17.8
计算标准自由能变化
在标准压力为 1 巴且压力为 298 K 时发生的反应的自由能变化称为标准自由能变化。 由于自由能是一个状态函数，因此其值仅取决于系统的初始和最终状态的条件。 计算自由能变化的物理和化学反应的一个方便而常见的方法是使用广泛可用的标准状态热动力学数据汇编。 一种方法是使用标准焓和熵来计算标准自由能变化…
对于标准状态下发生的化学反应，可以用以下三种方法之一来计算 它的标准自由能变化。第一种方法是使用反应的 标准ΔG 方程。请考虑氧化钙和二氧化碳 生成碳酸钙的过程。反应的标准焓变 可以从反应中化合物的 生成热来计算，标准熵变可以 由它们的标准摩尔熵来计算。使用参考表中的值，可以确定反应的标准焓变和 标准熵变。将这些值代入 标准自由能变化方程，298 开尔文时反应的标准ΔG 等于 130.7 10³ 焦耳。在第二种方法中，反应的标准自由能变化 由生成物的生成自由能之和 乘以它们的 化学计量系数，与反应物的生成自由能之和 乘以它们的化学计量系数 之差来计算。以氢气和氯气反应 生成两摩尔氯化氢气体为例。像生成焓一样，纯元素的 标准生成自由能为零。因此，氢气和氯气的 生成自由能为零，而氯化氢气体的生成自由能为 95.3 千焦。所以，这个反应的自由能 等于氯化氢气体生成自由能的 2 倍，或者 190.6 千焦。请回想一下盖斯定律，对于分步反应，净焓变可以 由每一步的焓之和来计算。第三种方法以类似方式计算 反应的净自由能变化。请考虑硫化锌燃烧 产生金属锌和二氧化硫气体，在已知标准自由能的情况下，这个反应可以分两步进行。将这些反应的自由能变化相加，得出的总ΔG 为 98.8 千焦耳。请注意，步骤 1 是非自发过程。通过与步骤 2 中的自发反应结合，净反应具有负ΔG，因此是自发过程。
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Q1: What are the three methods for calculating standard free energy change?
The three methods are: (1) using standard enthalpy and entropy values with the ΔG° equation, (2) calculating from standard free energies of formation of products and reactants, and (3) applying Hess's law principles by summing free energy changes of stepwise reactions. Each method yields the same result since free energy is a state function.
Q2: How do you calculate standard free energy change from enthalpy and entropy?
Determine the standard enthalpy change (ΔH°) from heats of formation and standard entropy change (ΔS°) from standard molar entropies using reference tables. Substitute these values into ΔG° = ΔH° − TΔS° at 298 K. For example, calcium carbonate formation yields ΔG° = −130.7 × 10³ J using this approach.
Q3: Why is the standard free energy of formation zero for pure elements?
By definition, the standard free energy of formation equals zero for elemental substances under standard state conditions because there is no net formation process—the element already exists in its standard state. This convention simplifies calculations, as only compounds contribute to the overall free energy change.
Q4: How do you use free energies of formation to find the standard free energy change of a reaction?
Calculate the sum of free energies of formation of products multiplied by their stoichiometric coefficients, then subtract the sum of free energies of formation of reactants multiplied by their coefficients. For hydrogen and chlorine forming hydrogen chloride, ΔG° = 2(−95.3 kJ) − 0 = −190.6 kJ.
Q5: What does a negative standard free energy change indicate about a reaction?
A negative ΔG° indicates the reaction is spontaneous under standard conditions and will proceed forward. Conversely, a positive ΔG° means the reaction is nonspontaneous. Coupling a nonspontaneous reaction with a spontaneous one can yield an overall negative ΔG°, making the combined process thermodynamically favorable.
Q6: How can Hess's law be applied to calculate free energy changes for stepwise reactions?
For a reaction carried out in multiple steps, sum the standard free energy changes of each individual step to obtain the overall ΔG°. This works because free energy is a state function. For zinc sulfide combustion in two steps with known ΔG° values, the net reaction yields ΔG° = −98.8 kJ.
Q7: What are standard state conditions for calculating free energy changes?
Standard state conditions are defined as 1 bar pressure and 298 K (25°C) temperature. The free energy change calculated under these conditions is called the standard free energy change (ΔG°). These standardized conditions allow chemists to use widely available thermodynamic data tables for consistent calculations.