Edexcel A-Level Chemistry: Bond Enthalpy Calculations (Topic 8.4) | 爱德思A-Level化学：键焓计算（8.4）

📚 Edexcel A-Level Chemistry: Bond Enthalpy Calculations (Topic 8.4) | 爱德思A-Level化学：键焓计算（8.4）

In Edexcel A-Level Chemistry Topic 8.4, mean bond enthalpies are used to estimate the enthalpy change of a reaction without carrying out a calorimetry experiment. Understanding how to count, break, and form covalent bonds is essential for both calculation questions and explanation questions in the exam.

在爱德思A-Level化学第8.4节中，平均键焓用于估算反应的焓变，而无需进行量热实验。掌握如何计算、断裂和形成共价键，对考试中的计算题和解释题都至关重要。

1. What Are Mean Bond Enthalpies? | 什么是平均键焓？

A mean bond enthalpy is the average enthalpy change when one mole of a specified covalent bond is broken in the gaseous state, averaged over a range of different compounds containing that bond. For example, the mean bond enthalpy of C-H is an average taken from many molecules such as methane, ethane, and larger hydrocarbons.

平均键焓是指气态下断开1摩尔某种共价键所需的平均焓变，该数值取自一系列含该键的不同化合物并取平均值。例如，C-H的平均键焓是从甲烷、乙烷以及更大的碳氢化合物等许多分子中取平均得到的。

Because the actual bond strength depends slightly on the surrounding atoms, mean bond enthalpies are quoted as positive values in kJ mol⁻¹. They are always positive because energy must be supplied to overcome the attraction between the bonding electrons and the two nuclei.

由于实际键能会略微受到周围原子的影响，平均键焓以正值列出，单位为 kJ mol⁻¹。它们总是正值，因为必须提供能量来克服成键电子与两个原子核之间的吸引作用。

2. Defining Bond Breaking and Bond Making | 定义断键与成键

Breaking a covalent bond always absorbs energy from the surroundings, so bond breaking is endothermic and is given a positive sign. Forming a covalent bond always releases energy to the surroundings, so bond making is exothermic and is given a negative sign.

断裂共价键总是从周围环境吸收能量，因此断键是吸热过程，符号为正。形成共价键总是向周围环境释放能量，因此成键是放热过程，符号为负。

In a chemical reaction, the overall enthalpy change is determined by the balance between the energy absorbed to break reactant bonds and the energy released when product bonds are formed. If more energy is released than absorbed, the reaction is exothermic; if more energy is absorbed, it is endothermic.

在化学反应中，总焓变取决于断裂反应物键所吸收的能量与形成产物键所释放的能量之间的平衡。如果释放的能量多于吸收的能量，反应为放热；如果吸收的能量更多，则为吸热。

3. The Core Equation for ΔH | 反应焓变的核心公式

The standard equation used to estimate the enthalpy change of a reaction from mean bond enthalpies is:

使用平均键焓估算反应焓变的标准公式为：

ΔH = ΣE(bonds broken) − ΣE(bonds formed)

In this equation, ΣE(bonds broken) is the total energy required to break all bonds in the reactants, and ΣE(bonds formed) is the total energy released when all bonds in the products are formed. The subtraction order is essential: broken first, formed second.

在该公式中，ΣE(bonds broken) 是断裂反应物中所有键所需的总能量，ΣE(bonds formed) 是形成产物中所有键所释放的总能量。相减顺序很关键：先断裂后形成。

This formula can also be written as ΔH = energy in − energy out, but the bond enthalpy version with broken and formed bonds is the most reliable for A-Level calculations because it automatically gives the correct sign for exothermic and endothermic reactions.

该公式也可以写成 ΔH = 输入能量 − 输出能量，但使用断裂键与形成键的键焓版本最可靠，因为它能自动给出放热和吸热反应的正负号，适合A-Level考试使用。

4. Worked Example: Combustion of Methane | 示例：甲烷燃烧

The complete combustion of methane is represented by the equation:

甲烷完全燃烧的方程式为：

CH₄(g) + 2O₂(g) → CO₂(g) + 2H₂O(g)

The mean bond enthalpies needed for this calculation are shown in the table below.

下表列出了该计算所需的平均键焓。

In the reactant molecules, CH₄ contains four C-H bonds, and each O₂ molecule contains one O=O bond. Since there are two O₂ molecules, two O=O bonds must be broken in total.

在反应物分子中，CH₄ 含有四个 C-H 键，每个 O₂ 分子含有一个 O=O 键。由于有两个 O₂ 分子，总共需要断裂两个 O=O 键。

5. Step-by-Step Calculation Method | 分步计算方法

Step 1: Calculate the total energy absorbed when bonds are broken. Energy absorbed = 4 × 413 + 2 × 498 = 1652 + 996 = 2648 kJ mol⁻¹.

第一步：计算断键时吸收的总能量。吸收能量 = 4 × 413 + 2 × 498 = 1652 + 996 = 2648 kJ mol⁻¹。

Step 2: Calculate the total energy released when new bonds are formed. In CO₂ there are two C=O bonds, and in two H₂O molecules there are four O-H bonds. Energy released = 2 × 799 +

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