# Explain why the reaction of nitrogen gas with oxygen gas to form nitrogen monoxide, N₂(g) + O₂(g) → 2NO(g), is non-spontaneous at standard conditions, yet becomes spontaneous at very high temperatures.

> IB DP Chemistry Higher Level (2023 syllabus) — R1.4 Entropy and spontaneity (HL only) · Explain · 4 marks

> The standard enthalpy change for the reaction N₂(g) + O₂(g) → 2NO(g) is +180 kJ mol⁻¹. The standard entropy change for the same reaction is approximately +25 J K⁻¹ mol⁻¹.

## Mark scheme (4 marks)

1. The reaction is non-spontaneous at standard conditions because ΔG is positive, arising from a large positive ΔH that outweighs the TΔS term at low temperatures.
2. The entropy change is positive (small but positive) because two moles of reactant gases produce two moles of product gases — there is a slight increase in disorder/the number of accessible microstates.
3. As temperature increases, the TΔS term increases in magnitude, because ΔS is positive; therefore ΔG = ΔH − TΔS decreases.
4. Above a critical/crossover temperature (where T > ΔH/ΔS), ΔG becomes negative and the reaction is spontaneous.

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