Chemistry · Hydrocarbons · chapter 9 of 9

Class 11 Chemistry Chapter 9: Hydrocarbons case study question with answers

One case-based question of the kind CBSE-pattern school exams ask: a passage and a table, then 3 parts worth 4 marks in all, with the answer and the marking scheme for each. Written from the NCERT chapter.

Case study · 4 marks

A student plots the boiling points of some alkanes against their molecular masses, using the values below. She notices that the straight-chain alkanes lie on a smooth rising curve, while isomers of the same molecular mass fall below it by different amounts. Her teacher reminds her that the attractions between alkane molecules are van der Waals forces, which depend on the area over which two molecules can touch.

Alkane	Molecular mass / u	b.p. / K
butane	58	272.4
2-methylpropane	58	261.0
pentane	72	309.1
2-methylbutane	72	300.9
2,2-dimethylpropane	72	282.5
hexane	86	341.9

Part 1 · 1 mark

From the table, the rise in boiling point from pentane to hexane is

(a) 8.2 K(b) 41.0 K(c) 32.8 K(d) 59.4 K

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Answer: (c) 32.8 K

341.9 − 309.1 = 32.8 K. 41.0 K is hexane minus 2-methylbutane, 59.4 K hexane minus 2,2-dimethylpropane, and 8.2 K pentane minus 2-methylbutane.

Part 2 · 1 mark

2,2-Dimethylpropane boils lower than pentane because

(a) it has a smaller molecular mass(b) its nearly spherical molecules have a smaller area of contact(c) it is more polar than pentane(d) its C–C bonds break more easily on heating

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Answer: (b) its nearly spherical molecules have a smaller area of contact

Both have molar mass 72 u and are almost non-polar. With more branching the molecule becomes nearly spherical, the area of contact is smaller, and the van der Waals forces are overcome at a lower temperature.

Part 3 · 2 marks

2-Methylpentane (molecular mass 86 u) boils at 333.4 K. Using the table, explain why this is below the boiling point of hexane but above that of pentane.

Answer and marking scheme

Below hexane: 2-methylpentane is a branched isomer of hexane with the same molecular mass, so its area of contact and van der Waals forces are smaller. Above pentane: it is a larger molecule (86 u against 72 u) with a larger surface area, and here that outweighs the effect of a single branch.

below hexane: branching at the same mass (1)
above pentane: larger molecular mass and surface area (1)

Grease stains on clothes are removed with petrol, but not with water. Explain, using the nature of the substances involved.

Answer and marking scheme

Grease is a mixture of higher alkanes: non-polar, hence hydrophobic, so it does not dissolve in polar water. Petrol is a mixture of non-polar hydrocarbons; like dissolves like, so non-polar grease dissolves in non-polar petrol.

grease non-polar, insoluble in polar water (1)
like dissolves like: dissolves in non-polar petrol (1)

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