Enthalpy changes for the following reactions can be...

Name: Problem 51, Chapter 5: Chemistry and Chemical Reactivity
Uploaded: 2023-11-22T14:39:23-08:00
Channel: Susan Hallstrom

Enthalpy changes for the following reactions can be determined experimentally: $$\begin{array}{l} \mathrm{N}_{2}(\mathrm{g})+3 \mathrm{H}_{2}(\mathrm{g}) \rightarrow 2 \mathrm{NH}_{3}(\mathrm{g}) \\ \quad \Delta_{r} H^{\circ}=-91.8 \mathrm{kJ} / \mathrm{mol}-\mathrm{r} \mathrm{xn} \\ 4 \mathrm{NH}_{3}(\mathrm{g})+5 \mathrm{O}_{2}(\mathrm{g}) \rightarrow 4 \mathrm{NO}(\mathrm{g})+6 \mathrm{H}_{2} \mathrm{O}(\mathrm{g}) \\ \quad \Delta_{\mathrm{r}} H^{\circ}=-906.2 \mathrm{kJ} / \mathrm{mol}-\mathrm{rxn} \end{array}$$ $\mathrm{H}_{2}(\mathrm{g})+1 / 2 \mathrm{O}_{2}(\mathrm{g}) \rightarrow \mathrm{H}_{2} \mathrm{O}(\mathrm{g})$ $$\Delta_{\mathrm{r}} H^{\circ}=-241.8 \mathrm{kJ} / \mathrm{mol} \text { -rxn }$$ Use these values to determine the enthalpy change for the formation of $\mathrm{NO}(\mathrm{g})$ from the elements (an enthalpy change that cannot be measured directly because the reaction is reactant-favored). $$^{1 / 2} \mathrm{N}_{2}(\mathrm{g})+1 / 2 \mathrm{O}_{2}(\mathrm{g}) \rightarrow \mathrm{NO}(\mathrm{g}) \quad \Delta_{\mathrm{r}} H^{\circ}=?$$

Enthalpy changes for the following reactions can be determined experimentally:
$$\begin{array}{l}
\mathrm{N}_{2}(\mathrm{g})+3 \mathrm{H}_{2}(\mathrm{g}) \rightarrow 2 \mathrm{NH}_{3}(\mathrm{g}) \\
\quad \Delta_{r} H^{\circ}=-91.8 \mathrm{kJ} / \mathrm{mol}-\mathrm{r} \mathrm{xn} \\
4 \mathrm{NH}_{3}(\mathrm{g})+5 \mathrm{O}_{2}(\mathrm{g}) \rightarrow 4 \mathrm{NO}(\mathrm{g})+6 \mathrm{H}_{2} \mathrm{O}(\mathrm{g}) \\
\quad \Delta_{\mathrm{r}} H^{\circ}=-906.2 \mathrm{kJ} / \mathrm{mol}-\mathrm{rxn}
\end{array}$$
$\mathrm{H}_{2}(\mathrm{g})+1 / 2 \mathrm{O}_{2}(\mathrm{g}) \rightarrow \mathrm{H}_{2} \mathrm{O}(\mathrm{g})$
$$\Delta_{\mathrm{r}} H^{\circ}=-241.8 \mathrm{kJ} / \mathrm{mol} \text { -rxn }$$
Use these values to determine the enthalpy change for the formation of $\mathrm{NO}(\mathrm{g})$ from the elements (an enthalpy change that cannot be measured directly because the reaction is reactant-favored).
$$^{1 / 2} \mathrm{N}_{2}(\mathrm{g})+1 / 2 \mathrm{O}_{2}(\mathrm{g}) \rightarrow \mathrm{NO}(\mathrm{g}) \quad \Delta_{\mathrm{r}} H^{\circ}=?$$

Key Concepts

Reaction Manipulation

Reaction manipulation involves reversing, scaling, or combining chemical equations in order to derive a target reaction from known reactions. Adjustments made to the reactions must also be reflected in the corresponding enthalpy changes.

Standard Enthalpy of Formation

The standard enthalpy of formation is defined as the change in enthalpy when one mole of a compound is formed from its constituent elements in their standard states. It serves as a fundamental thermodynamic reference for calculating the enthalpy of reactions.

Hess's Law

Hess's Law states that the overall enthalpy change for a reaction is the same, regardless of the number of intermediate steps in the reaction pathway. This principle allows for the calculation of unknown enthalpy changes by algebraically combining reactions with known enthalpy values.

Enthalpy Change

Enthalpy change refers to the heat absorbed or released during a chemical reaction at constant pressure. It is a state function, meaning its value only depends on the initial and final states of the system and not on the path taken between them.

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