IGCSE WJEC Chemistry: Enthalpy Changes | IGCSE WJEC 化学：焓变考点精讲

📚 IGCSE WJEC Chemistry: Enthalpy Changes | IGCSE WJEC 化学：焓变考点精讲

In IGCSE WJEC Chemistry, understanding enthalpy changes is crucial for grasping how energy is transferred in chemical reactions. This topic covers exothermic and endothermic processes, energy diagrams, experimental measurement, bond energy calculations, and Hess’s law. Mastery of these concepts will not only help you excel in exams but also in practical applications. This revision guide breaks down the key points you need to know for success.

在IGCSE WJEC 化学中，理解焓变对于掌握化学反应中的能量传递至关重要。本主题涵盖放热与吸热过程、能级图、实验测量、键能计算以及赫斯定律。熟练掌握这些概念不仅能帮助你在考试中取得高分，也有助于实际应用。本复习指南将逐一解析你需要掌握的关键考点。

1. What Is Enthalpy Change? | 什么是焓变？

Enthalpy (H) is a measure of the total heat content of a system at constant pressure. The enthalpy change (ΔH) of a reaction is the heat energy absorbed or released under these conditions. It can be expressed as:

焓 (H) 是系统在恒压下的总热含量。反应的焓变 (ΔH) 是在这种条件下吸收或释放的热量。其表达式为：

ΔH = H(products) – H(reactants)

If the products have less enthalpy than the reactants, ΔH is negative (exothermic). If more, ΔH is positive (endothermic). The standard unit is kilojoules per mole (kJ mol⁻¹). This means we often compare changes on a per‑mole basis for a specified equation.

如果生成物的焓值低于反应物，ΔH 为负数（放热）。反之，ΔH 为正数（吸热）。标准单位是千焦每摩尔 (kJ mol⁻¹)。这意味着我们通常针对特定方程式，基于每摩尔进行比较。

The standard enthalpy change of reaction (ΔHᶿ) refers to the enthalpy change when all substances are in their standard states (298 K, 100 kPa). The symbol ‘ᶿ’ indicates standard conditions.

标准摩尔反应焓变 (ΔHᶿ) 指所有物质都处于标准状态（298 K, 100 kPa）时的焓变。符号 ‘ᶿ’ 表示标准条件。

2. Exothermic vs Endothermic Reactions | 放热反应与吸热反应的对比

An exothermic reaction releases heat energy to the surroundings, causing the temperature of the surroundings to rise. The enthalpy change (ΔH) is negative. Common examples include combustion of fuels and neutralisation between acids and alkalis. In an exothermic reaction, the energy released by bond formation in the products exceeds the energy required to break bonds in the reactants.

放热反应向环境释放热量，导致环境温度升高。焓变 (ΔH) 为负值。常见例子包括燃料的燃烧和酸碱中和。在放热反应中，生成物成键释放的能量大于反应物断键所需的能量。

An endothermic reaction absorbs heat energy from the surroundings, causing the temperature of the surroundings to drop. ΔH is positive. Examples include photosynthesis and the thermal decomposition of calcium carbonate. Here, the energy needed to break reactant bonds exceeds the energy released during product bond formation.

吸热反应从环境中吸收热量，导致环境温度下降。ΔH 为正值。例子包括光合作用和碳酸钙的热分解。此时，反应物断键所需的能量大于生成物成键释放的能量。

Feature	Exothermic \| 放热	Endothermic \| 吸热
Sign of ΔH	Negative (-)	Positive (+)
Temperature change	Rises	Falls
Energy of products	Lower than reactants	Higher than reactants

3. Energy Level Diagrams | 能级图

Energy level diagrams (reaction profiles) show the relative enthalpies of reactants and products, as well as the activation energy. For an exothermic reaction, the products’ energy level is drawn lower than that of the reactants. The arrow from reactants to products points downward, and ΔH is labelled as negative.

能级图（反应曲线）显示反应物和生成物的相对焓值以及活化能。对于放热反应，生成物的能级低于反应物。从反应物到生成物的箭头向下，标有负的 ΔH。

For an endothermic reaction, the products’ energy level is higher, the arrow points upward, and ΔH is positive. The difference in height represents the enthalpy change.

对于吸热反应，生成物能级更高，箭头向上，ΔH 为正。高度差代表焓变。

A typical diagram also includes a ‘hump’ that represents the activation energy (Eₐ). This is the minimum energy required for a collision to be successful. In an exothermic profile, Eₐ is the energy from reactants to the top of the hump; in an endothermic profile, the hump is drawn higher relative to both reactants and products.

典型的能级图还包含一个’凸起’，代表活化能 (Eₐ)。这是碰撞成功所需的最低能量。在放热曲线中，Eₐ 是从反应物到凸起顶部的能量；在吸热曲线中，该凸起相对于反应物和生成物都更高。

A catalyst provides an alternative pathway with a lower activation energy, so the hump is drawn smaller. However, the enthalpy change (ΔH) remains the same. You must be able to sketch and interpret these diagrams.

催化剂提供一条活化能较低的替代路径，因此凸起画得更小。但焓变 (ΔH) 保持不变。你必须能够绘制和解读这些图。

4. Activation Energy | 活化能

Activation energy (Eₐ) is the minimum energy colliding particles must possess for a reaction to occur. Even a vigorously exothermic reaction, such as burning hydrogen, requires an initial spark to overcome Eₐ.

活化能 (Eₐ) 是碰撞粒子发生反应所必须具备的最低能量。即使像氢气燃烧这样剧烈的放热反应，也需要一个初始火花来克服 Eₐ。

In energy level diagrams, Eₐ is the difference in enthalpy between the reactants and the transition state (the peak of the curve). A larger Eₐ usually means a slower reaction at a given temperature because fewer particles have the required energy. For an exothermic reaction:

在能级图中，Eₐ 是反应物与过渡态（曲线顶点）之间的焓差。较大的 Eₐ 通常意味着在给定温度下反应较慢，因为拥有足够能量的粒子较少。对于放热反应：

ΔH = Eₐ(forward) – Eₐ(reverse)

In practice, catalysts lower Eₐ without being used up, increasing reaction rate but not altering ΔH.

实际上，催化剂在自身不被消耗的情况下降低 Eₐ，从而提高反应速率，但不会改变 ΔH。

5. Measuring Enthalpy Changes Experimentally | 实验测量焓变

In the IGCSE WJEC practical, a simple calorimeter is often used: a polystyrene cup with a lid, a thermometer, and a known volume of solution. The reaction is carried out inside the cup, and the temperature change is recorded. Polystyrene is a good insulator, but heat loss to the surroundings is still the main source of error.

在IGCSE WJEC 实验中，常用简易量热计：一个带盖的聚苯乙烯杯、一支温度计和已知体积的溶液。反应在杯内进行，记录温度变化。聚苯乙烯是良好的绝热体，但向环境散热仍是主要误差来源。

Typical steps: measure initial temperature of the reactants every minute for a few minutes, mix them (or add a solid), stir, and record the maximum (or minimum) temperature reached. For a neutralisation reaction, you can mix acid and alkali in the cup. For a displacement reaction, add a metal powder to a salt solution.

典型步骤：几分钟内每隔一分钟测量一次反应物的初始温度，混合（或加入固体），搅拌，记录达到的最高（或最低）温度。对于中和反应，可以在杯中混合酸和碱。对于置换反应，将金属粉末加入盐溶液中。

Often the reaction is not instantaneous, so you should plot the temperature against time and extrapolate the cooling curve back to the time of mixing to correct for heat loss – this is known as the graphical method for determining ΔT.

反应通常不是瞬间完成的，因此应绘制温度-时间图，并将冷却曲线外推回混合时刻，以校正热损失——这称为确定 ΔT 的图解方法。

6. Calculations: q = mcΔT and ΔH | 计算：q = mcΔT 与 ΔH

The heat energy transferred, q, is calculated using the equation:

传递的热量 q 用以下公式计算：

q = mcΔT

where m is the mass of the solution (in g, assume the density of dilute solutions is 1 g cm⁻³, so volume in cm³ ≈ mass in g), c is the specific heat capacity of the solution (usually taken as 4.18 J g⁻¹ °C⁻¹, the same as water), and ΔT is the temperature change (final – initial) in °C or K. Note that ΔT should be a positive number when calculating q; the sign of ΔH is assigned later.

其中 m 是溶液的质量（单位 g，假设稀溶液密度为 1 g cm⁻³，因此体积 cm³ ≈ 质量 g），c 是溶液的比热容（通常取水的比热容 4.18 J g⁻¹ °C⁻¹），ΔT 是温度变化（最终 – 初始），单位为 °C 或 K。注意计算 q 时 ΔT 取正值；ΔH 的符号稍后确定。

To find the molar enthalpy change ΔH, you need the amount in moles (n) of the limiting reactant:

要计算摩尔焓变 ΔH，需要知道限制反应物的物质的量 (n)：

ΔH = –q / n

The negative sign is included for exothermic reactions because the temperature rises (exothermic) and we want ΔH negative. If the temperature falls (endothermic), q is still calculated as a positive number, but then ΔH becomes positive without the negative sign – you must check the temperature change to assign the correct sign. Alternatively, you can always use ΔH = – (mcΔT) / n and let the temperature change carry its sign (negative if cooling). The WJEC mark scheme often expects ΔT as a magnitude and then a separate judgement of sign.

对于放热反应，因温度升高而加入负号，使 ΔH 为负。如果温度下降（吸热），q 仍计算为正，但此时 ΔH 为正，无需再加负号——你必须根据温度变化确定符号。或者也可以始终使用 ΔH = – (mcΔT) / n 并让温度变化带有符号（冷却时为负）。WJEC 评分标准通常要求将 ΔT 作为绝对值计算，然后单独判断符号。

Example: When 50 cm³ of 1.0 mol dm⁻³ HCl is neutralised by 50 cm³ of 1.0 mol dm⁻³ NaOH, the temperature rises by 6.5 °C. Total volume = 100 cm³, so m = 100 g. q = 100 × 4.18 × 6.5 = 2717 J. Moles of HCl = 0.050 dm³ × 1.0 mol dm⁻³ = 0.050 mol. ΔH = –2717 J / 0.050 mol = –54340 J mol⁻¹ ≈ –54.3 kJ mol⁻¹.

示例：用 50 cm³ 1.0 mol dm⁻³ HCl 中和 50 cm³ 1.0 mol dm⁻³ NaOH，温度升高 6.5 °C。总体积 = 100 cm³，故 m = 100 g。q = 100 × 4.18 × 6.5 = 2717 J。HCl 物质的量 = 0.050 dm³ × 1.0 mol dm⁻³ = 0.050 mol。ΔH = –2717 J / 0.050 mol = –54340 J mol⁻¹ ≈ –54.3 kJ mol⁻¹。

7. Bond Energies and Enthalpy Changes | 键能与焓变

Enthalpy changes can be estimated using average bond energies. Bond breaking requires energy (endothermic), so bond-breaking enthalpies are positive (+). Bond forming releases energy (exothermic), so bond-making enthalpies are negative (–). The overall ΔH is therefore:

焓变可以用平均键能估算。断键需要能量（吸热），因此断键焓为正值 (+)。成键释放能量（放热），因此成键焓为负值 (–)。因此总 ΔH 为：

ΔH = Σ (bond energies broken) – Σ (bond energies formed)

This formula ensures that energy released during bond formation is subtracted. If broken bonds require 1000 kJ and new bonds release 1200 kJ, ΔH = 1000 – 1200 = –200 kJ, an exothermic reaction. Always draw displayed formulae to count each bond correctly.

该公式确保将成键释放的能量减去。如果断裂的键需要 1000 kJ，形成的新键释放 1200 kJ，则 ΔH = 1000 – 1200 = –200 kJ，为放热反应。务必画出结构式以正确统计每个键。

Example: H₂ + Cl₂ → 2HCl. Bond energies: H–H = 436, Cl–Cl = 243, H–Cl = 432 kJ mol⁻¹. Bonds broken: 1×H–H + 1×Cl–Cl = 436 + 243 = 679 kJ. Bonds formed: 2×H–Cl = 2×432 = 864 kJ. ΔH = 679 – 864 = –185 kJ mol⁻¹ for 2 moles of HCl. For 1 mole of HCl, ΔH = –92.5 kJ mol⁻¹.

示例：H₂ + Cl₂ → 2HCl。键能：H–H = 436，Cl–Cl = 243，H–Cl = 432 kJ mol⁻¹。断裂的键：1×H–H + 1×Cl–Cl = 436 + 243 = 679 kJ。形成的键：2×H–Cl = 2 × 432 = 864 kJ。ΔH = 679 – 864 = –185 kJ mol⁻¹（对于 2 mol HCl）。对于 1 mol HCl，ΔH = –92.5 kJ mol⁻¹。

Remember that bond energy values are averages from many compounds; results from calculation are estimates and may differ from experimental values.

请记住，键能值是多种化合物的平均值；计算结果为估算值，可能与实验值不同。

8. Hess’s Law and Simple Enthalpy Cycles | 赫斯定律与简单焓循环

Hess’s law states that the total enthalpy change of a reaction is independent of the route taken. This allows you to calculate an unknown ΔH for a reaction by combining known enthalpy changes of other reactions. A common WJEC IGCSE application is using enthalpies of combustion or formation to construct an enthalpy cycle.

赫斯定律指出，反应的总焓变与采取的路径无关。这允许你通过组合其他已知反应的焓变来计算未知的 ΔH。WJEC IGCSE 常见的应用是利用燃烧焓或生成焓构建焓循环。

For example, to find the enthalpy change for the reaction C(s) + ½O₂(g) → CO(g) (which is difficult to measure directly), you can use the combustion enthalpies of C and CO:

例如，求反应 C(s) + ½O₂(g) → CO(g) 的焓变（难以直接测量），可利用 C 和 CO 的燃烧焓：

ΔH₁ = ΔH_combustion(C) – ΔH_combustion(CO)

In an energy cycle, the direct route from reactants to products is equivalent to an indirect route going through common intermediates (often the combustion products, CO₂ and H₂O). When using formation enthalpies, the cycle is drawn with elements as the reference level:

在能量循环中，从反应物直接到生成物的路径等同于通过共同中间体（通常是燃烧产物 CO₂ 和 H₂O）的间接路径。当使用生成焓时，循环以单质为基准水平绘制：

ΔH_reaction = ΣΔHᶿ_f(products) – ΣΔHᶿ_f(reactants)

You are expected to draw a simple cycle or apply the above formula, showing arrows and enthalpy changes clearly. Always check whether the arrows follow the direction of reaction or reverse; reversing a step changes the sign of ΔH.

你需要绘制一个简单循环或应用上述公式，清晰地标出箭头和焓变。始终检查箭头方向是否与反应方向一致；倒转一步会改变 ΔH 的符号。

9. Common Exam Mistakes and Tips | 常见考点错误与技巧

One frequent mistake is forgetting to convert q from joules to kilojoules before calculating ΔH in kJ mol⁻¹. Since q = mcΔT gives joules, divide by 1000 to get kJ. Always check unit consistency.

一个常见错误是在计算 ΔH（单位为 kJ mol⁻¹）之前忘记将 q 从焦耳转换为千焦。因为 q = mcΔT 给出的是焦耳，所以要除以 1000 得到千焦。务必检查单位的一致性。

Another error is failing to identify the limiting reactant correctly when two solutions are mixed. If one reactant is in excess, the moles n used to calculate ΔH must be the moles of the limiting reactant. For example, in the neutralisation of 1.0 mol dm⁻³ HCl with 2.0 mol dm⁻³ NaOH, HCl is limiting, so use its moles.

另一个错误是在混合两种溶液时未能正确识别限制反应物。如果某反应物过量，用于计算 ΔH 的物质的量 n 必须是限制反应物的量。例如，用 2.0 mol dm⁻³ NaOH 中和 1.0 mol dm⁻³ HCl 时，HCl 是限制剂，故使用其物质的量。

When using bond energies, double-check the balancing of the equation and count each separate bond. For instance, CO₂ contains two C=O double bonds; water has two O–H single bonds. Missing a bond here is a common slip.

使用键能时，要仔细核对方程式的配平并统计每个独立的键。例如，CO₂ 含有两个 C=O 双键；水分子有两个 O–H 单键。漏计一个键是常见失分点。

In energy level diagrams, ensure you label the enthalpy change correctly as either negative or positive and include the value. Also show the activation energy with and without a catalyst if asked.

在能级图中，确保正确标明焓变是负值还是正值，并注明数值。如果题目要求，还应画出有催化剂和无催化剂时的活化能。

Finally, when describing an experiment, always mention how you minimise heat loss, such as using a lid, stirring, and insulating the cup. This often earns marks in long-answer questions.

最后，在描述实验时，总要提及如何减少热损失，例如使用盖子、搅拌、杯子保温。这在长答题中常能得分。

Published by TutorHao | Chemistry Revision Series | aleveler.com

更多咨询请联系16621398022（同微信）

IGCSE WJEC Chemistry: Enthalpy Changes | IGCSE WJEC 化学：焓变 考点精讲