Alkanes are widely utilized as fuels, and their physical and chemical properties show distinct trends as the carbon chain length increases. Which of the following statements regarding the physical properties and combustion of alkanes is correct?
The viscosity of liquid alkanes increases with chain length primarily because longer chains have stronger intramolecular covalent bonds, which physically restrict the molecules from flowing.
When comparing structural isomers of pentane (C5H12\text{C}_5\text{H}_{12}C5H12), the highly branched 2,2-dimethylpropane has a higher boiling point than the straight-chain pentane because its spherical shape allows closer molecular packing.
During the complete combustion of gaseous alkanes, the amount of oxygen gas required for the complete combustion of one mole of the alkane increases by exactly 1.5 mol1.5\text{ mol}1.5 mol for each successive member of the homologous series.
As the molecular size of alkanes increases, their volatility increases and their ease of ignition increases because larger molecules contain more C–H bonds that can be broken to release energy.
130 exam-style questions on Edexcel GCSE Chemistry 9.1 Fuels, covering 9.1.1 Hydrocarbons and crude oil, 9.1.2 Fractional distillation of crude oil and its fractions, 9.1.3 The alkanes as a homologous series, 9.1.4 Complete and incomplete combustion, 9.1.5 Pollutants from burning fuels, 9.1.6 Hydrogen as a fuel and non-renewable fossil fuels, and 9.1.7 Cracking of larger alkanes. Each one has a worked solution and a mark scheme showing where the marks go.