Heat of Formation & Hess's Law Worked Cheat Sheet

Key Facts

• Standard enthalpy of formation, $\Delta H_f^\circ$, is the enthalpy change when $1\ \text{mol}$ of a compound forms from its elements in their standard states.
• The standard enthalpy of formation of any pure element in its standard state is $\Delta H_f^\circ = 0\ \text{kJ mol}^{-1}$.
• The reaction enthalpy from formation values is $\Delta H_{\text{rxn}}^\circ = \sum n\Delta H_f^\circ(\text{products}) - \sum n\Delta H_f^\circ(\text{reactants})$.
• Hess's law states that $\Delta H_{\text{overall}} = \Delta H_1 + \Delta H_2 + \Delta H_3 + \cdots$ for any set of steps that add to the target reaction.
• If a thermochemical equation is reversed, the enthalpy sign changes, so $\Delta H_{\text{reverse}} = -\Delta H_{\text{forward}}$.
• If every coefficient in a thermochemical equation is multiplied by $k$, the enthalpy change is also multiplied, so $\Delta H_{\text{new}} = k\Delta H_{\text{old}}$.
• An exothermic reaction has $\Delta H < 0$ because heat is released to the surroundings.
• An endothermic reaction has $\Delta H > 0$ because heat is absorbed from the surroundings.

Vocabulary

Enthalpy

Enthalpy, $H$, is the heat content of a system at constant pressure.

Standard enthalpy of formation

Standard enthalpy of formation, $\Delta H_f^\circ$, is the enthalpy change for forming $1\ \text{mol}$ of a substance from its elements in standard states.

Hess's law

Hess's law says the total enthalpy change of a reaction depends only on the initial and final states, not the pathway.

Thermochemical equation

A thermochemical equation is a balanced chemical equation that includes the enthalpy change, such as $\Delta H = -286\ \text{kJ}$.

Standard state

A standard state is the most stable form of a substance at $1\ \text{bar}$ pressure, usually with solutions at $1\ \text{M}$ concentration.

Reaction enthalpy

Reaction enthalpy, $\Delta H_{\text{rxn}}$, is the heat change for a reaction as written with its balanced coefficients.