Bonding

Draw a diagram to show the shape of the BF₄^- ion. State its shape. (2 marks)

Draw a diagram to show the shape of the BF₄^- ion. State its shape. (2 marks)

Name the type of bond formed in the reaction between BF₃ and F^- to form BF₄^-.

Explain how this bond is formed in terms of electrons involved. (2 marks)

• co-ordinate bond

• lone pair of electrons on F^- donated to B(F₃)

The boiling point of pentan-2-ol is 119 °C. The boiling point of pent-1-ene is 30 °C. Explain why pentan-2-ol has a higher boiling point than pent-1-ene. (3 marks)

• Pent-2-ol has stronger intermolecular forces

• As pent-1-ene has van der Waals’ forces only

• Pent-2-ol also has hydrogen bonds

Hydrogen peroxide dissolves in water.

State the strongest type of interaction that occurs between molecules of hydrogen peroxide and water. (1 mark)

Hydrogen bonding

This question is about elements in period 3 of the periodic table. The table shows some properties of magnesium, chlorine and magnesium chloride.

Use your knowledge of structure and bonding to explain the electrical conductivity and melting point of the three substances in the table. (6 marks)

Structure and bonding:

• Mg: giant metallic lattice OR metallic bonding

• Cl₂: simple covalent bonding

• MgCl₂: giant ionic lattice OR ionic bonding

Conductivity:

• Mg: delocalised electrons can carry electrical charge

• Cl₂: no mobile charge carriers

• MgCl₂: mobile ions can carry electrical charge

Melting point:

• Mg: metallic bonds are strong AND metallic bonds require large amounts of energy to break

• Cl₂: intermolecular forces are weak AND intermolecular forces require little energy to overcome

• MgCl₂: ionic bonds are strong AND ionic bonds require large amounts of energy to break

Name the type of bond formed between N and Al in H₃NAlCl₃. (1 mark)

Coordinate covalent bond

Explain how the value of the Cl-Al-Cl bond angle in AlCl₃ changes, if at all, on formation of the compound H₃NAlCl₃. (2 marks)

• Cl-Al-Cl bond angle decreases to 109.5°

• Because Al is tetrahedral

The Si-Cl bond is polar.

Explain why SiCl₄ is not a polar molecule.

• The molecule is tetrahedral OR there is an even distribution of electron density

• The dipoles cancel out

Explain how induced dipole-dipole forces arise in a sample of SiCl₄. 

• Electron movement in first SiCl₄ molecule results in a temporary dipole in first SiCl₄ molecule

• Induces a dipole in a second SiCl₄ molecule

• Attraction between adjacent molecules

Draw a diagram of hydrogen bonding between water molecules.

Why are boiling points of group 4 hydrides lower than hydrides of groups 5,6 and 7? (3 marks)

• Group 4 hydrides are non polar and so only have van der Waals forces between molecules

• Hydrides of group 5,6 and 7 are polar and so have dipole-dipole attraction between molecules

• Stronger than van der Waals forces

Suggest, in terms of intermolecular forces, why SiCl₄ has a higher boiling point than SiHCl₃. (4 marks)

• SiCl4 has induced dipole-dipole forces between molecules

• SiHCl3 has induced dipole-dipole forces AND SiHCl3 has permanent dipole-dipole forces

• The intermolecular forces in SiCl4 are stronger than the intermolecular forces in SiHCl3

  • Because SiCl4 has more electrons (than SiHCl3)

    allow a larger mass or a larger electron cloud

Why is the melting temperature of sodium lower than the melting temperature of magnesium?

The number of delocalised electrons per atom is fewer in sodium than in magnesium

Draw a diagram to show the strongest type of interaction between a molecule of ammonia and a molecule of water.

Include all lone pairs and partial charges in your diagram. (3 marks)