Question
20. The enthalpy change under standard conditions for which of the reactions below would be equal to the \( \Delta H_{\mathrm{f}}^{\circ} \) of \( \mathrm{NaOH}(s) \) ? (A) \( \mathrm{Na}(s)+\mathrm{H}_{2} \mathrm{O}(l) \rightarrow \mathrm{NaOH}(s)+1 / 2 \mathrm{H}_{2}(g) \) (B) \( \mathrm{Na}(s)+1 / 2 \mathrm{O}_{2}(g)+1 / 2 \mathrm{H}_{2}(g) \rightarrow \mathrm{NaOH}(s) \) (C) \( \mathrm{Na}(s)+1 / 2 \mathrm{H}_{2} \mathrm{O}_{2}(l) \rightarrow \mathrm{NaOH}(s) \) (D) \( \mathrm{Na}^{+}(a q)+\mathrm{OH}^{-}(a q) \rightarrow \mathrm{NaOH}(s) \)
Ask by Joseph Bernard. in the United Arab Emirates
Jan 22,2025
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Answer
答案：选项 B
Solution
标准生成焓（\( \Delta H_{\mathrm{f}}^{\circ} \)）定义为在标准条件下，从各元素的标准态生成1摩尔化合物所伴随的焓变。
对于氢氧化钠（\( \mathrm{NaOH}(s) \)），其标准生成焓反应应为：
\[ \mathrm{Na}(s) + \frac{1}{2} \mathrm{O}_{2}(g) + \frac{1}{2} \mathrm{H}_{2}(g) \rightarrow \mathrm{NaOH}(s) \]
查看选项，选项 **(B)** 符合上述反应，所以答案是：
**答案：选项 B**
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The enthalpy change under standard conditions for reaction (B) \( \mathrm{Na}(s)+1 / 2 \mathrm{O}_{2}(g)+1 / 2 \mathrm{H}_{2}(g) \rightarrow \mathrm{NaOH}(s) \) directly results in the formation of solid sodium hydroxide (\( \mathrm{NaOH}(s) \)) from its elemental components, and thus corresponds to the standard enthalpy of formation \( \Delta H_{\mathrm{f}}^{\circ} \) of \( \mathrm{NaOH}(s) \). The other options either involve decomposing sodium hydroxide, producing hydrogen gas, or consist of already formed ions, which don’t align with the definition of the standard enthalpy of formation as a reaction starting from elements in their standard states.
