Name: using standard enthalpy values in chemistry
Uploaded: 2020-09-03T13:38:27.000Z
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K12 Standards

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North Carolina K-12 Science Standard Course of Study

9-12

Chemistry

Energy

PS.Chm.7.2

Standard Enthalpy of Formation

反应的焓变 可以用量热计测量，也可以通过计算 反应物和产物之间的焓差 得到。然而，反应物和产物的绝对焓 不能直接测量；因此，化学家通常使用焓的变化，或 ΔH，相对于在约定标准状态下的 参考物质而言。标准状态由一组特定的条件来定义。这包括设定的温度，通常为 25 摄氏度或 298 开尔文，对于气体来说是 1 巴的恒定压力。对于溶液，标准状态 是溶剂中 1 摩尔浓度的纯溶质。物质的标准状态 还包括物质在这些条件下 存在的物质的物理状态。例如，氯化钠作为固体，水银作为液体，或氦气作为气体。如果一种元素在这些条件下以多种形式 存在，那么元素最稳定的形式 被定义为标准状态。例如，碳可以以石墨晶体 或金刚石的形式存在，但石墨是最稳定的形式，因此它是碳的标准状态。当处于标准状态的元素 结合形成 1 摩尔的纯化合物时，反应的焓称为标准生成焓 或标准生成热。这个量用 H 零，以及下标 f 表示，其中零表示组成元素的 标准状态，而 f 表示形成。标准状态下 纯元素的标准生成焓 总是为零，因为当元素已经处于 其标准状态时，没有反应，因此焓并没有变化。化合物的标准生成焓 单位：千焦每摩尔）的值，可在参考表格中找到。这些物质包括 非标准状态下的元素，如气态钠，和化合物，如氯化钠。一个反应的标准焓变 可以由产物和反应物的差值 来计算，反过来，也可以 用参考表中的焓值 来计算。

<ol>
	<li>霍克，埃里克·利。"烷烃形成标准焓的计算:说明分子力学和电子表格程序。"化学教育杂志 76，第 2 期（1999）:278。</li>
	<li>Mazzuca、James W.、Alexis R. Downing 和 Christopher Potter。"应用于燃烧焓的物理化学实验室的经验校正电子结构计算。"化学教育杂志 96，第 6 期（2019）:1165-1170。</li>
	<li>Jansen， Michael P. "将汽油动力汽车转换为丙烷的成本。高中或化学入门的实用复习题。化学教育杂志 77，第 12 期（2000 年）:1578. 
	 </li>
</ol>

标准生成焓

通常将反应物和产物处于相同条件下的反应的焓变制成表格。标准状态是一组普遍接受的条件，用作确定其他不同条件下的特性的参考点。对于化学家，IUPAC标准状态是指材料在1 bar的压力下和1 M的溶液，并且未指定温度。许多热化学表格列出了标准状态为1 atm的值。因为在如此小的压力变化（1 bar = 0.987 atm）下，反应的Δ H 变化很小，所以Δ H 值（最精确测量的除外）值）在两组标准条件下基本相同。上标“ o”焓变符号中的表示标准状态。由于通常的（但不是技术标准）温度是298.15 K，因此，除非指定其他温度，否则将假定该温度。因此，符号（ΔH°）用于指示在这些条件下发生的过程的焓变。 （符号ΔH用于表示在非标准条件下发生的反应的焓变。）
 
 许多类型的化学和物理过程的焓变可在参考文献中找到，包括燃烧反应，相变和形成反应的焓变。由于给定反应的焓变与所涉及物质的量成正比，因此可以在此基础上进行报告（即，特定反应物的量为ΔH）。但是，我们经常发现，将一个广泛的属性（ΔH）除以另一个（物质的数量），并报告每个数量的密集值ΔH，通常是“归一化”到每摩尔的基础上。
 
 <h4>标准形成焓</ h4>
 
 标准形成焓ΔHf°是反应的焓变，在该反应中，在标准状态下，由最稳定状态的自由元素精确地形成1摩尔纯物质。这些值对于计算或预测不切实际或危险的化学反应或难以进行测量的过程的焓变特别有用。使用已知的标准形成焓值，可以确定任何反应的焓变。
 
 CO 2 （ g ）的形成标准焓为-393.5 kJ / mol。这是放热反应的焓变：
 
 <img alt="Eq1" src="https://www.jove.com/files/core/CDNSource/lessons/801/801_Equations-01.png" style="height: 40px;" />
 
 以1个大气压和25摄氏度的压力开始反应（碳以石墨形式存在，这是在这些条件下最稳定的碳形式），最后以一摩尔CO 2 ，也处于1 atm和25℃。对于二氧化氮，NO 2 （ g ），Δ H f ＆deg;为33.2 kJ / mol。这是吸热反应的焓变：
 
 <img alt="Eq2" src="https://www.jove.com/files/core/CDNSource/lessons/801/801_Equations-02.png" style="height: 40px;" />
 
 具有 1 / 2 摩尔N 2 和1摩尔O 2 的反应方程是正确的在这种情况下，因为标准的形成焓总是指1摩尔的产物：NO 2 （ g ）。
 
 根据定义，最稳定形式的元素形成的标准焓在标准条件下等于零。例如，在标准条件下，碳（石墨烯），双原子氧气，双原子氮气，钠金属和液态汞的形成标准焓为零。
 
 本文改编自<a href="https://openstax.org/books/chemistry-2e/pages/5-3-enthalpy"> Openstax，化学2e，第5.3节：焓。 / a>

焓变通常以表格形式表示反应物和生成物处于相同条件的反应。标准状态是一种常见的 ...

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using standard enthalpy values in chemistry

Enthalpy changes are often listed for reactions that start and end under the same conditions. Chemists use a standard state as a reference set of conditions for comparing properties. In IUPAC terms, the standard state means 1 bar of pressure and 1 M solutions, but it does not set a temperature.

Many thermochemical tables still use 1 atm as the reference pressure. Because the difference between 1 bar and 1 atm is very small, enthalpy change usually stays almost the same. A superscripted o in the enthalpy symbol shows that the value is for standard state. The usual temperature is 298.15 K, so ΔH° is often used for a process under these conditions. ΔH is used for nonstandard conditions.

Standard enthalpy values are available for many chemical and physical processes. These include combustion reactions, phase changes, and formation reactions. Enthalpy change depends on the amount of substance involved, so it can be reported for a specific reaction amount. It is often more useful to divide by moles and give a per-mole value.

The standard enthalpy of formation, ΔHf°, is the enthalpy change when 1 mole of a pure substance forms from its elements in their most stable states under standard conditions. This value is useful when a reaction is hard to measure directly or would be unsafe to run. Known formation enthalpies can also be used to calculate the enthalpy change of other reactions.

For example, the standard enthalpy of formation of CO2(g) is −393.5 kJ/mol. That value matches the exothermic formation of one mole of carbon dioxide from graphite, the most stable form of carbon under these conditions, with all substances at 1 atm and 25 °C. For NO2(g), ΔHf° is 33.2 kJ/mol. Here the balanced formation reaction uses 1/2 mole of N2 and 1 mole of O2 because the value must refer to 1 mole of NO2 product.

By definition, an element in its most stable form has a standard enthalpy of formation of zero. Examples include carbon as graphene, diatomic oxygen gas, diatomic nitrogen gas, sodium metal, and liquid mercury. These zero values help make thermochemical calculations consistent.

Using Standard Enthalpy Values in Chemistry

using-standard-enthalpy-values-in-chemistry

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Using Standard Enthalpy Values in Chemistry | JoVE K12

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JoVE Core 化学通过简单明了且通俗易懂的动画视频课程，将普通化学概念带入到现实生活当中。此外，在科学家行动视频中，我们演示了在实验室中进行的经典和原始研究实验。

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Using Standard Enthalpy Values in Chemistry

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