The general formula
The alkanes are a series of compounds, and the series has some common features. One of them is that every compound in the series can be represented by a general formula.
The alkane general formula
CnH2n+2
n is the number of carbon atoms in one molecule of the compound
Using it
Methane: n = 1, so C1H1×2+2 = CH4
Ethane: n = 2, so C2H2×2+2 = C2H6
Assignment 14.2 — worked
Derive the formulas of the alkanes with 1 to 5 carbon atoms using the general formula.
n = 1: 2n + 2 = 4, so CH4
n = 2: 2n + 2 = 6, so C2H6
n = 3: 2n + 2 = 8, so C3H8
n = 4: 2n + 2 = 10, so C4H10
n = 5: 2n + 2 = 12, so C5H12
Why 2n + 2
Picture the chain of n carbons. Every carbon has four bonds. The two carbons at the ends each use one bond on the chain, so they have three left for hydrogens; the carbons in the middle use two on the chain, so they have two left. Count it another way: each carbon brings two hydrogens (2n), and the two ends each carry one extra (+2). That gives 2n + 2 — whatever the length of the chain. Each alkane has one more CH2 than the one before it: methane CH4, ethane C2H6, propane C3H8, each step adding one carbon and two hydrogens.
Double n, then add two
A common slip is to write 2(n + 2). For propane that gives 10 hydrogens instead of 8. The formula is 2n, then + 2.
The alkane formula
Every alkane is CnH2n+2: for 1 to 5 carbons, CH4, C2H6, C3H8, C4H10, C5H12.