IB DP Chemistry Reactivity 2.2 How fast? The rate of chemical change HL Paper 2- Exam Style Questions - New Syllabus
Question
Most-appropriate topic codes (IB Chemistry 2025):
• Reactivity 3.2: When is it useful to use half-equations? — parts (a)(i), (a)(iv)
• Structure 3.1: The periodic table: Classification of elements — parts (a)(ii), (a)(iii)
• Structure 2.1: The ionic model — part (b)(i)
• Structure 2.2: The covalent model — parts (b)(ii), (b)(iii), (b)(iv)
▶️ Answer/Explanation
(a)(i)
In [Fe(S₂O₃)₂(H₂O)₂]⁻, the thiosulfate ion (S₂O₃²⁻) acts as a ligand. The oxidation state of sulfur in thiosulfate is +2.
\(\boxed{+2}\)
(a)(ii)
The complex is coloured because:
• The iron ion has partially filled d-orbitals
• The ligands cause splitting of the d-orbitals into different energy levels
• Electrons can absorb visible light energy and undergo d-d transitions between these split levels
• The colour observed is complementary to the wavelength of light absorbed
\(\boxed{\text{d-d transitions in split d-orbitals absorb visible light.}}\)
(a)(iii)
CoCl₂ acts as a catalyst by:
• Providing an alternative reaction pathway with a lower activation energy
• This allows more reactant particles to have sufficient energy to react during collisions
\(\boxed{\text{Provides alternative pathway with lower activation energy.}}\)
(a)(iv)
Oxidation half-equation:
S₂O₃²⁻ + H₂O → 2SO₂ + 2H⁺ + 4e⁻
Overall redox equation:
S₂O₃²⁻ + 4Fe³⁺ + H₂O → 2SO₂ + 4Fe²⁺ + 2H⁺
\(\boxed{\text{See equations above.}}\)
(a)(v)
The energy profile should show:
• Reactants at higher energy than products (exothermic)
• Two peaks representing two transition states
• One energy minimum between peaks representing the intermediate
• Eₐ labelled for both steps
• ΔH shown as negative (products lower than reactants)
\(\boxed{\text{Two-humped curve with intermediate, showing exothermic reaction.}}\)
(b)(i)
Ionic bonding: Electrostatic attraction between oppositely charged ions (Fe³⁺ cations and NO₃⁻ anions)
Covalent bonding: Sharing of electron pairs between atoms within the nitrate ions
\(\boxed{\text{Ionic: between ions; Covalent: shared electrons within nitrate.}}\)
(b)(ii)
The Lewis formula for NO₃⁻ shows nitrogen bonded to three oxygen atoms with one double bond and two single bonds, with resonance and a formal charge of -1.
\(\boxed{\text{Resonance structure with N central, one N=O and two N-O bonds.}}\)
(b)(iii)
The nitrate ion has trigonal planar geometry.
\(\boxed{\text{Trigonal planar}}\)
(b)(iv)
The bond lengths in the nitrate ion are all equal at approximately 124 pm due to resonance. The electron delocalization across all three N-O bonds results in bond lengths intermediate between single and double bonds.
\(\boxed{\text{All bonds 124 pm due to resonance/delocalization.}}\)