📚 Energy from Fuels | 燃料产生的能量
Fuels are at the heart of modern society, powering transportation, industry and homes through the release of stored chemical energy. In IB Chemistry, understanding how to quantify and compare this energy, and how to evaluate fuels in a sustainable world, is an essential skill that bridges thermodynamics and real‑life applications.
燃料是现代社会的核心，通过释放储存的化学能为交通、工业和家庭提供动力。在 IB 化学中，理解如何量化和比较这些能量，以及如何在可持续发展的世界中评估燃料，是一项将热力学与现实应用紧密联系的关键能力。
1. The Nature of Fuels and Chemical Energy | 燃料的本质与化学能
A fuel is a substance that reacts with oxygen in a combustion reaction to release useful energy, primarily as heat. During combustion, chemical bonds are broken and new bonds form, and the overall process transfers energy from the chemical system to the surroundings.
燃料是一种在燃烧反应中与氧气反应并释放有用能量（主要是热量）的物质。在燃烧过程中，化学键断裂并形成新的化学键，整个过程将能量从化学体系传递到周围环境。
Most common fuels are hydrocarbons or alcohols, whose molecules store energy in C–C and C–H bonds. When these bonds are replaced by stronger C=O and O– ds in CO₂ and H₂O, the overall potential energy decreases, making the reaction exothermic.
大多数常见燃料是碳氢化合物或醇类，分子中的 C–C 和 C–H 键储存着能量。当这些键被 CO₂ 和 H₂O 中更强的 C=O 和 O–H 键取代时，整体势能降低，反应表现为放热。
2. Enthalpy Changes of Combustion | 燃烧反应的焓变
The standard enthalpy change of combustion (ΔHc°) is the enthalpy change when one mole of a substance is completely burned in oxygen under standard conditions (298 K, 100 kPa). All combustion enthalpies are negative, meaning heat is released to the surroundings.
标准燃烧焓变 (ΔHc°) 是指在标准条件（298 K，100 kPa）下，一摩尔物质在过量氧气中完全燃烧时的焓变。所有燃烧焓均为负值，意味着热量被释放到周围环境。
For example, the complete combustion of methane can be written with its standard enthalpy change:
例如，甲烷的完全燃烧可表示为：
CH₄(g) + 2O₂(g) → CO₂(g) + 2H₂O(l) ΔHc° = -890 kJ mol⁻¹
This value tells us that 890 kJ of thermal energy is transferred to the surroundings for every mole of methane burned. Comparisons between different fuels are then based on the magnitude of their ΔHc° values.
该数值告诉我们，每燃烧一摩尔甲烷将有 890 kJ 的热能传递到周围环境。不同燃料之间的比较正是基于它们 ΔHc° 数值的大小。
3. Measuring Energy: Calorimetry | 能量测量：量热法
water, c is its specific heat capacity (4.18 J g⁻¹ K⁻¹) and ΔT is the temperature change.
实验室中测定燃料释放焓的一个简单方法，是将少量燃料在装有水的金属罐下方燃烧，并测量水温的升高。这种技术称为量热法，水吸收的热量由 q = m c ΔT 计算，其中 m 为水的质量，c 为水的比热容 (4.18 J g⁻¹ K⁻¹)，ΔT 为温度变化。
A spirit burner containing ethanol, for instance, is weighed before and after combustion to find the mass of fuel burnt. The experimental ΔHc can then be estimated by dividing the heat absorbed by the moles of fuel consumed, with a negative sign to indicate an exothermic release.
例如，盛有乙醇的燃烧灯在燃烧前后称重，可得出消耗的燃料质量。然后用水吸收的热量除以消耗燃料的物质的量，再加上负号表示放热，即可估算出实验 ΔHc。
4. Calculations of ΔH from Calorimetry | 量热法计算焓变
e heat absorbed by the water is q = 100 g × 4.18 J g⁻¹ K⁻¹ × 32.0 K = 13 376 J (13.38 kJ).
假设 0.46 g 乙醇 (C₂H₅OH) 使 100 g 水的温度升高 32.0 °C。水吸收的热量为 q = 100 g × 4.18 J g⁻¹ K⁻¹ × 32.0 K = 13 376 J (13.38 kJ)。
46.0 g mol⁻¹, so the moles burnt = 0.46 g / 46.0 g mol⁻¹ = 0.010 mol. The experimental ΔHc ¹. This is less exothermic than the accepted value (-1 367 kJ mol⁻¹) due to heat losses, incomplete combustion and evaporation of fuel.
乙醇的摩尔质量为 46.0 g mol⁻¹，因此燃烧的物质的量为 0.46 g / 46.0 g mol⁻¹ = 0.010 mol。实验 ΔHc° = -13.38 kJ / 0.010 mol = -1 338 kJ mol⁻¹。由于热量损失、不完全燃烧和燃料蒸发，该值比理论值 (-1 367 kJ mol⁻¹) 偏正。
5. Fossil Fuels and Their Impacts | 化石燃料及其影响
Coal, crude oil and natural gas are fossil fuels formed from ancient organic matter over millions of years. Their combustion releases large amounts of energy, with coal being mainly carbon, oil a mixture of hydrocarbons, and natural gas primarily methane (CH₄).
煤、原油和天然气是历经数百万年由远古有机物形成的化石燃料。它们的燃烧释放大量能量：煤的主要成分是碳，石油是碳氢化合物的混合物，天然气则以甲烷 (CH₄) 为主。
While fossil fuels have a high energy density and are relatively cheap, burning them introduces CO₂, a greenhouse gas, and pollutants such as SO₂ and NOₓ. Additionally, they are non‑renewable, and their extraction causes environmental damage.
虽然化石燃料具有高能量密度且相对低廉，但燃烧它们会产生温室气体 CO₂，以及 SO₂ 和 NOₓ 等污染物。此外，它们不可再生，开采过程也会造成环境破坏。
6. Biofuels: Renewable Carbon Sources | 生物燃料：可再生碳源
Bioethanol, produced by fermentation of sugars from crops like sugarcane or corn, can be blended with petrol. Biodiesel is obtained from vegetable oils. They are considered renewable because the CO₂ released during combustion is offset by the CO₂ absorbed during plant growth.
生物乙醇由甘蔗或玉米等作物的糖分经发酵制得，可与汽油混合使用。生物柴油源自植物油。它们被认为是可再生的，因为燃烧释放的 CO₂ 被植物生长过程中吸收的 CO₂ 所抵消。
However, the carbon neutrality is debated when considering fertiliser production, land‑use change and processing energy. Moreover, large‑scale biofuel cultivation can compete with food supply and lead to deforestation.
但当考虑到化肥生产、土地利用变化和加工能耗时，其碳中和性存在争议。而且，大规模种植生物燃料作物会与粮食供应竞争，并可能导致森林砍伐。
7. Hydrogen and Fuel Cells | 氢与燃料电池
Hydrogen gas (H₂) can be burned directly or used in a fuel cell to produce electricity, with water as the only product. Its specific energy is about 142 MJ kg⁻¹, making it an extremely attractive clean energy carrier.
氢气 (H₂) 可直接燃烧或用于燃料电池发电，产物只有水。其比能量约为 142 MJ kg⁻¹，使其成为一种极具吸引力的清洁能源载体。
The main challenges are storage and production – hydrogen is a low‑density gas and must be compressed or liquefied, which requires energy. Most hydrogen today is produced from methane (grey hydrogen), limiting its overall environmental benefit unless green hydrogen from electrolysis using renewables is used.
主要挑战在于储存和制备——氢是一种低密度气体，必须压缩或液化，这需要消耗能量。目前大多数氢气通过甲烷制备（灰氢），除非使用可再生能源电解水得到的绿氢，否则其整体环境效益有限。
8. Comparing Fuels: Energy Density and Specific Energy | 比较燃料：能量密度与比能量
Two key metrics for evaluating fuels are specific energy (energy per unit mass, often in MJ kg⁻¹) and energy density (energy per unit volume, in MJ L⁻¹). Both are important for applications where weight or space is a constraint, such as in aviation or vehicles.
评估燃料的两个关键指标是比能量（单位质量的能量，常以 MJ kg⁻¹ 表示）和能量密度（单位体积的能量，以 MJ L⁻¹ 表示）。在重量或空间受限的应用中，如航空或车辆，这两个指标都非常重要。
| Fuel | Specific energy (MJ kg⁻¹) | Energy density (MJ L⁻¹) |
|---|---|---|
| Hydrogen | 142 | 0.01 (gas, 1 bar) |
| Natural gas (CH₄) | 55 | 0.04 (gas, 1 bar) |
| Petrol / gasoline | 46 | 34 |
| Diesel | 45 | 38 |
| Ethanol | 30 | 24 |
| Coal (bituminous) | ~24 | ~24 |
While hydrogen has an exceptional specific energy, its low volumetric energy density makes storage bulky unless heavily compressed. Liquid fuels like petrol remain convenient because they combine reasonable mass‑ and volume‑based energy values.
虽然氢具有出色的比能量，但其低体积能量密度使得储存体积庞大，除非进行高度压缩。像汽油这样的液体燃料因能兼顾质量和体积能量值而使用便捷。
9. Bond Enthalpies and Combustion Enthalpy | 键焓与燃烧焓
the form: ΔH ≈ Σ (bond enthalpies of bonds broken) − Σ (bond enthalpies of bonds formed).
平均键焓提供了一种估算燃烧反应焓变的方法。其过程遵循赫斯定律的形式：ΔH ≈ Σ (断裂键的键焓) − Σ (形成键的键焓)。
For methane, the bonds broken are 4 C– mol⁻¹ each) and 2 O=O (498 kJ mol⁻¹). Bonds formed are 2 C=O (799 kJ mol⁻¹ in CO₂) and 4 O– though differences arise because average bond enthalpies are not exact for every molecule.
对于甲烷，断裂的键有 4 个 C–H (各 413 kJ mol⁻¹) 和 2 个 O=O (498 kJ mol⁻¹)。形成的键有 2 个 C=O (在 CO₂ 中，799 kJ mol⁻¹) 和 4 个 O–H (463 kJ mol⁻¹)。估算 ΔH = [4×413 + 2×498] − [2×799 + 4×463] = -818 kJ mol⁻¹，接近实验值 -890 kJ mol⁻¹。差异源于平均键焓并非对所有分子都精确。
10. Hess's Law Applied to Fuels | 赫斯定律在燃料反应中的应用
Hess's law states that the total enthalpy change for a reaction is independent of the route taken. This principle allows us to calculate the enthalpy of combustion for a fuel if its enthalpy of formation is known, or to combine known thermochemical equations.
赫斯定律指出，一个反应的总焓变与反应路径无关。该原理使我们能够在已知生成焓的情况下计算燃料的燃烧焓，或组合已知的热化学方程式。
ΔH_combustion = Σ ΔHf°(products) − Σ ΔHf°(reactants)
For example, using standard enthalpies of formation (ΔHf l⁻¹, H₂O(l) = -286 kJ mol⁻¹, CH₄ = -75 kJ mol⁻¹), the combustion enthalpy of methane is ΔHc° = [ (-394) + 2(-286) ] − (-75) = -891 kJ mol⁻¹, consistent with the experimental value.
例如，利用标准生成焓 (CO₂ 的 ΔHf° = -394 kJ mol⁻¹, H₂O(l) = -286 kJ mol⁻¹, CH₄ = -75 kJ mol⁻¹)，甲烷的燃烧焓为 ΔHc° = [ (-394) + 2(-286) ] − (-75) = -891 kJ mol⁻¹，与实验值一致。
11. Efficiency and Incomplete Combustion | 效率与不完全燃烧
In real engines and furnaces, combustion is rarely 100% Efficiency can be defined as (useful energy output / total energy released by fuel) × 100%.
在实际发动机和炉子中，燃烧几乎不会 100% 有效。大量能量以废热的形式散失到排气和机体中。效率可定义为（有用能量输出 / 燃料释放的总能量）× 100%。
ustion of methane: 2CH₄ + 3O₂ → 2CO + 4H₂O releases less energy than the complete pathway.
不完全燃烧时，由于氧气供应不足会产生一氧化碳 (CO) 甚至碳（黑烟），而不是 CO₂，这会降低每摩尔释放的能量并产生有毒物质。例如，甲烷的不完全燃烧：2CH₄ + 3O₂ → 2CO + 4H₂O 比完全燃烧路径释放的能量少。
12. Choosing Fuels for a Sustainable Future | 为可持续未来选择燃料
Decision‑making about fuels extends beyond enthalpy values. Criteria include: renewability, lifecycle CO₂ emissions, specific energy and energy density, storage safety, cost, and infrastructure compatibility. In many scenarios, a mix of fuels is the most practical solution.
关于燃料的决策不仅仅基于焓值。评价标准还包括：可再生性、全生命周期 CO₂ 排放、比能量与能量密度、储存安全性、成本以及基础设施兼容性。在许多情况下，混合使用多种燃料是最实际的解决方案。
While hydrogen promises zero tailpipe emissions, the energy penalty of compression and the need for green production pathways must be solved. Biofuels reduce reliance on fossil reserves but raise sustainability concerns. Ultimately, a combination of improved efficiency, renewable electricity and cleaner chemical fuels will be necessary to meet global energy demands responsibly.
虽然氢有望实现零尾气排放，但压缩过程的能量损失和绿色制氢路径有待解决。生物燃料减少了对化石储量的依赖，但也引发了可持续性的问题。最终，必须结合提高效率、可再生电力以及更清洁的化学燃料，才能负责任地满足全球能源需求。
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