Calculate ΔH^∘ for each of the following reactions using the...

Calculate $\Delta H^{\circ}$ for each of the following reactions using the data in Appendix 4: $$ \begin{array}{c} 4 \mathrm{Na}(s)+\mathrm{O}_{2}(g) \longrightarrow 2 \mathrm{Na}_{2} \mathrm{O}(s) \\ 2 \mathrm{Na}(s)+2 \mathrm{H}_{2} \mathrm{O}(l) \longrightarrow 2 \mathrm{NaOH}(a q)+\mathrm{H}_{2}(g) \\ 2 \mathrm{Na}(s)+\mathrm{CO}_{2}(g) \longrightarrow \mathrm{Na}_{2} \mathrm{O}(s)+\mathrm{CO}(\mathrm{g}) \end{array} $$ Explain why a water or carbon dioxide fire extinguisher might not be effective in putting out a sodium fire.

Calculate $\Delta H^{\circ}$ for each of the following reactions using the data in Appendix 4:
$$
\begin{array}{c}
4 \mathrm{Na}(s)+\mathrm{O}_{2}(g) \longrightarrow 2 \mathrm{Na}_{2} \mathrm{O}(s) \\
2 \mathrm{Na}(s)+2 \mathrm{H}_{2} \mathrm{O}(l) \longrightarrow 2 \mathrm{NaOH}(a q)+\mathrm{H}_{2}(g) \\
2 \mathrm{Na}(s)+\mathrm{CO}_{2}(g) \longrightarrow \mathrm{Na}_{2} \mathrm{O}(s)+\mathrm{CO}(\mathrm{g})
\end{array}
$$
Explain why a water or carbon dioxide fire extinguisher might not be effective in putting out a sodium fire.

Key Concepts

Standard Enthalpy Change (?H°)

The standard enthalpy change of a reaction quantifies the net heat absorbed or released when a reaction occurs under standard conditions. It is determined by subtracting the sum of the standard enthalpies of formation of the reactants from that of the products and provides insight into the energy profile of chemical processes.

Hess's Law

Hess's Law states that the total enthalpy change for a reaction is independent of the pathway taken from reactants to products. This principle allows the calculation of reaction enthalpy changes using known standard enthalpies of formation, even when a reaction occurs in several steps.

Enthalpy of Formation

The enthalpy of formation is defined as the change in enthalpy when one mole of a compound is formed from its individual elements in their standard states. It serves as a fundamental building block in thermochemistry, enabling the calculation of reaction enthalpies through the combination of formation values.

Reactivity of Alkali Metals

Alkali metals, such as sodium, are characterized by their high reactivity, particularly with water and other substances that can act as proton donors. This high reactivity is due to their low ionization energies, meaning they readily lose electrons, which often leads to rapid and highly exothermic reactions.

Firefighting Considerations for Reactive Metals

In fires involving reactive metals, typical extinguishing agents like water or carbon dioxide may not be effective and can even exacerbate the situation. Water can trigger violent reactions with reactive metals, releasing flammable hydrogen, while carbon dioxide may not provide sufficient cooling or chemical quenching needed, necessitating the use of specialized dry chemical extinguishers tailored for such metals.

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