Use the data from Table 6.2 to calculate ΔH^∘ for the following reactions. On the basis of their

step 1 We can't tell whether or not the process will be spontaneous solely based upon standard delta H.

Use the data from Table 6.2 to calculate ΔH^∘ for the...

Use the data from Table 6.2 to calculate $\Delta H^{\circ}$ for the following reactions. On the basis of their values of $\Delta H^{\circ},$ which are favored to occur spontaneously? $$ \begin{array}{l} \text { (a) } \mathrm{NH}_{4} \mathrm{Cl}(s) \longrightarrow \mathrm{NH}_{3}(g)+\mathrm{HCl}(g) \\ \text { (b) } 2 \mathrm{C}_{2} \mathrm{H}_{2}(g)+5 \mathrm{O}_{2}(g) \longrightarrow 4 \mathrm{CO}_{2}(g)+2 \mathrm{H}_{2} \mathrm{O}(g) \end{array} $$ (c) $\mathrm{C}_{2} \mathrm{H}_{2}(g)+5 \mathrm{~N}_{2} \mathrm{O}(g) \longrightarrow$ $$ 2 \mathrm{CO}_{2}(g)+\mathrm{H}_{2} \mathrm{O}(g)+5 \mathrm{~N}_{2}(g) $$

Use the data from Table 6.2 to calculate $\Delta H^{\circ}$ for the following reactions. On the basis of their values of $\Delta H^{\circ},$ which are favored to occur spontaneously?
$$
\begin{array}{l}
\text { (a) } \mathrm{NH}_{4} \mathrm{Cl}(s) \longrightarrow \mathrm{NH}_{3}(g)+\mathrm{HCl}(g) \\
\text { (b) } 2 \mathrm{C}_{2} \mathrm{H}_{2}(g)+5 \mathrm{O}_{2}(g) \longrightarrow 4 \mathrm{CO}_{2}(g)+2 \mathrm{H}_{2} \mathrm{O}(g)
\end{array}
$$
(c) $\mathrm{C}_{2} \mathrm{H}_{2}(g)+5 \mathrm{~N}_{2} \mathrm{O}(g) \longrightarrow$
$$
2 \mathrm{CO}_{2}(g)+\mathrm{H}_{2} \mathrm{O}(g)+5 \mathrm{~N}_{2}(g)
$$

Key Concepts

Exothermic and Endothermic Reactions

These terms describe reactions based on their heat exchange with the surroundings. Exothermic reactions release heat and typically have a negative ?H°, while endothermic reactions absorb heat and have a positive ?H°. Understanding these concepts is crucial for predicting temperature changes during chemical reactions and for assessing reaction feasibility under specific conditions.

Spontaneity in the Context of Enthalpy

While spontaneity is ultimately governed by Gibbs free energy (?G), the enthalpy change provides valuable insight into the heat flow of a reaction. Generally, exothermic reactions (with negative ?H°) tend to be more thermodynamically favorable under constant pressure, although other factors like entropy and temperature also play significant roles.

Calculation of Reaction Enthalpy (?H°)

The reaction enthalpy, often represented as ?H°, is calculated by taking the difference between the sum of the enthalpies of formation of the products and that of the reactants. This calculation helps in predicting whether a reaction is endothermic or exothermic, providing insights into the thermal energy changes involved in chemical reactions.

Hess's Law

Hess's Law states that the total enthalpy change for a chemical reaction is the same, regardless of the pathway by which the reaction occurs. This principle allows for the calculation of reaction enthalpies using standard enthalpies of formation, by subtracting the sum of the enthalpy of formation of reactants from that of products, making it an essential tool for thermochemical computations.

Standard Enthalpy of Formation

This concept refers to the heat change that accompanies the formation of one mole of a substance from its elements in their standard states under standard conditions. It is a fundamental value used to compute reaction enthalpies, as each compound's heat of formation is used in Hess’s Law to determine the overall enthalpy change of a reaction.

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