and
nitrogen dioxide.
Step 2: Rearrange the equations and multiply by coefficients so they can be cancelled
to form the base equation. Add ΔHs.