Intermolecular Forces

Pentane, C₅H₁₂, has three structural isomers (compounds with the same molecular formula but a different structural formula).

Rearrange them in order of increasing boiling point

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Answer explanation

Consider the shape of the molecule!!

Straight chain pentane has the largest surface area, hence most extensive LDF, which contributed to the highest BP.

Highly-branched 2-2,dimethylpropane has the smallest surface area, least extensive LDF, and lowest BP

These five compounds have identical or very similar molar masses. Arrange them from the LOWEST BP to HIGHEST BP

• ethanol, CH₃CH^₂OH

• methanoic acid, HCOOH

• methoxymethane, CH₃OCH₃

• propane, C₃H₈,

Answer explanation

1) Propane (LDF, B.P. -42.2 ^oC)

2) methoxymethane (pd-d, B.P -

-25^oC)

3) ethanol (hydrogen bonding, B.Pt. 78 ^oC)

4)methanoic acid (strong hydrogen bonding, B.P 101 oC).

Assuming the size of the electron cloud are similar, arrange the following forces of attraction in order of INCREASING strength.

• London Dispersion Forces

• Ionic Bonding

• Hydrogen Bonding

• Dipole-dipole

Answer explanation

LDF < Pd-d < H Bonding < Ionic

Ethanol has a relative molar mass of 46 and boils at 78.5 ^oC.

Water has a relative molar mass of 18 and boils at 100 ^oC under the same conditions. Both molecules show hydrogen bonding.

Explain why water has a higher boiling point.

Answer explanation

Water has MORE EXTENSIVE Hydrogen bonding as compared to ethanol.

Water has on average 2 Hydrogen Bonding per molecule. (2 H and 2 LP)

Ethanol has on average 1 H bonding per molecule (1 H and 2 LP)

Which of the following molecules form Hydrogen Bonds with each other?

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Which of the following molecules forms hydrogen bonds with water?

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