A student dissolves a dark brown solid, dinitrogen tetroxide (N₂O₄), in a sealed flask. The following reversible reaction occurs: N₂O₄(g) ⇌ 2NO₂(g). N₂O₄ is colourless and NO₂ is brown. When the flask is placed in ice-cold water, the brown colour fades but does not disappear completely. Explain these observations in terms of equilibrium.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
N₂O₄(g) ⇌ 2NO₂(g) The forward reaction is endothermic. N₂O₄ is colourless; NO₂ is brown.
Model answer (4 marks)
The reaction is reversible, so both the forward and reverse reactions occur simultaneously.
At equilibrium the rates of the forward and reverse reactions are equal, giving constant concentrations of N₂O₄ and NO₂; this explains why some brown colour remains.
Lowering the temperature favours the exothermic reverse reaction, shifting the equilibrium to the left, towards N₂O4.
Consequently more N₂O₄ is formed and less NO₂ remains, so the concentration of brown NO₂ decreases, making the colour fade but not disappear completely.
At equilibrium the rates of the forward and reverse reactions are equal, giving constant concentrations of N₂O₄ and NO₂; this explains why some brown colour remains.
Lowering the temperature favours the exothermic reverse reaction, shifting the equilibrium to the left, towards N₂O4.
Consequently more N₂O₄ is formed and less NO₂ remains, so the concentration of brown NO₂ decreases, making the colour fade but not disappear completely.
Examiner tips
- Use the term "equilibrium" and "Le Chatelier’s principle". Show the shift to the left when temperature is lowered. Explain why the colour does not vanish – equilibrium still exists. Keep the answer concise, 4 points total.
Common mistakes
- Saying the reaction stops or that the colour disappears completely. Forgetting that the reaction is reversible. Using the wrong direction for the temperature shift (i.e. saying it shifts to the right).
Mark scheme (4 marks)
- The reaction is reversible, so both forward and reverse reactions occur simultaneously / at the same time
- At equilibrium, the rates of the forward and reverse reactions are equal, so concentrations of N₂O₄ and NO₂ remain constant (explaining why some brown colour persists)
- Lowering the temperature favours the exothermic (reverse) reaction, shifting the equilibrium position to the left / towards N₂O₄
- More N₂O₄ is produced / less NO₂ remains, so the concentration of brown NO₂ decreases, causing the colour to fade (but not disappear, as equilibrium is still established)
Key terms in this question
reversible reaction · equilibrium
Related
- All Cambridge International IGCSE Chemistry (0620) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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