In simple terms
A friendly intro before the formal notes — no formulas yet.
Alkanes: Building Blocks and Reactions
Alkanes are simple, saturated hydrocarbons that form the backbone of fuels. They react by breaking their strong bonds, either through combustion with oxygen or substitution with halogens.
Imagine alkanes are like a basic chain of paperclips (carbon atoms) with smaller clips attached (hydrogen atoms). You can't easily add more clips to the chain without first removing one (substitution), but you can burn the whole chain (combustion) or snap it into smaller pieces (cracking).
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Alkanes: CₙH₂ₙ₊₂ — only C–C and C–H σ bonds; saturated. | Sim hint: Build methane CH₄ — tetrahedral sp³ carbon.
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Free-radical substitution: initiation → propagation → termination. | Sim hint: Replace one H on ethane — note single substitution product.
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Combustion: complete → CO₂ + H₂O; incomplete → CO or C. | Sim hint: Count C and H in alkane for O₂ needed in balanced equation.
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Cracking: long alkane → shorter alkane + alkene (C=C). | Sim hint: Compare saturated vs unsaturated product structures.
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Key formulas
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Full topic notes
Formal explanation with the rigour you need for the exam.
Structure and Bonding in Alkanes
Alkanes are described by the general formula . Each carbon atom in an alkane is hybridised, forming four single covalent bonds. These bonds are arranged in a tetrahedral geometry around the carbon atom, with bond angles of approximately 109.5°. This arrangement maximises the distance between electron pairs, minimising repulsion according to VSEPR theory. The bonds are all (sigma) bonds, formed by the head-on overlap of orbitals, which are strong and not easily broken.
General Formula:
Bonding: Only C-C and C-H single covalent bonds ( bonds).
Hybridisation: All carbons are hybridised.
Geometry: Tetrahedral shape around each carbon.
Bond Angle: 109.5°.
Combustion of Alkanes
Combustion is the most important reaction of alkanes, releasing large amounts of energy, which is why they are excellent fuels. The type of combustion depends on the amount of available oxygen. Complete combustion occurs in a plentiful supply of oxygen, producing only carbon dioxide and water. Incomplete combustion occurs in a limited oxygen supply and produces toxic carbon monoxide and/or solid carbon (soot), alongside water.
Complete Combustion:
Free-Radical Substitution
Due to the strength and low polarity of C-C and C-H bonds, alkanes are generally unreactive. However, in the presence of ultraviolet (UV) light, they can react with halogens (like chlorine or bromine) via a free-radical substitution mechanism. This is a chain reaction that proceeds in three distinct stages: initiation, propagation, and termination.
Initiation: UV light provides energy for homolytic fission of the halogen molecule () to form two highly reactive halogen radicals (). Example:
Propagation: A chain reaction occurs. A halogen radical reacts with an alkane molecule, creating an alkyl radical and a hydrogen halide. The alkyl radical then reacts with another halogen molecule, regenerating the halogen radical. Example: ; then
Termination: The reaction stops when two radicals collide and combine to form a stable molecule, removing radicals from the system. Example: ; or ; or
In exams, you must be precise. State that UV light is needed for initiation. Use dots (•) to clearly show the unpaired electron on free radicals. Remember that propagation involves one radical on each side of the equation, while termination involves two radicals reacting to form zero radicals.
Cracking of Alkanes
Crude oil contains a large proportion of long-chain alkanes, which are not very useful and have low economic value. Cracking is a process that breaks these large hydrocarbon molecules into smaller, more useful ones by breaking C-C bonds. The products are always a smaller alkane and at least one alkene. There are two main types: thermal cracking (high temperature and pressure) and catalytic cracking (high temperature, slight pressure, and a zeolite catalyst).
Purpose: To convert long-chain alkanes into high-demand shorter-chain alkanes (for petrol) and alkenes (for polymers).
Conditions (Thermal): High temperature (e.g., 700-1200 K) and high pressure (e.g., 7000 kPa). Produces a high proportion of alkenes.
Conditions (Catalytic): High temperature (e.g., 900 K), low pressure, and a zeolite catalyst (aluminosilicates). Produces aromatic hydrocarbons and branched alkanes, useful for high-octane petrol.
Example Equation: (Decane cracks to Octane and Ethene).
Worked examples
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Butane () undergoes complete combustion. (a) Write a balanced chemical equation for this reaction. (b) Calculate the volume of carbon dioxide produced, measured at room temperature and pressure (RTP), when 120 cm³ of butane gas is completely combusted.
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(a) First, balance the carbons, then hydrogens, then oxygens. Balance C: Balance H: Balance O: There are (4 × 2) + 5 = 13 oxygen atoms on the right. So we need 13/2 . To use whole numbers:
Propane reacts with bromine in the presence of UV light. Write equations for the mechanism to form 1-bromopropane (). Include one initiation step, two propagation steps, and one possible termination step.
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Initiation: The Br-Br bond is broken by UV light to form two bromine radicals.
How it all connects
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Glossary
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Quick check
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Revision flashcards
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What is an alkane?
A saturated hydrocarbon containing only single C-C and C-H bonds.
Key takeaways
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General Formula:
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Bonding: Only C-C and C-H single covalent bonds ( bonds).
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Hybridisation: All carbons are hybridised.
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Geometry: Tetrahedral shape around each carbon.
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Bond Angle: 109.5°.
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