📚 Standard Enthalpy Changes | 标准焓变
Enthalpy change is one of the most important quantitative ideas in A-Level Chemistry. It allows chemists to compare the energy absorbed or released by different reactions under the same reference conditions. This article explains the main standard enthalpy change definitions, their signs, and how to use them in Hess’s law calculations.
焓变是 A-Level 化学中最重要的定量概念之一。它使化学家能够在相同的参考条件下比较不同反应吸收或释放的能量。本文解释主要的标准焓变定义、它们的正负号，以及如何在赫斯定律计算中使用它们。
1. Enthalpy and Enthalpy Change | 焓与焓变
Enthalpy H is a state function defined as H = U + PV, where U is internal energy, P is pressure and V is volume. Most chemical reactions are carried out at constant atmospheric pressure. Under these conditions, the heat absorbed or released by a system equals its enthalpy change.
焓 H 是一个状态函数，定义为 H = U + PV，其中 U 是内能，P 是压强，V 是体积。大多数化学反应在恒定的常压条件下进行。在这些条件下，体系吸收或放出的热量等于其焓变。
H = U + PV
Because H is a state function, the enthalpy change ΔH depends only on the initial and final states, not on the route taken. This is the basis of Hess’s law, which is used to calculate enthalpy changes that cannot be measured directly.
由于 H 是状态函数，焓变 ΔH 只取决于初态和终态，与所经过的途径无关。这是赫斯定律的基础，该定律用于计算无法直接测量的焓变。
ΔH = H(products) − H(reactants)
2. Standard Conditions and State Symbols | 标准条件与状态符号
To compare enthalpy changes fairly, chemists use an agreed set of standard conditions. The standard pressure is 100 kPa, the standard temperature is 298 K, and any solution involved must have a concentration of 1 mol dm⁻³. A substance must be in its most stable physical state under these conditions.
为了公平地比较焓变，化学家使用一套约定的标准条件。标准压强为 100 kPa，标准温度为 298 K，所涉及的溶液浓度必须为 1 mol dm⁻³。物质在这些条件下必须处于其最稳定的物理状态。
| Quantity | 项目 | Standard condition | 标准条件 |
|---|---|
| Pressure | 压强 | 100 kPa |
| Temperature | 温度 | 298 K |
| Solution concentration | 溶液浓度 | 1 mol dm⁻³ |
| Physical state | 物理状态 | Most stable state at 100 kPa and 298 K |
State symbols must always be shown in thermochemical equations. For example, carbon is written as C(s, graphite), not as diamond, because graphite is the most stable allotrope under standard conditions. Water is written as H₂O(l) at 298 K.
热化学方程式中必须始终标注状态符号。例如，碳应写为 C(s, graphite)，而不是金刚石，因为石墨在标准条件下是最稳定的同素异形体。在 298 K 时，水写为 H₂O(l)。
3. Exothermic and Endothermic Reactions | 放热反应与吸热反应
An exothermic reaction transfers energy from the system to the surroundings, so the enthalpy of the products is lower than that of the reactants. Its ΔH value is negative. Combustion of methane and neutralisation of a strong acid by a strong base are common exothermic examples.
放热反应将能量从体系传递到环境，因此生成物的焓低于反应物的焓。其 ΔH 值为负值。甲烷的燃烧和强酸与强碱的中和反应是常见的放热例子。
An endothermic reaction absorbs energy from the surroundings, so the enthalpy of the products is higher than that of the reactants. Its ΔH value is positive. Thermal decomposition of calcium carbonate and photosynthesis are endothermic processes.
吸热反应从环境吸收能量，因此生成物的焓高于反应物的焓。其 ΔH 值为正值。碳酸钙的热分解和光合作用都是吸热过程。
Enthalpy profile diagrams show exothermic reactions with products at a lower energy level than reactants, while endothermic reactions have products at a higher energy level. The activation energy is always drawn as the energy barrier between reactants and products.
焓变曲线图显示，放热反应的生成物能级低于反应物能级，而吸热反应的生成物能级高于反应物能级。活化能始终被画为反应物与生成物之间的能量障碍。
4. Standard Enthalpy Change of Reaction ΔH°r | 标准反应焓变
The standard enthalpy change of reaction, written as ΔH°r, is the enthalpy change when the reaction occurs in the molar quantities shown in the balanced equation, under standard conditions. Its value depends on the coefficients in the equation.
标准反应焓变，写为 ΔH°r，是指在标准条件下，按照配平方程式中所示的摩尔量发生反应时的焓变。其数值取决于方程式中的系数。
2H₂(g) + O₂(g) → 2H₂O(l) ΔH°r = −572 kJ mol⁻¹
This value refers to the reaction of exactly 2 mol of hydrogen gas with 1 mol of oxygen gas to form 2 mol of liquid water. If the equation were divided by 2, the value would become −286 kJ mol⁻¹.
该数值表示恰好 2 mol 氢气与 1 mol 氧气反应生成 2 mol 液态水。如果将方程式除以 2，数值将变为 −286 kJ mol⁻¹。
5. Standard Enthalpy Change of Formation ΔH°f | 标准生成焓变
The standard enthalpy change of formation, ΔH°f, is the enthalpy change when 1 mole of a compound is formed from its constituent elements in their standard states under standard conditions. For example, the formation of carbon dioxide from graphite and oxygen is written as:
标准生成焓变 ΔH°f 是指在标准条件下，由处于标准状态的组成元素生成 1 mol 化合物时的焓变。例如，由石墨和氧气生成二氧化碳可写为：
C(s, graphite) + O₂(g) → CO₂(g) ΔH°f = −394 kJ mol⁻¹
By definition, the standard enthalpy change of formation of any element in its standard state is zero. This is because forming an element from itself involves no chemical change. For example, ΔH°f for O₂(g) and C(s, graphite) is exactly 0.
根据定义，任何处于标准状态的元素的标准生成焓变为零。这是因为由元素本身生成元素不涉及化学变化。例如，O₂(g) 和 C(s, graphite) 的 ΔH°f 正好为 0。
6. Standard Enthalpy Change of Combustion ΔH°c | 标准燃烧焓变
The standard enthalpy change of combustion, ΔH°c, is the enthalpy change when 1 mole of a substance is completely burned in excess oxygen under standard conditions. The products must be the usual oxides, with carbon forming CO₂(g) and hydrogen forming H₂O(l).
标准燃烧焓变 ΔH°c 是指在标准条件下，1 mol 物质在过量氧气中完全燃烧时的焓变。产物必须是通常的氧化物，碳生成 CO₂(g)，氢生成 H₂O(l)。
CH₄(g) + 2O₂(g) → CO₂(g) + 2H₂O(l) ΔH°c = −890 kJ mol⁻¹
Combustion reactions are always exothermic, so standard enthalpies of combustion are always negative. They are widely used in energy cycles because many organic compounds burn cleanly and their combustion enthalpy changes can be measured directly in a bomb calorimeter.
燃烧反应总是放热的，因此标准燃烧焓变始终为负值。燃烧焓变在能量循环中被广泛使用，因为许多有机化合物可以完全燃烧，其燃烧焓变可以在弹式量热计中直接测定。
7. Standard Enthalpy Change of Neutralisation ΔH°neut | 标准中和焓变
The standard enthalpy change of neutralisation, ΔH°neut, is the enthalpy change when 1 mole of water is formed from the reaction of an acid and an alkali under standard conditions, using dilute aqueous solutions. For strong acids and strong alkalis, the value is almost constant at about −57 kJ mol⁻¹.
标准中和焓变 ΔH°neut 是指在标准条件下，使用稀的水溶液，由酸与碱反应生成 1 mol 水时的焓变。对于强酸和强碱，该数值几乎恒定，约为 −57 kJ mol⁻¹。
H⁺(aq) + OH⁻(aq) → H₂O(l) ΔH°neut = −57 kJ mol⁻¹
The reason for this constant
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