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Alkanes are saturated hydrocarbons that contain only single bonds between carbon atoms, with the general formula CnH2n+2. Free GCSE Chemistry notes.
Understanding alkanes as saturated hydrocarbons is part of Reactions of alkenes and alcohols (chemistry only) in GCSE Edexcel Higher Chemistry.
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Alkanes are saturated hydrocarbons that contain only single bonds between carbon atoms, with the general formula CnH2n+2. Free GCSE Chemistry notes.
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Use this lesson to stay inside Reactions of alkenes and alcohols (chemistry only) while connecting the detail back to the wider Topic 9: Separate chemistry 2 topic.
Read the core explanations from Understanding alkanes as saturated hydrocarbons before testing yourself from memory.
When you fill up a car with petrol or heat your home with natural gas, you're using alkanes. These simple molecules make up most of crude oil and are the backbone of our energy system. But what exactly are they?
The key word here is 'saturated' - this means every carbon atom forms four single bonds, with no double or triple bonds anywhere. Think of it like a carbon skeleton that's completely filled up with hydrogen atoms.
**Trap**: Many students think 'saturated' means 'dissolved in water'. Actually, it means the carbon chain is saturated with hydrogen - no more can be added without breaking bonds.
Alkanes form what we call a homologous series. Each member has the same general formula but differs by exactly one CH2 group. Let's meet the first four members of this family.
Methane (CH4): The simplest alkane with just one carbon atom. This is natural gas - what heats many homes and powers gas hobs.
Ethane (C2H6): Two carbon atoms joined together. Found in natural gas mixtures and used to make plastics.
Propane (C3H8): Three carbons in a chain. This is the gas in camping stoves and BBQ cylinders.
Butane (C4H10): Four carbons long. Found in lighter fuel and some aerosols.
Notice the pattern? Each time we add one carbon, we also add two hydrogens. This is where the general formula comes from.
Every alkane follows the same pattern. Once you know this general formula, you can work out the molecular formula for any alkane.
Worked example: Using the General Formula
What is the molecular formula for the alkane with 5 carbon atoms?
Answer: C5H12
This formula works because of how carbon bonds. Each carbon forms exactly four bonds - in a chain, the end carbons bond to three hydrogens each, while middle carbons bond to two hydrogens each.
Crude oil formed millions of years ago from dead sea creatures and plants. Under high pressure and temperature, the complex organic molecules broke down into simpler, more stable compounds - mainly alkanes.
Alkanes are chemically stable because all their bonds are single bonds. This stability meant they survived the geological processes that destroyed other types of molecules. That's why when we drill for oil, we find mostly alkanes of different chain lengths.
The different alkanes in crude oil have different boiling points, which is why we can separate them by fractional distillation. Shorter alkanes like methane and ethane boil at low temperatures, while longer chains like those in diesel and fuel oil need much higher temperatures.
As alkanes get longer, their properties change in predictable ways. This is crucial for understanding how crude oil is processed and used.
Boiling Point: Increases as the chain gets longer. Methane boils at -162°C (gas at room temperature), while longer alkanes like those in petrol are liquids.
Viscosity: Longer alkanes are more viscous (thicker). Methane flows like air, petrol flows easily, while heavy fuel oil is quite thick.
Flammability: Shorter alkanes ignite more easily. That's why gas (methane) lights instantly, but diesel needs compression ignition.
Alkane questions often test your ability to use the general formula and recognise patterns. Here's how to nail these questions every time.
Common Exam Mistakes: Don't confuse alkanes (CnH2n+2) with alkenes (CnH2n). Don't say alkanes are 'unsaturated' - they're saturated. Don't forget that crude oil contains mainly alkanes, not all hydrocarbons.
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Move to the related lessons or back to the topic page when you need the wider sequence before exam-style practice.
Crude oil formed millions of years ago from dead sea creatures and plants. Under high pressure and temperature, the complex organic molecules broke down into simpler, more stable compounds, mainly alkanes.
The general formula of alkanes is CnH2n+2, where n is the number of carbon atoms. For an alkane with 5 carbon atoms, 2(5) + 2 = 12, so the formula is C5H12, which is pentane.
Alkanes are saturated hydrocarbons that contain only single bonds between carbon atoms, with the general formula CnH2n+2. They make up most of crude oil and include methane, ethane, propane and butane.
Crude oil is mainly alkanes because alkanes are chemically stable, as all their bonds are single bonds. When dead sea creatures and plants broke down under high pressure and temperature over millions of years, the alkanes survived the geological processes that destroyed other molecules.
Alkanes are chemically stable because all their bonds are single bonds. This stability meant they survived the geological processes that destroyed other types of molecules, which is why crude oil is mostly alkanes.
Alkanes in crude oil are separated by fractional distillation, because alkanes of different chain lengths have different boiling points. Shorter alkanes like methane and ethane boil at low temperatures, while longer chains need much higher temperatures.
Each alkane in the homologous series differs from the next by exactly one CH₂ group. Adding one carbon also adds two hydrogens, which is where the general formula CnH2n+2 comes from.