Question
Nitrogen(IV) oxide, NO2, 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 Gibbs free energy change, \(∆G^{\theta}\), for this equilibrium at 100°C.
Use sections 1 and 2 of the data booklet.
(iii) Calculate the value of \(K_c\) at 100°C for the equilibrium:
\(N_2O_4 (g) \rightleftharpoons 2NO_2 (g)\)
(iv) Calculate the standard enthalpy change, in kJ \(mol^{−1}\), for the reaction:
\(N_2O_4(g) \rightarrow 2NO_2(g)\)
(v) Calculate the standard entropy change, in J \(mol^{−1}\), for the reaction:
\(N_2O_4(g) \rightarrow 2NO_2(g)\)
(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) \(\delta G^Θ\) = «−RTlnK = −8.31 x 373 x ln(0.0665) =»
8.40 «kJ \(mol^{−1}\)»
(iii) «\(K_c = \frac{1}{0.0665} =» 15.0\)
(iv) «\(\delta H^{\theta}\) = 2(33.18) – 9.16 =» «+» 57.20 «kJ \(mol^{−1}\)»
(v) «\(\delta S^{\theta}\) = 2(240.06) – 304.29 =»
«+»175.83 «J \(K^{-1} mol^{−1}\)»
(b) 
(c) (i) it has resonance structures
(ii) 110-\(120^o\)
(d) \(2NO_2(g) + H_2O(l) \rightarrow HNO_2(aq) + HNO_3(aq)\)
Question
(a) Explain why a colorimeter set at a wavelength of 500nm is not suitable to investigate reactions of \(Zn^{2+}\) compounds. Use section 3 of the data booklet.
(b) Nitrogen(II) oxide radicals (NO•) catalyse the decomposition of ozone \((O_3)\).
(i) Formulate equations showing how NO• acts as a catalyst in this reaction.
Chlorine also forms free radicals; the bond enthalpy for \(Cl_2\) is \(4.02 × 10^{-19}\) J.
(ii) Calculate the minimum frequency of light needed to break this bond.
Use sections 1 and 2 of the data booklet.
(iii) Calculate the formal charge on each atom in the two Lewis structures of the \(NO_2\)•(g) radical.
(iv) Lewis structure A is more stable. Suggest, giving one reason, whether the formal charge model supports this.
Answer/Explanation
Answer:
(a) \(Zn^{2+}\) does not form coloured compounds/ has a complete d subshell/orbital
500 nm/«the setting on the colorimeter» in visible region AND no absorbance will
be seen
(b) (i) «\(O_3\)(g) \(\rightarrow O_2\)(g) + O• (g)»
NO• (g) + \(O_3\) (g) \(\rightarrow NO_2\)• (g) + \(O_2\) (g)
OR
\(NO_2\)• (g) + O• (g) \(\rightarrow \) NO• (g) + \(O_2\) (g)
(ii) «v = E/h = \(4.02 x 10^{-19}\)/\(6.63 x 10^{-34}\) =» \(6.06 x 10^{14}\) «Hz»
(iii) 
(iv) No
AND
Structure B has all atoms of formal charge 0