3.
Dinitrogen tetroxide (N2O4) breaks down into nitrogen dioxide (NO2). If the reaction is
reversible and endothermic, which condition will give the largest yield of NO2?
a. High temperature and high pressure
b. High temperature and low pressure
c. Low temperature and high pressure
d. Low temperature and low pressure
High temperature and low pressure give the largest yield of NO₂ in the endothermic dissociation of N₂O₄.
Reaction Overview
The reversible reaction is N₂O₄(g) ⇌ 2NO₂(g), where the forward dissociation is endothermic (ΔH > 0), absorbing heat. NO₂ appears brown, while colorless N₂O₄ dominates at equilibrium under certain conditions. Le Chatelier’s principle predicts shifts: systems counteract changes in temperature, pressure, or moles of gas.
Option Analysis
High temperature favors the endothermic forward reaction, increasing NO₂ yield by shifting equilibrium right (heat acts as reactant). Low pressure favors more moles of gas (2 NO₂ vs 1 N₂O₄), shifting right. High pressure shifts left (fewer moles), reducing NO₂. Low temperature favors exothermic reverse, decreasing NO₂.
| Option | Temperature Effect | Pressure Effect | Net NO₂ Yield |
|---|---|---|---|
| a. High T, High P | Increases NO₂ | Decreases NO₂ | Moderate |
| b. High T, Low P | Increases NO₂ | Increases NO₂ | Highest |
| c. Low T, High P | Decreases NO₂ | Decreases NO₂ | Lowest |
| d. Low T, Low P | Decreases NO₂ | Increases NO₂ | Low |
Correct Answer
Option b maximizes NO₂ yield.
The N2O4 dissociation to NO2 reaction is a classic reversible endothermic equilibrium: N₂O₄(g) ⇌ 2NO₂(g). For maximum NO2 yield, specific conditions apply per Le Chatelier’s principle—high temperature drives the heat-absorbing forward reaction, while low pressure favors the side with more gas moles (2 vs 1). This N2O4 to NO2 equilibrium is key for CSIR NET Life Sciences and chemistry exams.
Factors Affecting N2O4 to NO2 Equilibrium
Temperature shifts dominate for endothermic reactions. Heating produces more brown NO₂ gas; cooling favors colorless N₂O₄. Pressure changes affect mole ratios: low pressure increases NO₂, high pressure reverses it by crowding molecules toward N₂O₄.
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High temperature: Endothermic forward favored
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Low pressure: More moles (NO₂) side shifts right
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Visual demo: Brown color intensifies with heat/low P
Why High Temperature Low Pressure Maximizes NO2 Yield
In N2O4 breaks down to NO2 endothermic systems, combine effects: high T absorbs heat forward, low P expands to 2 moles NO₂. Experiments confirm: hot/low P yields darkest brown (most NO₂). Opposite (low T/high P) minimizes yield.
Exam Tips for N2O4-NO2 Questions
CSIR NET aspirants: Always check ΔH sign and Δn_gas. Practice: Predict shifts for options like high T/high P (conflicting, moderate yield).
