Covalent bonding and dot-and-cross diagramsEdexcel A-Level Chemistry: Subtopic test

1. 1
   Nitrogen makes up about 78% of the atmosphere and is very unreactive. The two atoms in a nitrogen molecule are joined by a triple bond of length 0.110 nm, whereas a typical nitrogen–nitrogen double bond is 0.125 nm long.

   (a)

   How many electrons are shared between the two nitrogen atoms in one molecule of nitrogen, N₂?

   [1 mark]

   • A6
   • B3
   • C10
   • D2

   (b)

   Compared with a nitrogen–nitrogen double bond, which row describes the triple bond in N₂?

   [1 mark]

   • ALonger and stronger
   • BLonger and weaker
   • CShorter and weaker
   • DShorter and stronger

   (c)

   Explain why the N≡N bond is shorter and stronger than the N=N bond.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   Ammonia gas, NH₃, reacts with hydrogen chloride gas to form solid ammonium chloride, NH₄Cl. In the ammonium ion all four N–H bonds are identical in length and strength.

   (a)

   What type of bond forms when an ammonia molecule bonds to a hydrogen ion, H⁺?

   [1 mark]

   • AIonic
   • BDative covalent
   • CDouble covalent
   • DMetallic

   (b)

   Which species contains a dative covalent bond?

   [1 mark]

   • ACH₄
   • BH₂O
   • CH₃O⁺
   • DCO₂

   (c)

   Explain how the fourth N–H bond in the ammonium ion forms, and why it is identical to the other three.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   Aluminium chloride can exist as AlCl₃ molecules in the vapour at high temperature. At lower temperatures the vapour contains Al₂Cl₆ molecules, formed when two AlCl₃ molecules join together. In an AlCl₃ molecule each aluminium atom has only six electrons in its outer shell.

   (a)

   Explain how two AlCl₃ molecules join to form Al₂Cl₆.

   [3 marks]

   (b)

   Describe the dot-and-cross diagram of Al₂Cl₆. State the total number of Al–Cl bonds, how many of them are dative covalent, which atom provides both electrons in each dative bond, and the number of electrons in the outer shell of each aluminium atom.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   Bond lengths and mean bond enthalpies for three carbon–oxygen bonds are: the C–O bond in methanol, 0.143 nm and 358 kJ mol⁻¹; the C=O bond in carbon dioxide, 0.116 nm and 805 kJ mol⁻¹; and the C≡O bond in carbon monoxide, 0.113 nm and 1077 kJ mol⁻¹. Carbon monoxide is a toxic gas formed by incomplete combustion.

   (a)

   Describe and explain the relationship between the bond length and the strength of these three carbon–oxygen bonds.

   [6 marks]

   (b)

   Carbon monoxide contains a triple bond that includes one dative covalent bond. Explain how the bonding in a carbon monoxide molecule arises, in terms of the outer electrons of carbon and oxygen.

   [6 marks]

   Total for question 4: 12 marks

End of questions