Hess's Law and Manipulating Elementary Reactions

Vocabulary terms, operational rules, and example calculations for determining overall reaction enthalpies using elementary reaction steps.

Elementary steps

A series of given chemical reactions that are manipulated and added together to produce an overall target reaction.

Intermediates

Chemical species that appear in the elementary steps but do not appear in the overall reaction, requiring them to cancel out by having equal molar amounts on both sides of the reaction arrow.

Sign reversal rule for $\Delta H$

The rule that when an elementary reaction is reversed to run in the opposite direction, the algebraic sign of its enthalpy value ($\Delta H$) must be flipped.

Enthalpy multiplication rule

The rule that if an elementary reaction is halved, doubled, or multiplied by any stoichiometric factor, the heat ($\Delta H$) must be multiplied by that exact same factor.

Overall $\Delta H$ for $C + H_2O \rightarrow CO + H_2$

The overall enthalpy change obtained by calculating $-393.5 + 566 \times \frac{1}{2} + 483.6 \times \frac{1}{2}$, which equals $131.3\,\text{kJ}$.

Reverse of a combustion reaction

A reaction running in the opposite direction of heat-releasing combustion, requiring an input of heat such as positive $2043\,\text{kJ}$ for propane synthesis.

Overall $\Delta H$ for $N_2O + NO_2 \rightarrow 3NO$

The overall enthalpy change calculated as $113.1 \times \frac{1}{2} + 182.6 - 163.2 \times \frac{1}{2}$, resulting in $157.55\,\text{kJ}$.

Overall $\Delta H$ for $2S + 3O_2 \rightarrow 2SO_3$

The overall enthalpy change calculated as $-296.8 \times 2 - 197.8$, resulting in $-791.4\,\text{kJ}$.

Spring 2024 intermediate cancellation problem

A targeted reaction ($Z + Q \rightarrow X + 2R$) where the second given reaction is flipped to $+17.1\,\text{kJ}$ solely to cancel intermediates ($E + 2D$ and $J + A$), yielding a total $\Delta H$ of $-14\,\text{kJ}$.