Shapes of molecules and ionsEdexcel International A Level Chemistry: Subtopic test

1. 1
   Ammonia gas is bubbled into dilute acid to make ammonium salts, which are widely used as fertilisers. In the reaction, the lone pair on the nitrogen atom of each ammonia molecule forms a bond with a hydrogen ion, H⁺, producing an ammonium ion.

   (a)

   What is the shape of the ammonium ion, NH₄⁺?

   [1 mark]

   • ATrigonal pyramidal
   • BTetrahedral
   • CSquare planar
   • DTrigonal planar

   (b)

   Which value is closest to the H–N–H bond angle in an ammonia molecule?

   [1 mark]

   • A90°
   • B104.5°
   • C107°
   • D120°

   (c)

   The H–N–H bond angle is 107° in an ammonia molecule but 109.5° in an ammonium ion. Explain this difference.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   Sulfur hexafluoride, SF₆, is an unreactive gas used as an electrical insulator in high-voltage switchgear. Phosphorus pentachloride, PCl₅, is a solid that vaporises on heating. In both the gaseous PCl₅ molecule and the SF₆ molecule the central atom is surrounded by more than eight electrons.

   (a)

   What is the shape of a gaseous PCl₅ molecule?

   [1 mark]

   • ATrigonal bipyramidal
   • BOctahedral
   • CSquare pyramidal
   • DTetrahedral

   (b)

   How many bonding pairs and how many lone pairs of electrons surround the sulfur atom in SF₆?

   [1 mark]

   • A4 bonding pairs and 2 lone pairs
   • B5 bonding pairs and 1 lone pair
   • C5 bonding pairs and 0 lone pairs
   • D6 bonding pairs and 0 lone pairs

   (c)

   Explain why an SF₆ molecule is octahedral with F–S–F bond angles of 90°.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   A chemical supplier lists three gases in its catalogue: ethene, C₂H₄, which is released by ripening fruit; beryllium chloride, BeCl₂, which is a gas at high temperature and is used in the manufacture of beryllium; and sulfur dichloride, SCl₂, a red liquid used in the production of rubber chemicals.

   (a)

   Deduce the shape of an ethene molecule around each carbon atom and the H–C–H bond angle, and explain your answer.

   [3 marks]

   (b)

   Predict the shape and the Cl–X–Cl bond angle of the BeCl₂ molecule and of the SCl₂ molecule (where X is the central atom), explaining each answer.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   Methane, ammonia and water are hydrides of neighbouring second-period elements. Their measured bond angles are 109.5° (H–C–H), 107° (H–N–H) and 104.5° (H–O–H). Separately, boron trifluoride, BF₃, reacts with a fluoride ion, F⁻, to form the tetrafluoroborate ion, BF₄⁻, in which the fluoride ion donates a lone pair of electrons to the boron atom.

   (a)

   Explain why the bond angle decreases in the order CH₄, NH₃, H₂O.

   [6 marks]

   (b)

   Deduce the shapes of BF₃ and BF₄⁻, and explain how and why the F–B–F bond angle changes when BF₃ forms BF₄⁻.

   [6 marks]

   Total for question 4: 12 marks

End of questions