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
Consider the following equilibrium reaction:
2SO2 (g) + O2 (g) 2SO3 (g)
(a )State the equilibrium constant expression, Kc, for the reaction above. [1]
(b) State and explain how the equilibrium would be affected by increasing the volume of the reaction container at a constant temperature. [3]
(c) SO2 (g), O2 (g) and SO3 (g) are mixed and allowed to reach equilibrium at 600 °C.
| SO2 | O2 | SO2 |
Initial concentration / mol dm–3 | 2.00 | 1.50 | 3.00 |
Equilibrium concentration / mol dm–3 | 1.50 |
|
|
Determine the value of Kc at 600 °C. [2]
Answer/Explanation
Ans:
a «Kc = »
b
pressure decrease «due to larger volume»
reaction shifts to side with more moles/molecules «of gas»
reaction shifts left/towards reactants
c
[O2] = 1.25 «mol dm−3» AND [SO3] = 3.50 «mol dm−3»
«Kc = =» 4.36