📚 Combustion Reactions of Organic Compounds and Their Equations | 有机物燃烧反应及方程式
Combustion is one of the most important chemical reactions of organic compounds. In IB Chemistry, you need to understand the general equations, the stoichiometry of complete and incomplete combustion, and how to balance combustion equations for alkanes, alkenes, alcohols, and other organic molecules.
燃烧是有机化合物最重要的化学反应之一。在 IB 化学中，你需要掌握完全燃烧与不完全燃烧的通式、化学计量关系，以及如何配平烷烃、烯烃、醇和其他有机分子的燃烧方程式。
1. What Is Combustion? | 什么是燃烧
Combustion is an exothermic redox reaction between a substance and an oxidant, usually oxygen. For organic compounds, combustion typically produces carbon dioxide and water when complete. The reaction releases large amounts of energy, which is why organic fuels are widely used.
燃烧是物质与氧化剂（通常为氧气）之间发生的放热氧化还原反应。对于有机化合物，完全燃烧通常生成二氧化碳和水。该反应释放大量能量，因此有机燃料被广泛使用。
In a combustion reaction, the organic compound is oxidised: carbon increases its oxidation state to +4 in CO₂, and hydrogen goes to +1 in H₂O. Oxygen is reduced from 0 in O₂ to −2 in the products.
在燃烧反应中，有机化合物被氧化：碳的氧化态升高到 CO₂ 中的 +4，氢变为 H₂O 中的 +1。氧气从 O₂ 中的 0 被还原为产物中的 −2。
2. Complete Combustion of Hydrocarbons | 烃的完全燃烧
Complete combustion occurs when there is sufficient oxygen. For a hydrocarbon with the general formula CₓHᵧ, the balanced equation is:
完全燃烧发生在氧气充足时。对于通式为 CₓHᵧ 的烃，配平方程式为：
CₓHᵧ + (x + y/4) O₂ → x CO₂ + (y/2) H₂O
For example, the complete combustion of methane is:
例如，甲烷的完全燃烧为：
CH₄ + 2 O₂ → CO₂ + 2 H₂O
For ethane:
乙烷的完全燃烧为：
2 C₂H₆ + 7 O₂ → 4 CO₂ + 6 H₂O
The products are always CO₂ and H₂O, and the equation can be balanced by adjusting coefficients.
产物始终是 CO₂ 和 H₂O，通过调整系数即可配平。
3. Incomplete Combustion | 不完全燃烧
When oxygen is limited, hydrocarbons burn incompletely, producing carbon monoxide (CO), carbon (soot, C), or both, along with water. Incomplete combustion releases less energy per mole of fuel and produces hazardous products.
当氧气不足时，烃发生不完全燃烧，生成一氧化碳（CO）、碳（烟灰，C）或两者，同时生成水。不完全燃烧每摩尔燃料释放的能量较少，且产生有害产物。
A common example is incomplete combustion of methane:
甲烷不完全燃烧的常见例子：
2 CH₄ + 3 O₂ → 2 CO + 4 H₂O
Or with soot formation:
或生成烟灰：
CH₄ + O₂ → C + 2 H₂O
In IB exam questions, you must specify complete or incomplete combustion because the products and enthalpies differ.
在 IB 考试题中，你必须指明完全燃烧还是不完全燃烧，因为产物和焓变不同。
4. Balancing Combustion Equations by Inspection | 用观察法配平燃烧方程式
The standard method is: balance carbon first, then hydrogen, then oxygen. For example, balance C₃H₈ (propane) combustion:
标准方法是：先配平碳，再配平氢，最后配平氧。例如，配平 C₃H₈（丙烷）的燃烧：
Step 1 – Balance carbon: 1 C₃H₈ → 3 CO₂. Step 2 – Balance hydrogen: 1 C₃H₈ → 4 H₂O. Step 3 – Count oxygen atoms on the right: 3×2 + 4 = 10 O atoms, so 5 O₂ molecules are needed.
第一步——配平碳：1 C₃H₈ → 3 CO₂。第二步——配平氢：1 C₃H₈ → 4 H₂O。第三步——统计右边氧原子数：3×2 + 4 = 10 个 O 原子，因此需要 5 个 O₂ 分子。
C₃H₈ + 5 O₂ → 3 CO₂ + 4 H₂O
For larger molecules, fractional coefficients may appear when balancing oxygen; multiply the entire equation by 2 to get whole numbers. Example: C₆H₁₄ combustion:
对于较大的分子，配平氧时可能出现分数系数；将整个方程式乘以 2 以得到整数。例如 C₆H₁₄ 的燃烧：
2 C₆H₁₄ + 19 O₂ → 12 CO₂ + 14 H₂O
5. Combustion of Alkanes | 烷烃的燃烧
Alkanes are saturated hydrocarbons (CₙH₂ₙ₊₂). Their complete combustion follows a simple pattern. Butane combustion is commonly used as an example:
烷烃是饱和烃（CₙH₂ₙ₊₂）。其完全燃烧遵循简单的规律。丁烷燃烧是常用的例子：
2 C₄H₁₀ + 13 O₂ → 8 CO₂ + 10 H₂O
General equation for alkane complete combustion:
烷烃完全燃烧通式：
CₙH₂ₙ₊₂ + (3n+1)/2 O₂ → n CO₂ + (n+1) H₂O
For n = 1 (methane), oxygen coefficient is (3+1)/2 = 2. For n = 4, coefficient is (12+1)/2 = 6.5, so double: 2 C₄H₁₀ + 13 O₂.
当 n = 1（甲烷）时，氧气系数为 (3+1)/2 = 2。当 n = 4 时，系数为 (12+1)/2 = 6.5，因此乘以 2：2 C₄H₁₀ + 13 O₂。
6. Combustion of Alkenes | 烯烃的燃烧
Alkenes are unsaturated hydrocarbons with the general formula CₙH₂ₙ. They burn with a more sooty flame than alkanes because of their higher carbon-to-hydrogen ratio. The complete combustion equation is:
烯烃是不饱和烃，通式为 CₙH₂ₙ。由于碳氢比更高，它们燃烧时比烷烃产生更多烟灰。完全燃烧方程式为：
CₙH₂ₙ + (3n/2) O₂ → n CO₂ + n H₂O
For ethene:
乙烯的燃烧：
C₂H₄ + 3 O₂ → 2 CO₂ + 2 H₂O
For propene:
丙烯的燃烧：
2 C₃H₆ + 9 O₂ → 6 CO₂ + 6 H₂O
7. Combustion of Alcohols | 醇的燃烧
Alcohols contain an –OH group. Their general formula is CₙH₂ₙ₊₁OH or CₙH₂ₙ₊₂O. Complete combustion produces CO₂ and H₂O. The oxygen in the alcohol is included when balancing.
醇含有 –OH 基团。其通式为 CₙH₂ₙ₊₁OH 或 CₙH₂ₙ₊₂O。完全燃烧产生 CO₂ 和 H₂O。配平时需计入醇中的氧。
For methanol:
甲醇的燃烧：
2 CH₃OH + 3 O₂ → 2 CO₂ + 4 H₂O
For ethanol:
乙醇的燃烧：
C₂H₅OH + 3 O₂ → 2 CO₂ + 3 H₂O
Notice that ethanol has the same ratio as ethene combustion, but with one molecule less O₂ because ethanol already contains an oxygen atom.
注意乙醇与乙烯燃烧的产物比例相似，但乙醇少消耗 1 个 O₂，因为乙醇本身已含有一个氧原子。
8. Combustion of Other Oxygen-Containing Compounds | 其他含氧化合物的燃烧
For any organic compound containing C, H, and O, the combustion equation can be balanced by treating the formula as CₐHᵦO꜀. The complete combustion equation is:
对于任何含 C、H 和 O 的有机化合物，可将分子式视为 CₐHᵦO꜀ 来配平燃烧方程式：
CₐHᵦO꜀ + (a + β/4 − γ/2) O₂ → a CO₂ + (β/2) H₂O
Example: glucose, C₆H₁₂O₆. Here a = 6, β = 12, γ = 6. Oxygen coefficient = 6 + 3 − 3 = 6.
例如葡萄糖 C₆H₁₂O₆。其中 a = 6，β = 12，γ = 6。氧系数 = 6 + 3 − 3 = 6。
C₆H₁₂O₆ + 6 O₂ → 6 CO₂ + 6 H₂O
For ethanoic acid, CH₃COOH (C₂H₄O₂): coefficient = 2 + 1 − 1 = 2.
对于乙酸 CH₃COOH（C₂H₄O₂）：系数 = 2 + 1 − 1 = 2。
CH₃COOH + 2 O₂ → 2 CO₂ + 2 H₂O
9. O₂ Requirement and Air-to-Fuel Ratio | 氧气需求与空燃比
The volume of oxygen needed for complete combustion can be calculated using stoichiometry. For example, at STP, 1 mole of any gas occupies 22.7 dm³ in IB (or 22.4 dm³ at STP in some curricula). Air is approximately 21% oxygen by volume.
完全燃烧所需的氧气体积可以用化学计量关系计算。例如，在标准状况下，1 摩尔任何气体在 IB 中占 22.7 dm³（某些课程在 STP 下为 22.4 dm³）。空气中氧气体积占比约为 21%。
Example: What volume of air is required to completely combust 50 dm³ of methane? CH₄ + 2 O₂ → CO₂ + 2 H₂O. Volume ratio of CH₄ to O₂ is 1:2, so O₂ needed = 100 dm³. Air volume = 100 / 0.21 ≈ 476 dm³.
示例：完全燃烧 50 dm³ 甲烷需要多少体积的空气？CH₄ + 2 O₂ → CO₂ + 2 H₂O。CH₄ 与 O₂ 的体积比为 1:2，因此需要 O₂ 100 dm³。空气体积 = 100 / 0.21 ≈ 476 dm³。
10. Heat of Combustion and Enthalpy Changes | 燃烧热与焓变
Combustion is exothermic, so ΔHc is negative. The enthalpy of combustion is defined as the enthalpy change when one mole of a substance is completely burned in excess oxygen under standard conditions. In IB, experimental determination uses a calorimeter, and bond enthalpy calculations can estimate ΔH.
燃烧是放热的，因此 ΔHc 为负值。燃烧焓定义为一摩尔物质在标准条件下于过量氧气中完全燃烧时的焓变。在 IB 中，可通过量热仪实验测定，也可用键焓估算 ΔH。
For ethanol, the enthalpy of combustion is approximately −1367 kJ mol⁻¹:
乙醇的燃烧焓约为 −1367 kJ mol⁻¹：
C₂H₅OH(l) + 3 O₂(g) → 2 CO₂(g) + 3 H₂O(g), ΔH = −1367 kJ mol⁻¹
Note that state symbols matter: heat of vaporisation of water changes the value if H₂O is liquid instead of gas.
注意状态符号很重要：若产物 H₂O 为液态而非气态，水的汽化热会影响数值。
11. Comparing Sootiness of Flames | 火焰黑烟程度比较
The nature of the flame depends on the percentage of carbon in the compound. A higher C:H ratio tends to produce a more yellow, sooty flame due to incomplete combustion of carbon particles. Alkenes and alkynes burn with sootier flames than alkanes of similar molar mass.
火焰的性质取决于化合物中碳的百分含量。较高的 C:H 比往往产生更黄、更黑烟的火焰，这是因为碳颗粒不完全燃烧。摩尔质量相近时，烯烃和炔烃的火焰比烷烃更黑烟。
| Compound 化合物 | Formula 分子式 | C:H ratio 碳氢比 | Flame type 火焰类型 |
|---|---|---|---|
| Methane 甲烷 | CH₄ | 1:4 | Blue, clean 蓝色，清洁 |
| Ethene 乙烯 | C₂H₄ | 1:2 | Yellow, sooty 黄色，黑烟 |
| Ethyne 乙炔 | C₂H₂ | 1:1 | Very sooty 黑烟很浓 |
12. Common Mistakes in IB Exams | IB 考试中的常见错误
Students often forget to balance oxygen correctly, especially when the organic molecule contains oxygen. Another common mistake is writing CO or C as a product without specifying incomplete combustion. Also, using fractional coefficients when the question requests whole numbers leads to lost marks.
学生经常在配平氧时出错，特别是当有机分子中含有氧时。另一个常见错误是未指明不完全燃烧就直接写出 CO 或 C 作为产物。此外，当题目要求整数系数时使用分数系数也会失分。
Remember to always include state symbols in enthalpy questions, and to check that the total number of atoms of each element is equal on both sides. Practice writing balanced equations for methane, ethane, ethene, ethanol, and glucose.
记住在焓变相关题目中始终包含状态符号，并检查每种元素在两侧的原子总数相等。练习书写甲烷、乙烷、乙烯、乙醇和葡萄糖的配平方程式。
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