IB DP Chemistry Structure 1.3 Electron configurations HL Paper 2- Exam Style Questions - New Syllabus

IBDP Chemistry HL Paper 2 - All Chapters

Question

In the stratosphere, ozone is decomposed by ultraviolet radiation.
\(\mathrm{O_3(g) \rightarrow O_2(g) + O(g)}\)
(a) State the full electron configuration of an oxygen atom and the number of unpaired electrons in that atom. [2]
(b) (i) Draw a Lewis (electron dot) structure for the ozone molecule. [1]
(ii) Deduce the formal charge on each of the three oxygen atoms by adding them to your Lewis (electron dot) structure in (b)(i). [2]
(iii) Predict the shape and bond angle of the ozone molecule. [2]
(iv) State the hybridization state of the central oxygen atom in ozone. [1]
(c) Suggest a value, in pm, for the bond lengths in the ozone molecule and explain your answer. Use data: \(\mathrm{O{=}O}\) double bond \(=121~\text{pm}\); \(\mathrm{O{-}O}\) single bond \(=148~\text{pm}\) (Data Booklet ). [2]
(d) Explain the dependence of the dissociation of diatomic oxygen, \(\mathrm{O_2}\), and ozone, \(\mathrm{O_3}\), on the wavelength of UV radiation. [2]
(e) The concentrations of ozone molecules and chlorine monoxide, ClO, free radicals were measured (graph provided in the paper).
Ozone and ClO Concentration Graph
(i) Outline the relationship between the concentrations of ozone and ClO, free radicals. [1]
(ii) Comment, based on this graph, on the conclusion that the hole in the ozone layer is caused by ClO free radicals. [2]
(iii) When a chlorine emission spectrum is produced, there is a strong line at \(453~\text{nm}\). Determine the energy of the photon of light emitted in J. Use constants: \(c=3.00\times10^8~\text{m s}^{-1}\), \(h=6.63\times10^{-34}~\text{J s}\) (Data Booklet). [2]
▶️ Answer/Explanation
Markscheme (with detailed working)
(a)
Electron configuration: \(1s^2\,2s^2\,2p^4\).  Unpaired electrons: \(2\). A1 A1
(b)
(i) One acceptable Lewis structure (no resonance shown): \(\mathrm{O{=}O{-}O}\) with the single bond drawn as a coordinate bond \(\mathrm{O{\rightarrow}O}\) from the central O; lone pairs shown on each O.
Ozone Lewis Structure A1
(ii) Formal charges (using \( \text{FC}=V-(N_\text{lone}+ \tfrac{1}{2}N_\text{bond}) \)): central O \(=+1\); single-bonded terminal O \(=-1\); double-bonded terminal O \(=0\).
Ozone Formal Charges A1 A1
(iii) Shape: bent (angular, V-shaped); bond angle \(\approx 110^\circ\!-\!119^\circ\) (accept any in this range; typical \(\sim117^\circ\)). M1 A1
(iv) Hybridization of central O: \(\mathrm{sp^2}\). A1
(c)
In ozone, the two \(\mathrm{O{-}O}\) bonds are equivalent by resonance (bond order \(\approx 1.5\)), so the bond length is intermediate between a single \(\mathrm{O{-}O}\) \((148~\text{pm})\) and a double \(\mathrm{O{=}O}\) \((121~\text{pm})\). A reasonable estimate is any value in \(122\text{–}147~\text{pm}\) (for example, \(\boxed{135~\text{pm}}\)), with resonance/delocalization as the explanation. M1 A1
(d)
UV with shorter wavelength (higher energy) is required to break the stronger \(\mathrm{O{=}O}\) bond in \(\mathrm{O_2}\); longer-wavelength UV can dissociate the weaker \(\mathrm{O{-}O}\) bond in \(\mathrm{O_3}\). Hence \(\mathrm{O_2}\) needs higher-energy (shorter-\(\lambda\)) photons than \(\mathrm{O_3}\). A1 A1
(e)
(i) There is a negative (inverse) correlation: as ClO increases, ozone decreases (and vice versa). A1
(ii) The graph supports the claim but does not prove causation; a mechanism is needed and other factors may contribute. (Accept mention of catalytic Cl•/ClO• ozone-depleting steps.) A1 A1
(iii) \(\lambda=453~\text{nm}=4.53\times10^{-7}~\text{m}\). \[ \nu=\frac{c}{\lambda}=\frac{3.00\times10^8}{4.53\times10^{-7}} =6.62\times10^{14}~\text{s}^{-1} \] \[ E=h\nu=(6.63\times10^{-34})(6.62\times10^{14}) =\boxed{4.39\times10^{-19}~\text{J}} \] (Accept answers to appropriate significant figures.) M1 A1
Total Marks: 14
More DP Chemistry HL Paper 2 Questions..
Scroll to Top