GCSE AQA Chemistry: Alkanes | GCSE AQA 化学：烷烃考点精讲

📚 GCSE AQA Chemistry: Alkanes | GCSE AQA 化学：烷烃考点精讲

Alkanes are a fundamental class of organic compounds that form the basis of fuels like natural gas, petrol and diesel. In GCSE AQA Chemistry, understanding alkanes involves learning their general formula, naming, structure, physical properties, and reactions – particularly combustion. This revision guide covers all the essential points you need to know for your exam.

烷烃是构成天然气、汽油和柴油等燃料基础的一类基本有机化合物。在 GCSE AQA 化学中，理解烷烃需要学习它们的通式、命名、结构、物理性质和反应——尤其是燃烧反应。本复习指南涵盖了你需要掌握的全部考试要点。

1. What Are Alkanes? | 什么是烷烃？

Alkanes are saturated hydrocarbons. This means they contain only carbon and hydrogen atoms, with all carbon–carbon bonds being single covalent bonds. The term ‘saturated’ indicates that no more hydrogen atoms can be added to the molecule, as each carbon atom has already formed four single bonds. They are obtained mainly from crude oil through fractional distillation.

烷烃是饱和烃。这意味着它们只含有碳和氢原子，所有的碳-碳键都是单共价键。“饱和”一词表示不能再向分子中加入更多的氢原子，因为每个碳原子已经形成了四个单键。它们主要通过分馏从原油中获得。

2. Homologous Series and General Formula | 同系物与通式

Alkanes form a homologous series. A homologous series is a family of organic compounds with the same functional group, similar chemical properties, and a gradual change in physical properties. Each successive member differs by a CH₂ unit. The general formula for alkanes is CₙH₂ₙ₊₂, where n is the number of carbon atoms. For the first four alkanes:

Methane: CH₄ (n=1)

Ethane: C₂H₆ (n=2)

Propane: C₃H₈ (n=3)

Butane: C₄H₁₀ (n=4)

烷烃组成一个同系物系列。同系物是一类具有相同官能团、相似化学性质且物理性质逐渐变化的有机化合物。每个相邻成员间相差一个 CH₂ 单元。烷烃的通式为 CₙH₂ₙ₊₂，其中 n 为碳原子数。对于前四种烷烃：

甲烷：CH₄ (n=1)

乙烷：C₂H₆ (n=2)

丙烷：C₃H₈ (n=3)

丁烷：C₄H₁₀ (n=4)

3. Naming of Alkanes | 烷烃的命名

Alkanes are named using a systematic prefix that indicates the number of carbon atoms, followed by the suffix ‘-ane’. The first four prefixes are meth- (1C), eth- (2C), prop- (3C) and but- (4C). This naming continues with pent- (5C), hex- (6C), etc. These rules are essential for identifying and communicating about organic molecules.

烷烃的命名采用表示碳原子数的系统前缀，后接词尾“-ane”。前四个前缀为 meth-（1个碳）、eth-（2个碳）、prop-（3个碳）和 but-（4个碳）。这种命名法延续到 pent-（5个碳）、hex-（6个碳）等。这些规则对于识别和交流有机分子至关重要。

4. Structural and Displayed Formulas | 结构式与显示式

In GCSE Chemistry, you need to be able to represent alkanes using molecular formulas, structural formulas and displayed formulas. The molecular formula simply counts atoms, e.g. C₂H₆. The structural formula shows how atoms are connected, e.g. CH₃–CH₃. The displayed formula shows every atom and every bond, often drawn with sticks for bonds. For methane, the displayed formula shows a central carbon atom with four single bonds to four hydrogen atoms, arranged tetrahedrally.

在 GCSE 化学中，你需要能够用分子式、结构式和显示式表示烷烃。分子式只计算原子个数，例如 C₂H₆。结构式显示原子如何连接，例如 CH₃–CH₃。显示式则画出所有原子和所有键，通常用短线表示键。对于甲烷，其显示式呈现一个中心碳原子通过四个单键连接四个氢原子，呈四面体排列。

5. Isomerism in Alkanes: Butane and Methylpropane | 烷烃的同分异构：丁烷与甲基丙烷

Isomers are molecules that have the same molecular formula but different structural formulas. Butane (C₄H₁₀) has two isomers: straight-chain butane (CH₃–CH₂–CH₂–CH₃) and branched-chain methylpropane (CH₃–CH(CH₃)–CH₃). The different structures lead to different physical properties; for example, methylpropane has a lower boiling point (-11.7 °C) than butane (-0.5 °C) due to weaker intermolecular forces in the branched shape.

同分异构体是具有相同分子式但不同结构式的分子。丁烷（C₄H₁₀）有两种同分异构体：直链丁烷（CH₃–CH₂–CH₂–CH₃）和支链甲基丙烷（CH₃–CH(CH₃)–CH₃）。不同的结构导致不同的物理性质；例如，甲基丙烷的沸点（-11.7 °C）低于丁烷（-0.5 °C），因为支链形状使分子间作用力较弱。

6. Trends in Physical Properties | 物理性质的变化趋势

As the carbon chain length increases in the alkane homologous series, several physical properties change noticeably. Boiling points and viscosity increase, while flammability decreases. This is because larger molecules have stronger intermolecular forces (van der Waals forces), requiring more energy to separate them. The shorter alkanes (methane to butane) are gases at room temperature, while longer ones are liquids or waxy solids. This trend is exploited during fractional distillation to separate crude oil into useful fractions.

随着烷烃同系物中碳链长度的增加，多项物理性质发生明显变化。沸点和粘度增大，而可燃性降低。这是因为较大的分子具有更强的分子间作用力（范德华力），需要更多能量才能将它们分开。较短的烷烃（甲烷至丁烷）在室温下为气体，而较长的则为液体或蜡状固体。这一趋势在分馏过程中被利用，将原油分离成有用的馏分。

7. Complete Combustion of Alkanes | 烷烃的完全燃烧

Alkanes burn in a plentiful supply of oxygen to produce carbon dioxide and water vapour – this is complete combustion. The reaction is highly exothermic, which is why alkanes are excellent fuels. A balanced equation for the complete combustion of methane is:

烷烃在充足的氧气中燃烧，生成二氧化碳和水蒸气——这就是完全燃烧。该反应高度放热，因此烷烃是优良的燃料。甲烷完全燃烧的配平方程式为：

CH₄ + 2O₂ → CO₂ + 2H₂O

You can write similar equations for any alkane once you know the number of carbon atoms. The general formula for complete combustion is: CₙH₂ₙ₊₂ + (1.5n+0.5)O₂ → nCO₂ + (n+1)H₂O, but you do not need to recall this for GCSE – balancing with the given formula is sufficient.

一旦知道碳原子数，你就可以为任何烷烃写出类似的方程式。完全燃烧的通式是：CₙH₂ₙ₊₂ + (1.5n+0.5)O₂ → nCO₂ + (n+1)H₂O，但在 GCSE 阶段你不必记住此式——根据给定分子式进行配平即可。

8. Incomplete Combustion and Its Hazards | 不完全燃烧及其危害

In a limited supply of oxygen, alkanes undergo incomplete combustion. Instead of producing only carbon dioxide and water, the products can include poisonous carbon monoxide (CO) and solid carbon particles (soot). For example, incomplete combustion of methane may be represented as:

在氧气供应有限的条件下，烷烃会发生不完全燃烧。产物不只是二氧化碳和水，还可能包括有毒的一氧化碳（CO）和固态碳微粒（烟灰）。例如，甲烷的不完全燃烧可表示为：

2CH₄ + 3O₂ → 2CO + 4H₂O

Carbon monoxide is dangerous because it binds irreversibly to haemoglobin in red blood cells, reducing the blood’s ability to carry oxygen. This can lead to fainting, brain damage or death. Soot particles cause respiratory problems and can blacken buildings. Appliances that burn alkanes, such as gas boilers, must be regularly serviced to ensure complete combustion and proper ventilation.

一氧化碳很危险，因为它与红细胞中的血红蛋白不可逆地结合，降低血液携带氧气的能力。这可能导致昏厥、脑损伤甚至死亡。烟灰微粒会引起呼吸系统问题，并熏黑建筑物。使用烷烃的器具，如燃气锅炉，必须定期检修以确保完全燃烧和良好通风。

9. Testing for Products of Combustion | 燃烧产物的检验

In the lab, you can test for the products of alkane combustion using simple chemical tests. Water vapour can be detected by passing the gas over anhydrous copper(II) sulfate, which turns from white to blue in the presence of water. Carbon dioxide gas turns limewater (calcium hydroxide solution) cloudy or milky, due to the formation of insoluble calcium carbonate. These tests can confirm that complete combustion has occurred.

在实验室中，你可以用简单的化学检验来检测烷烃燃烧的产物。将气体通过无水硫酸铜（II），遇到水会从白色变为蓝色，以此检测水蒸气。二氧化碳气体能使石灰水（氢氧化钙溶液）变浑浊或呈乳白色，这是因为生成了不溶于水的碳酸钙。这些检验可确认完全燃烧的发生。

10. Alkanes as Fuels – Source from Crude Oil | 作为燃料的烷烃 – 原油来源

Alkanes are primarily obtained from crude oil, a finite fossil fuel. Crude oil is separated into fractions by fractional distillation, where the mixture is heated and different alkanes vaporise and condense according to their boiling points. Short-chain alkanes like methane and ethane condense near the top of the column (lower boiling points), while longer-chain alkanes like those in diesel condense lower down. These fractions are then used as fuels: natural gas (mainly methane), LPG (propane and butane), petrol, kerosene, diesel, and fuel oil.

烷烃主要从原油（一种有限化石燃料）中获取。原油通过分馏分离成不同馏分：加热混合物，不同烷烃根据各自的沸点蒸发并在分馏塔中冷凝。短链烷烃如甲烷和乙烷在塔顶附近冷凝（沸点较低），而较长的烷烃如柴油中的烃则在较低处冷凝。这些馏分随后用作燃料：天然气（主要为甲烷）、液化石油气LPG（丙烷和丁烷）、汽油、煤油、柴油和燃料油。

11. Environmental Considerations of Using Alkanes | 使用烷烃的环境考量

Burning alkane fuels releases carbon dioxide, a greenhouse gas that contributes to global warming. Incomplete combustion adds toxic carbon monoxide and soot to the atmosphere. While pure alkanes contain no sulfur, crude oil often contains sulfur impurities; when burned, these produce sulfur dioxide, leading to acid rain. To reduce environmental impact, vehicles use catalytic converters, fuels undergo desulfurisation, and renewable energy sources are being developed. GCSE questions often link alkane combustion to these environmental issues.

燃烧烷烃燃料释放二氧化碳，这是一种导致全球变暖的温室气体。不完全燃烧会将有毒的一氧化碳和烟灰带入大气。虽然纯烷烃不含硫，但原油常含有硫杂质；燃烧时这些杂质会产生二氧化硫，导致酸雨。为了减少环境影响，车辆使用催化转化器，燃料进行脱硫处理，并且正在开发可再生能源。GCSE 考题常常将烷烃燃烧与这些环境问题联系起来。

12. Summary of Key Points | 关键点总结

Alkanes are saturated hydrocarbons with single bonds and the general formula CₙH₂ₙ₊₂. They form a homologous series with predictable trends in physical properties. Complete combustion produces CO₂ and H₂O, while incomplete combustion produces CO and soot. Their main source is crude oil, and their use as fuels has both benefits and environmental drawbacks. Make sure you can name, draw and write balanced combustion equations for the first four alkanes, and link their properties to their uses.

烷烃是具有单键的饱和烃，通式为 CₙH₂ₙ₊₂。它们构成一个同系物系列，物理性质有可预测的变化趋势。完全燃烧生成 CO₂ 和 H₂O，而不完全燃烧生成 CO 和烟灰。它们的主要来源是原油，作为燃料既有益处也有环境弊端。务必确保你能够对前四种烷烃进行命名、画图并书写配平的燃烧方程式，并将其性质与用途联系起来。

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GCSE AQA Chemistry: Alkanes | GCSE AQA 化学：烷烃 考点精讲