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Ch.6 - Thermochemistry WorksheetSee all chapters

# Hess's Law

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Sections
Nature of Energy
Kinetic & Potential Energy
First Law of Thermodynamics
Internal Energy
Endothermic & Exothermic Reactions
Heat Capacity
Constant-Pressure Calorimetry
Constant-Volume Calorimetry
Thermal Equilibrium
Thermochemical Equations
Formation Equations
Enthalpy of Formation
Hess's Law
Enthalpy (IGNORE)

Hess's Law involves the use of partial reactions in the determination of the overall enthalpy of reaction.

###### Hess's Law

Concept #1: Rearrangement of Thermochemical Equations

Multiplication of the original reaction results in multiplication of ∆Hrxn.

Division results in the division of ∆Hrxn..

Reversing the original reaction results in reversing the sign of ∆Hrxn.

Example #1: If the formation equation for boron trioxide is given as the following:

What will be the new enthalpy value when it is rearranged?

Practice: Calculate the ∆Hrxn for the following thermochemical equation:

When given the following:

Concept #2: Applying Hess’s Law

Addition of ∆Hrxn values of multiple steps gives us the ∆Hrxn of the overall reaction.

Example #2: Calculate the ∆Hrxn for overall reaction:

When given the following set of partial reactions:

Practice: Calculate the ∆Hrxn for

Given the following set of reactions:

Practice: Calculate the ∆Hrxn for

Given the following reactions: