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AS · Practice questions · Hydrogen bonding and bond properties

Hydrogen, held tight.

Six original Cambridge-style questions on hydrogen bonding and bond properties: conditions, diagrams, the hydride boiling points, ice, solubility and bond energy against bond length.

Original questions All questions on this page are original work, written in the Cambridge AS & A Level style. They are not from past papers. They test the same concepts and skills the syllabus rewards.
Keep these straight

H on N, O or F, pointing at a lone pair.

01
Recall
[2 marks]

State the two conditions needed for a hydrogen bond to form between two molecules.

  • One molecule has a hydrogen atom covalently bonded to N, O or F. ✓
  • The other molecule has a lone pair of electrons on an N, O or F atom. ✓
02
Explain
[3 marks]

Draw a diagram to show a hydrogen bond between two ammonia molecules. Include partial charges and lone pairs.

  • One NH3 with Nδ− and Hδ+ labelled; the second NH3 with its lone pair drawn on N. ✓
  • A dashed line from the lone pair on N of the second molecule to a δ+ H of the first. ✓
  • The N–H···N arrangement drawn roughly linear. ✓
03
Explain
[3 marks]

Using Fig. 1, explain why the boiling points of CH4, SiH4, GeH4 and SnH4 increase steadily while H2O has a much higher boiling point than H2S.

  • Down Group 14 the molecules have more electrons, so id-id forces get steadily stronger; CH4 cannot hydrogen bond. ✓
  • H2O forms hydrogen bonds because O is very electronegative and has lone pairs; H2S cannot. ✓
  • Hydrogen bonds are much stronger than the other intermolecular forces, so much more energy is needed to boil water. ✓
04
Explain
[2 marks]

Explain why ice floats on water.

  • In ice each water molecule forms four hydrogen bonds, holding the molecules in an open lattice. ✓
  • The molecules are further apart than in liquid water, so ice is less dense. ✓
05
Explain
[2 marks]

Explain why ethanol, C2H5OH, is completely miscible with water, but ethane is not.

  • Ethanol has an O–H group, so it can form hydrogen bonds with water molecules. ✓
  • Ethane is non-polar with only id-id forces, which cannot compete with the hydrogen bonds between water molecules. ✓
06
Analysis
[2 marks]

The bond lengths of C–C, C=C and C≡C are 0.154, 0.134 and 0.120 nm. Describe the relationship between bond length and bond energy, and explain it.

  • The shorter the bond, the higher the bond energy: C≡C > C=C > C–C. ✓
  • More shared pairs increase the electron density between the nuclei, pulling them closer and holding them more strongly. ✓

Mark this once you have attempted all six and checked your working. It records a Practiced badge on the topic and adds a one-time bonus. Revealing the solutions alone does not count.