Carbon and its Compounds
What the exam tests
Carbon and its Compounds tests covalent bonding, catenation and tetravalency, the homologous series, functional groups, isomerism, and the reactions of ethanol and ethanoic acid. Equation writing and structural formulae carry most marks.
Core rules
- Carbon has four valence electrons and forms four covalent bonds. It can link with itself (catenation) to form chains, branches and rings.
- Diamond, graphite and fullerene are allotropes of carbon. Graphite conducts because one electron per carbon is delocalised; diamond does not.
- Saturated hydrocarbons contain only single bonds; unsaturated hydrocarbons contain double or triple bonds.
- Alkanes: CnH2n+2; alkenes: CnH2n; alkynes: CnH2n−2.
- A homologous series has the same functional group and successive members differ by CH2.
- Functional groups: alcohol −OH, carboxylic acid −COOH, aldehyde −CHO, ketone −CO−, halo −X.
- Addition occurs at double/triple bonds; substitution occurs in saturated hydrocarbons, for example with chlorine in sunlight.
- Ethanol reacts with ethanoic acid to form an ester; soap is made by saponification of fats with alkali.
Worked example
Balance the combustion of ethanol: C2H5OH + O2 → CO2 + H2O. Carbon: 2 C gives 2CO2. Hydrogen: 6 H gives 3H2O. Oxygen on the right is 4 + 3 = 7; ethanol supplies 1, so 3O2 supply 6. Equation: C2H5OH + 3O2 → 2CO2 + 3H2O.
Traps that cost marks
- Saying graphite is soft because it has weak covalent bonds; the layers are held by weak forces, but the bonds within layers are strong.
- Writing soap as effective in hard water; it forms scum with Ca2+ and Mg2+.
- Confusing ethanol with ethanoic acid; only the acid turns blue litmus red.
- Forgetting that ethene decolourises bromine water, while ethane does not.