Give the definition of the standard enthalpy of formation...

Give the definition of the standard enthalpy of formation for a substance. Write separate reactions for the formation of $\mathrm{NaCl}$, $\mathrm{H}_{2} \mathrm{O}, \mathrm{C}_{6} \mathrm{H}_{12} \mathrm{O}_{6}$, and $\mathrm{PbSO}_{4}$ that have $\Delta H^{\circ}$ values equal to $\Delta H_{\mathrm{f}}^{\circ}$ for each compound.

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Key Concepts

Standard Enthalpy of Formation

The standard enthalpy of formation is defined as the heat change that accompanies the formation of one mole of a compound from its constituent elements, with all reactants in their most stable forms under standard conditions (1 bar and typically 298.15 K). It is a key thermodynamic quantity that provides a reference point for calculating reaction enthalpies using Hess's law.

Formation Reaction

A formation reaction is a balanced chemical equation that represents the synthesis of one mole of a substance from its elements in their standard states. The enthalpy change of this reaction is exactly the standard enthalpy of formation of the substance, serving as an essential concept in understanding reaction energetics.

Elements in Their Standard States

In the context of formation reactions, the elements are considered in their standard states, which are their most stable physical forms at 1 bar pressure and the standard temperature (usually 298.15 K). This consistency allows for reliable tabulation and comparison of thermodynamic data across different substances.

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