Use the values of ΔHf^∘ in Appendix 4 to calculate ΔH^∘ for...

Use the values of $\Delta H_{\mathrm{f}}^{\circ}$ in Appendix 4 to calculate $\Delta H^{\circ}$ for the following reactions. (See Exercise 67.) b. $\mathrm{Ca}_{3}\left(\mathrm{PO}_{4}\right)_{2}(s)+3 \mathrm{H}_{2} \mathrm{SO}_{4}(l) \longrightarrow 3 \mathrm{CaSO}_{4}(s)+2 \mathrm{H}_{3} \mathrm{PO}_{4}(l)$ c. $\mathrm{NH}_{3}(\mathrm{~g})+\mathrm{HCl}(\mathrm{g}) \longrightarrow \mathrm{NH}_{4} \mathrm{Cl}(s)$

Use the values of $\Delta H_{\mathrm{f}}^{\circ}$ in Appendix 4 to calculate $\Delta H^{\circ}$ for the following reactions. (See Exercise 67.)
b. $\mathrm{Ca}_{3}\left(\mathrm{PO}_{4}\right)_{2}(s)+3 \mathrm{H}_{2} \mathrm{SO}_{4}(l) \longrightarrow 3 \mathrm{CaSO}_{4}(s)+2 \mathrm{H}_{3} \mathrm{PO}_{4}(l)$
c. $\mathrm{NH}_{3}(\mathrm{~g})+\mathrm{HCl}(\mathrm{g}) \longrightarrow \mathrm{NH}_{4} \mathrm{Cl}(s)$

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Key Concepts

Standard Enthalpy of Formation

This is the enthalpy change that occurs when one mole of a compound is formed from its elements in their standard states under standard conditions. It is a fundamental concept used to calculate the overall enthalpy changes in reactions involving the formation or decomposition of substances.

Hess's Law

Hess's Law states that the total enthalpy change for a reaction is the same regardless of the number of steps in which the reaction occurs. This principle allows us to sum the enthalpy changes of multiple steps (or individual formation reactions) to determine the overall enthalpy change of a chemical reaction.

Reaction Enthalpy Calculation Using Formation Data

By applying the standard enthalpies of formation for all reactants and products, the overall reaction enthalpy can be calculated using the formula: ?H°(reaction) = ?(?H°f of products) - ?(?H°f of reactants). This method is widely used in thermochemistry to determine whether a reaction is endothermic or exothermic.

Thermochemical Equations

Thermochemical equations are balanced chemical equations that include the enthalpy change of the reaction. Writing these equations correctly, with proper stoichiometric coefficients, is critical for accurate calculations of reaction enthalpies using standard enthalpy of formation values.

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