Physical Properties of Alkanes

Melting Point and Boiling Point

Alkanes are covalent substances with simple molecular substances with weak forces of attraction between the molecules. Hence, in general alkanes have low melting and boiling points.

Melting and boiling points of alkanes gradually increase down the homologous series. This is because forces of attraction between alkane molecules increase as their size increases.

Name of Alkane	Number of Carbon Atoms	Melting Point	Boiling Point	Physical State at 25$°C$
Butane	4	-138	-0.5	gas
Pentane	5	-130	36	liquid
Hexane	6	-95	69	liquid
Heptadecane	17	22	302	liquid
Octadecane	18	28	316	solid
Nonadecane	19	32	330	solid

Sample

Explain why butane has higher melting and boiling points than ethane

• butane has larger molecular size than ethane
• there are more weak intermolecular forces of attraction between butane molecules compared to ethane molecules
• hence larger amount of energy required to overcome these forces in butane compared to ethane for melting and boiling to occur. OR

Substance	Butane	Ethane
molecular size	larger	smaller
amount of weak intermolecular forces of attraction between molecules of substance	greater	lesser
amount of energy required to overcome these forces to melt/boil substance	more	lesser

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Density

Density of liquid alkanes increase as molecular size increases. densities of liquid alkanes are less than $1g/c{m}^3$ → will float on water

Sample

Explain why liquid pentane has higher density than ethane at r.t.p.

• $M_r$ of pentane is 72 which is heavier than $M_r$ of ethane which is 30.
• at r.t.p, pentane is a liquid ad ethane is a gas.
• pentane molecules are closely packed together while ethane molecules are spaced far apart.
• there are a lot of pentane molecules than ethane molecules per unit volume of substance.

Substances	Pentame	Ethane
$M_r$	72	30
physical state at r.t.p	liquid	gas
arrangement of molecules in substance at r.t.p	closely packed together	far apart
number of molecules in per unit volume of substance at r.t.p	more	less

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Viscosity

• refers to fluidity of liquid. measure of fluids resistance to flow. the more viscous the liquid, the less easily it flows. Alkanes become more viscous as their molecular sizes increase. This is due to the stronger forces of attraction between the alkane molecules.

Sample

Explain why pentane is less viscous than octane

• pentane has smaller molecular size than octane
• forces of attraction between the pentane molecules is weaker than that of the octane molecules
• thus pentane will flow more easily than octane, thus more viscous

Substances	Octane	Pentane
Molecular size	larger	smaller
forces of attraction between molecules	greater	lesser
ease of flow of substance	flows less easily	flows more easily

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Flammability

Alkanes are flammable. As molecular size increases, percentage by mass of carbon increases hence flammability of alkanes decreases. flame also becomes Sootier (or smokier) → caused by incomplete combustion of alkanes due to insufficient oxygen as there is higher carbon content. Smoker flame is due to the soot produced during incomplete combustion of carbon containing fuels.

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Solubility

Alkanes are insoluble in water but soluble in most organic solvents such as tetrachloromethane ($CC{l}_4$) and benzene.

• solubility in organic solvents decreases as the molecular size increases. Liquid alkanes are often used as solvents for other organic compounds.

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