NCEA Level 2 Chemistry 91164 Bonding free practice questions with answers

Question 1 (15 marks) [Bonding, shape and polarity]

(a) Consider the molecule SO₂. (i) Draw its Lewis structure (electron dot diagram), showing all lone pairs.

(ii) State the shape of the molecule and the approximate bond angle. (iii) Explain whether the molecule is polar or non-polar.

(a) Answer: SO₂ is bent (V-shaped) with a bond angle of about 117°; it is polar

• Evidence level: Excellence (count valence electrons, place bonds and lone pairs, then decide shape and polarity from the symmetry).

• (i) Valence electrons: 6 (S) + 6 (O) + 6 (O) = 18. The central atom is S; it forms 2 bond(s) to O, O (with 2 multiple bond(s)).

• The central atom has 1 lone pair(s) and 3 region(s) of electron density.

• (ii) 3 regions of electron density with 1 lone pair(s): the shape is bent (V-shaped), with a bond angle of about 117°.

• Achievement: (iii) States whether the molecule is polar or non-polar.

• Merit: Gives a reason for the bond polarity: the bonds are polar because of the difference in electronegativity between S and the other atoms.

• Excellence: Full explanation: the bonds are polar and the bent (V-shaped) shape is not symmetrical, so the bond dipoles do not cancel and the molecule is polar.

(b) The electronegativity of Mg is 1.31 and the electronegativity of O is 3.44. Classify the bond between Mg and O as ionic, polar covalent or non-polar covalent, and explain how the electronegativity values lead to your answer, including the direction of any bond polarity.

(b) Answer: ionic bond

• Evidence level: Excellence (use the electronegativity difference to classify the bond and show the direction of polarity).

• Difference = |1.31 − 3.44| = 2.13.

• Achievement: States the bond type (ionic).

• Merit: Links to the difference: a difference of 2.13 is large (above about 1.7), so the bond is ionic.

• Excellence: Full explanation: the electrons are transferred (or almost completely pulled) to O, giving ions held by electrostatic attraction.

(c) Potassium forms an ionic compound with sulfur. (i) Write the electron arrangement change for each atom as the compound forms, and write the formula of the compound. (ii) Explain why the compound has a high melting point and why it only conducts electricity when molten or dissolved in water.

(c) Answer: (i) K loses 1, S gains 2 electron(s); formula K₂S; (ii) strong electrostatic attractions between ions in the lattice give a high melting point; ions are free to move only when molten or dissolved

• Evidence level: Excellence (describe electron transfer, balance the charges, and explain properties from the lattice structure).

• (i) K loses 1 electron(s) to form K⁺; S gains 2 electron(s) to form S²⁻. The charges must balance: 2 ×

(1) = 1 × (−2), so the formula is K₂S (potassium sulfide).

• Achievement: (ii) States that the compound has a lattice of ions with strong attractions between them.

• Merit: Links to the properties: a lot of energy is needed to overcome the strong electrostatic attractions between oppositely charged ions, so the melting point is high; the ions cannot move in the solid.