Question
Nitrogen(IV) oxide, \(NO_2\), is a brown gas found in photochemical smog and has a pollutant causing acid deposition.
(a) Nitrogen(IV) oxide exists in equilibrium with dinitrogen tetroxide, \(N_2O_4\) (g), which is colourless.
\(2NO_2 (g) \rightleftharpoons N_2O_4 (g)\)
(i) At 100°C \(K_c\) for this reaction is 0.0665. Outline what this indicates about the extent of this reaction.
(ii) Calculate the value of Kc at 100°C for the equilibrium:
\(N_2O_4 (g) \rightleftharpoons 2NO_2 (g)\)
(iii) Calculate the standard enthalpy change, in kJ \(mol^{−1}\), for the reaction:
\(N_2O_4 (g)\) → \(2NO_2 (g)\)
(b) Deduce the Lewis structure of \(N_2O_4\).
(c) The NO bond lengths in \(N_2O_4\) are all \(1.19 × 10^{−10}\)m.
(i) Suggest what the bond lengths indicate about the structure of \(N_2O_4\).
(ii) Predict the ONN bond angle in \(N_2O_4\).
(d) Acid deposition is formed when nitrogen oxides dissolve in water. Write an equation for nitrogen(IV) oxide reacting with water to produce two acids.
Answer/Explanation
Answer:
(a) (i) reaction hardly proceeds
OR
reverse reaction/formation of \(NO_2\) is favoured
OR
«concentration of» reactants greater than «concentration of» products «at
equilibrium»
(ii) «\(K_c = \frac{1}{0.0665}\) =» 15.0
(iii) «\(\Delta H^{\theta}\) = 2(33.18) – 9.16 =» «+» 57.20 «kJ \(mol^{−1}\)»
(b) 
(c) (i) it has resonance structures
(ii) 110-\(120^o\)
(d) \(2NO_2(g) + H_2O(l) → HNO_2(aq) + HNO_3(aq)\)
Question
The periodic table is a useful tool in explaining trends of chemical behaviour.
(a) (i) Annotate and label the ground state orbital diagram of boron, using arrows to represent electrons.
(ii) Sketch the shapes of the occupied orbitals identified in part (a)(i).
(b) Explain the decrease in first ionization energy from Li to Cs, group 1.
(c) (i) State the electron domain geometry of the ammonia molecule.
(ii) Deduce the Lewis (electron dot) structure of ammonia and sketch its 3D molecular shape.
(iii) Explain, with reference to the forces between molecules, why ammonia has a higher boiling point than phosphine (\(PH_3\)).
(d) (i) Ammonia is manufactured by the Haber process.
\(N_2 (g) + 3H_2 (g) \rightleftharpoons 2NH_3 (g)\) \(ΔH^{\theta}_t\) = – 92.0 kJ \(mol^{-1}\)
Outline what is meant by dynamic equilibrium.
(ii) Deduce the \(K_c\) expression for the reaction in part (d)(i).
(iii) The Haber process requires a catalyst. State how a catalyst functions.
(iv) Sketch a Maxwell–Boltzmann distribution curve showing the activation energies with and without a catalyst.
(v) Suggest how the progress of the reaction could be monitored.
Answer/Explanation
Answer:
(a) (i) 
(ii) 
(b) valence electron further from nucleus/«atomic» radius larger «down the group»
«electron» more shielded/ less attractive force/easier to remove
(c) (i) tetrahedral
(ii) 
(iii) ammonia has intermolecular/IMF hydrogen bonds «phosphine does not»
phosphine «and ammonia» dipole-dipole/London dispersion forces/instantaneous
dipole attractions/Van der Waals forces
hydrogen bonds stronger
(d) (i) «in a closed system» the rate of the forward reaction equals the rate of the reverse reaction.
(ii) \([NH_3]^2/([N_2][H_2]^3)\)
(iii) alternate pathway AND lowers activation energy/\(E_a\)
(iv) 
correct shape curve starting at the origin, without touching the x axis at high
energy.
(\(E_a\)) catalysed <(\(E_a\)) uncatalysed on x axis.
(v) change in AND
volume
OR
pressure
OR
temperature
OR
concentration of \(H_2\)/\(N_2\) /reactants/\(NH_3\) /product