Edexcel International A Level (IAL) Chemistry (YCH11) - Unit 2 - 7.5 IMF and physical properties-Study Notes - New Syllabus

7.5 Intermolecular Forces and Boiling Temperature

(i) Trend in Boiling Temperatures of Alkanes with Increasing Chain Length

Observation

• Boiling temperature increases as carbon chain length increases.
• Example:
• \( \mathrm{CH_4 < C_2H_6 < C_3H_8 < C_4H_{10}} \)

Explanation

• Alkanes are non-polar molecules.
• Only intermolecular forces present are London dispersion forces.

• As chain length increases:
  • Number of electrons increases
  • Molecular size increases
  • Surface area increases

• This leads to:
  • Stronger temporary dipoles
  • Stronger London forces

• More energy required to separate molecules → higher boiling temperature.

Key Conclusion

Increasing chain length → stronger intermolecular forces → higher boiling temperature.

(ii) Effect of Branching on Boiling Temperatures of Alkanes

Observation

• Increased branching → lower boiling temperature.
• Example:
• \( \mathrm{C_5H_{12}} \):
  • Pentane (straight chain) → highest b.p.
  • 2-methylbutane → lower b.p.
  • 2,2-dimethylpropane → lowest b.p.

Explanation 

• Branching makes molecules more compact and spherical.
• This reduces surface area of contact between molecules.

• As a result:
  • London dispersion forces become weaker
  • Less energy required to separate molecules

• Therefore, boiling temperature decreases.

Conclusion

More branching → smaller surface area → weaker intermolecular forces → lower boiling temperature.

Summary

• Longer chains → stronger London forces → higher boiling point.
• More branching → weaker London forces → lower boiling point.

Therefore, boiling temperature in alkanes depends on molecular size and shape.

(iii) Higher Boiling Temperatures of Alcohols vs Alkanes

Observation

• Alcohols have higher boiling temperatures than alkanes with similar number of electrons.
• Example:
• \( \mathrm{C_2H_5OH} \) vs \( \mathrm{C_2H_6} \)

Explanation

• Alkanes: only London dispersion forces.
• Alcohols:
  • London dispersion forces
  • Permanent dipole–dipole interactions
  • Hydrogen bonding (strongest)

• Hydrogen bonding occurs due to:
  • Highly polar O–H bond
  • Attraction between \( \mathrm{H^{\delta+}} \) and lone pair on O

• Stronger intermolecular forces → more energy needed to break → higher boiling temperature.

Conclusion

Presence of hydrogen bonding in alcohols significantly increases boiling temperature compared to alkanes.

(iv) Trends in Boiling Temperatures of Hydrogen Halides (HF → HI)

Observation

• General trend (HCl → HI): boiling temperature increases down the group.
• However, HF has an anomalously high boiling temperature.

From HCl to HI:

• Increasing number of electrons.
• Stronger London dispersion forces.
• Therefore, boiling temperature increases.

Why HF is Anomalous

• HF molecules form hydrogen bonds.
• Fluorine is highly electronegative.
• Strong intermolecular attraction between molecules.

• Therefore:
  • HF has much higher boiling temperature than expected.

Trend Summary

\( \mathrm{HF > HI > HBr > HCl} \) (in terms of boiling temperature)

Conclusion

• HF: hydrogen bonding dominates.
• HCl → HI: London forces dominate.

Summary

• Alcohols have higher boiling temperatures due to hydrogen bonding.
• Hydrogen halides show increasing boiling temperature down the group due to London forces.
• HF is anomalous due to hydrogen bonding.

Therefore, boiling temperature depends on the type and strength of intermolecular forces.