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

When given the following:

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

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 ∆H_{rxn} for overall reaction:

When given the following set of partial reactions:

Practice: Calculate the ∆H_{rxn} for

Given the following set of reactions:

Practice: Calculate the ∆H_{rxn} for

Given the following reactions: