A-Level Chemistry October/November 2024 Q5(a): Label one bond angle on each of complexes P, Q, R and S, and identify the size of the a…

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  The bond angles are identified based on the VSEPR geometry of each complex. The angle is labelled on the diagram provided in the question paper for each complex.

  Complex P

  • Shape: Tetrahedral
  • Labelled angle: Any L–M–L bond angle.
  • Bond Angle = 109.5°

  Complex Q

  • Shape: Octahedral
  • Labelled angle: The angle between two adjacent ligands (e.g., an axial and an equatorial ligand).
  • Bond Angle = 90°

  Complex R

  • Shape: Octahedral
  • Labelled angle: The angle between two adjacent ligands.
  • Bond Angle = 90°

  Complex S

  • Shape: Linear
  • Labelled angle: The L–M–L bond angle.
  • Bond Angle = 180°

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  How the marks are awarded

  • B1 — Correctly identifying the bond angle for any two of the four complexes. For example, stating P is 109.5° and S is 180°.
  • B1 — Correctly identifying the bond angles for all four complexes: P (109.5° or 109°), Q (90°), R (90°), and S (180°). Note: For Q and R, 180° is also acceptable if the angle between opposite ligands is labelled.

  Common mistakes

  • Incorrectly stating the bond angle for the tetrahedral complex P as 90°, confusing it with a square planar geometry.
  • Providing an imprecise value for the tetrahedral angle, such as 120° (trigonal planar) instead of the correct 109.5°.
  • Forgetting the bond angles in an octahedral complex (Q and R), often mixing up the 90° and 180° angles or guessing an incorrect value like 60°.
  • Stating the bond angle for the linear complex S is 90° instead of 180°.

  Examiner tip: Memorise the names, 3D shapes, and ideal bond angles for common coordination numbers (2, 4, and 6) to quickly solve geometry questions.

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