Intermolecular Attractions

NH₃ molecules can form...

• London dispersion forces

• Dipole-dipole forces

• Hydrogen bonds

HF molecules can form...

• London dispersion forces

• Dipole-dipole forces

• Hydrogen bonds

HCl molecules can form...

• London dispersion forces

• Dipole-dipole forces

• Hydrogen bonds

CH₄ molecules can form...

• London dispersion forces

• Dipole-dipole forces

• Hydrogen bonds

CH₃Cl molecules can form...

• London dispersion forces

• Dipole-dipole forces

• Hydrogen bonds

Which one has the highest boiling point?

• CH₄

• SO₂

• H₂O

Answer explanation

H₂O has the highest boiling point because it can form the strongest type of intermolecular force - HYDROGEN BONDING.

Which of the following non-polar molecules has the highest boiling point? (Refer Page 15 - 17 of Intermolecular Forces)

• CH₄; molar mass = 16.04 g/mol

• CCl₄; molar mass = 153.82 g/mol

• PF₅; molar mass = 125.966 g/mol

• Rn; molar mass = 222.01758 g/mol

Answer explanation

The bigger the molar mass, the more electron a species has.

The more electrons, the more distance over which

they can move, the bigger the possible temporary

dipoles and therefore the bigger the dispersion

forces.

London dispersion forces strength is stronger.

H₂O and NH₃ are polar molecules that can form the strongest type of intermolecular force (hydrogen bonding). However, the boiling point of H₂O is 100.00 °C whereas NH₃ is -33.34 °C.

Why is that so? (Refer Page 32 of Intermolecular Forces)

• H₂O can form more hydrogen bonds than NH₃

• H₂O can form less hydrogen bonds than NH₃

• O on the H₂O is more electronegative than N on the NH₃

Between HF and H₂O... which one has a higher boiling point? (Refer Page 32 of Intermolecular Forces)

• H₂O

• HF

Answer explanation

Even though fluorine (F) is more electronegative than

oxygen, H₂O has higher boiling point than HF

because there are 4 hydrogen bonding per H₂O molecule

compared to only 2 per HF molecule.

H₂O can from more hydrogen bonds than HF.

Factor(s) that influence the strength of London dispersion forces

• Molecular size

• Molecular shape

• Molecular polarity

Answer explanation

Molecular polarity is for dipole-dipole forces