Crude oil, hydrocarbons and alkanes: C₂H₆ or

The idea

C₂H₆ is the molecular formula of ethane, the two-carbon alkane. It tells you there are two carbon atoms and six hydrogen atoms, but not how they are joined. Ethane is saturated: the two carbons share a single covalent bond, and each carbon also bonds to three hydrogen atoms, giving each carbon four single bonds. The same molecule can be shown as a displayed formula with all atoms and bonds drawn, or as the condensed structural formula CH₃CH₃. Checking against the general formula CₙH₂ₙ₊₂ with n = 2 gives C₂H₆, confirming ethane is an alkane. Recognising C₂H₆ as ethane, and linking the formula to its structure, is the key skill.

What earns the marks

• State that C₂H₆ is the molecular formula of ethane and contains two carbon atoms and six hydrogen atoms.
• Describe the bonding in ethane as two carbon atoms joined by a single covalent bond, with each carbon also bonded to three hydrogen atoms.
• Explain that ethane is saturated because all its carbon–carbon and carbon–hydrogen bonds are single bonds.
• Check that C₂H₆ fits the alkane general formula CₙH₂ₙ₊₂ when n = 2.
• Write the condensed structural formula CH₃CH₃ or draw the displayed formula for ethane.

Common mistakes and how to put them right

• Reading C₂H₆ as two carbon atoms and six separate hydrogen molecules.

  Put it right: The subscript 6 refers to six hydrogen atoms within the molecule.

• Drawing a double bond between the carbons in ethane.

  Put it right: Ethane is saturated, so the C–C bond is single.

• Confusing C₂H₆ with C₂H₄, which is ethene and has a carbon–carbon double bond.

  Put it right: Check the hydrogen count against CₙH₂ₙ₊₂.

Exam tips

• Use the general formula CₙH₂ₙ₊₂ to confirm a molecular formula belongs to an alkane.
• When drawing ethane, count four bonds around each carbon to check the structure is complete.
• Link the name ethane to the two-carbon chain, remembering the prefix eth- means two carbons.