Bonding

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: metallic bonding

• Cl₂: simple covalent bonding

• MgCl₂: 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 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. (3 marks)

• 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 Van der Waal forces between molecules

• SiHCl3 has Van der Waal 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)

Q8. Which molecule has a permanent dipole? (1 mark)

A BF₃

B NH₃

C SiCl₄

D SO₃

B NH₃

Which polymer has hydrogen bonding between the polymer chains?

A Kevlar

B PVC

C poly(phenylethene)

D Terylene

A Kevlar

Sodium fluoride contains sodium ions (Na⁺) and fluoride ions (F^-). Na⁺ and F^- have the same electron configuration.

Explain why a fluoride ion is larger than a sodium ion. (2 marks)

• Fluoride ion has (two) fewer protons/lower nuclear charge

• Weaker attraction between nucleus and (outer) electrons

The melting point of XeF₄ is higher than the melting point of PF₃.

Explain why the melting points of these two compounds are different.

In your answer you should give the shape of each molecule, explain why each molecule has that shape and how the shape influences the forces that affect the melting point. (6 marks)

Stage 1 electron pairs:

• XeF₄ 4bp and 2lp around Xe

• PF₃ 3bp and 1lp around P

Stage 2 explanation of shapes:

• XeF₄ is square planar

• PF₃ is pyramidal

• Electron pairs repel as far as possible or Lone pair repels more than bonding pairs

Stage 3 IMF:

• XeF₄ has vdw forces and PF₃ has dipole-dipole forces (and vdw)

• Stronger/more intermolecular forces in XeF₄

• Due to more electrons or larger molecules

Which compound has the highest boiling point? (1 mark)

A CH₃CH₂CH₂Br

B CH₃CH₂CH₂F

C CH₃CH₂CHO

D CH₃CH₂COOH

D CH₃CH₂COOH

Draw a diagram to show the strongest type of interaction between two molecules of ethanol (C2H5OH) in the liquid phase. Include all lone pairs and partial charges in your diagram. (3 marks)

Methoxymethane (CH3OCH3) is an isomer of ethanol. The table shows the boiling points of ethanol and methoxymethane.

In terms of the intermolecular forces involved, explain the difference in boiling points. (3 marks)

• Hydrogen bonds between ethanol molecules

• Dipole-dipole OR van der Waals force between methoxymethane molecules

• Hydrogen bonds are a stronger intermolecular force

Which is not responsible for conduction of electricity? (1 mark)

A The sodium ions in molten sodium chloride

B The electrons between layers of carbon atoms in graphite

C The bonding electrons in a metal

D The lone pair electrons on water molecules

D The lone pair electrons on water molecules

Which row shows the bonding in ammonium chloride? (1 mark)

C (Covalent, Dative covalent, and Ionic)

Silicon tetrafluoride (SiF₄) is a tetrahedral molecule.

Deduce the type of intermolecular forces in SiF₄. Explain how this type of intermolecular force arises and why no other type of intermolecular force exists in a sample of SiF_4. (3 marks)

• Van der Waals forces

• Uneven distribution of electrons in one molecule induces dipole in neighbouring molecule

• symmetrical molecule / dipoles cancel so no dipole-dipole attraction

Deduce why the bonding in nitrogen oxide is covalent rather than ionic. (1 mark)

Small electronegativity difference

Suggest the type of crystal shown by OF₂ (1 mark)

Simple molecular