Crude Oil and Hydrocarbon Properties
This topic covers the fundamentals of organic chemistry, focusing on hydrocarbons derived from crude oil. It explores their physical properties, key reactions like cracking and combustion, and the systematic rules for representing and naming their structures.
Part of the ESAT Chemistry syllabus — revision for the Engineering and Science Admissions Test (ESAT), the UAT-UK admissions test for Cambridge, Imperial, Oxford and UCL.
Key points
- Crude oil is a mixture of hydrocarbons, separated into fractions by fractional distillation, where substances with lower boiling points rise higher in the column.
- As the carbon chain length of a hydrocarbon increases, its boiling point and viscosity increase due to stronger intermolecular forces, while its flammability decreases.
- Cracking is a thermal decomposition process that breaks down large, less valuable long-chain alkanes into smaller, more useful molecules, including a shorter-chain alkane and at least one alkene.
- Structural isomers are molecules that share the same molecular formula but have different arrangements of atoms, leading to different physical and chemical properties.
- Complete combustion of a hydrocarbon in excess oxygen produces only carbon dioxide (CO2) and water (H2O).
- Incomplete combustion occurs in a limited supply of oxygen and produces water along with carbon monoxide (CO) and/or solid carbon (soot) instead of, or in addition to, carbon dioxide.
Formulae
Alkane + O2 → CO2 + H2O Represents the general reaction for the complete combustion of any alkane. The equation must be balanced for the specific alkane used.
Long-chain alkane → Shorter-chain alkane + Alkene(s) Represents a typical cracking reaction. The total number of carbon and hydrogen atoms must be conserved on both sides of the equation.
Definitions
- Homologous Series
- A series of organic compounds with the same functional group and similar chemical properties, in which each successive member differs by a CH2 group. Alkanes are a common example.
- Functional Group
- A specific atom or group of atoms within a molecule that is responsible for the characteristic chemical reactions of that molecule. For example, the C=C double bond in an alkene.
- Structural Isomers
- Molecules that have the same molecular formula but a different structural formula, meaning the atoms are connected in a different order.
- Displayed Formula
- A 2D representation of a molecule showing all the atoms and all the bonds connecting them. Also known as a full structural formula.
- Condensed Structural Formula
- A representation of a molecule where the symbols of atoms are listed in order as they appear in the molecule's structure with hydrogen atoms grouped with the carbon they are bonded to (e.g., CH3CH2CH3 for propane).
Worked example
A hydrocarbon has the molecular formula C6H14. One of its branched-chain isomers is named 2,3-dimethylbutane. Write a balanced chemical equation for the complete combustion of this isomer.
- 1
Identify the reactants and products.
The reactant is the hydrocarbon (C6H14) and oxygen (O2).
For complete combustion, the products are carbon dioxide (CO2) and water (H2O).
- 2
Write the unbalanced equation:
C6H14 + O2 → CO2 + H2O.
- 3
Balance the carbon atoms.
There are 6 carbons on the left, so we need 6 molecules of CO2 on the right:
C6H14 + O2 → 6CO2 + H2O.
- 4
Balance the hydrogen atoms.
There are 14 hydrogens on the left, so we need 7 molecules of H2O on the right (since 7 x 2 = 14):
C6H14 + O2 → 6CO2 + 7H2O.
- 5
Balance the oxygen atoms.
The right side now has (6 x 2) + (7 x 1) = 12 + 7 = 19 oxygen atoms.
To get 19 atoms on the left, we need 19/2 (or 9.5) molecules of O2.
- 6
The equation is now:
C6H14 + (19/2)O2 → 6CO2 + 7H2O.
To eliminate the fraction, multiply all coefficients by 2.
- 7
The final balanced equation is 2C6H14 + 19O2 → 12CO2 + 14H2O.
Answer: 2C6H14 + 19O2 → 12CO2 + 14H2O
Common mistakes
- ×When naming branched alkanes, failing to identify the longest possible continuous carbon chain, which may not be written horizontally.
- ×Numbering the carbon chain from the wrong end, resulting in higher numbers for the side groups than necessary. Always number to give the lowest possible locants.
- ×Mistaking two different drawings of the same molecule for structural isomers. If the atom connectivity is identical, they are not isomers.
- ×In cracking equations, forgetting to conserve atoms. The sum of carbons and hydrogens in the products must equal the amount in the original reactant.
- ×Incorrectly balancing oxygen atoms in combustion equations, especially when an odd number of oxygen atoms is required on the product side, leading to a fractional coefficient for O2.
No-calculator tips
- ✓For balancing combustion, always follow the order: Carbon, then Hydrogen, then Oxygen (CHO). This systematic approach prevents confusion.
- ✓If balancing oxygen results in a half-integer (like 5.5), don't panic. Simply multiply the entire equation by 2 to clear the fraction and obtain whole number coefficients.
- ✓To quickly check if a cracking equation is balanced, do a fast atom count. Sum the carbons in the products, and then the hydrogens. Ensure they match the starting molecule.