Hydrocarbons and Homologous Series
Simple Explanation
A hydrocarbon is a compound made of only carbon and hydrogen. A homologous series is a family of compounds (like the alkanes or alkenes) that share the same general formula and similar chemical properties, differing only by the number of CH₂ units in the chain.
Why Do We Need It?
Recognizing a homologous series lets you predict the formula, and even roughly predict the properties, of ANY member of that family just from knowing the general pattern — you don't need to memorize every single compound individually.
See It
A ball-and-stick diagram of methane, showing one central carbon atom bonded to four hydrogen atoms
Formula
General Formula of Alkanes
CₙH₂ₙ₊₂
Gives the molecular formula of any alkane (a saturated hydrocarbon with only single C–C bonds) directly from its number of carbon atoms.
- n
- — the number of carbon atoms in the molecule
- CₙH₂ₙ₊₂
- — the resulting molecular formula — n carbon atoms and (2n + 2) hydrogen atoms
When to use it: Whenever you know how many carbon atoms an alkane has and need its full molecular formula, or vice versa.
Worked Example
Find the molecular formula of an alkane
Find the molecular formula of the alkane with 6 carbon atoms (hexane).
Why Does This Work?
Every alkane has the same basic structure — a chain of carbons each bonded to as many hydrogens as possible (fully "saturated") — so adding one more carbon to the chain always adds exactly one more carbon and two more hydrogens (a CH₂ unit), which is exactly what the general formula CₙH₂ₙ₊₂ captures.
Real-Life Example
Methane, propane, and butane as fuels
Methane (natural gas), propane (gas grills), and butane (lighters) are all alkanes with different chain lengths.
Methane (CH₄, n=1), propane (C₃H₈, n=3), and butane (C₄H₁₀, n=4) are all members of the same homologous series — their formulas all fit CₙH₂ₙ₊₂, and their fuel properties change predictably as chain length increases.
Practice
What is the molecular formula of the alkane with 8 carbon atoms (octane)?
MediumCommon mistake
Confusing the alkane general formula (CₙH₂ₙ₊₂) with the alkene general formula (CₙH₂ₙ) — alkenes have one fewer pair of hydrogens per carbon count because a C=C double bond uses up bonding positions that would otherwise hold hydrogen atoms.
Quick Review
- A hydrocarbon contains only carbon and hydrogen.
- A homologous series shares a general formula and similar properties.
- Alkanes: CₙH₂ₙ₊₂. Alkenes: CₙH₂ₙ.