Paper 1 Q8 — Al₂Cl₆ → AlCl₃: shape and bond angle

Bonding & shapes — dimer to monomer changes geometry and bond angle

What the examiners said

“Fewer candidates appreciated that the change from Al₂Cl₆ to AlCl₃ alters the structure from three-dimensional to planar or that the change involves an increase in bond angle.”

Try the question

When the dimer Al₂Cl₆ dissociates into two AlCl₃ molecules on heating, which change occurs at each aluminium atom?

Examiner report: “Few appreciated the 3D→planar change and the increase in bond angle.”

Explore — break the dimer

Slide from the dimer to two monomers. Watch each aluminium go from four bonds (tetrahedral, 3D) to three bonds (trigonal planar) and the bond angle open up.

Dissociation: 0%

Two effects the exam wanted together: 4 bonds → 3 bonds (so 3D → planar) and the angle widening ~109° → 120° as an electron domain is removed.

Original reconstruction inspired by the misconceptions described in the Singapore-Cambridge GCE A-Level H1 Chemistry 8873 November 2023 Examiner Report. Not the actual examination question. For classroom exploration.

Made by lookang, using Claude Fable 5. More resources: iwant2study.org