Nitrogen dioxide decomposes as follows - VCE - SSCE Chemistry - Question 5 - 2012 - Paper 1

To determine the enthalpy change, we start with the given reaction:

$2NO₂(g) → N₂(g) + 2O₂(g) \\ ΔH = -66 kJ mol⁻¹$

For the reverse reaction, we flip the equation:

$N₂(g) + 2O₂(g) → 2NO₂(g) \\ ΔH = +66 kJ mol⁻¹$

Now, we want the reaction:

$\frac{1}{2}N₂(g) + O₂(g) → NO₂(g)$

This is half of the reversed reaction. When we divide the enthalpy change by 2, we find:

$ΔH = \frac{+66 kJ mol⁻¹}{2} = +33 kJ mol⁻¹$

Thus, the correct answer is option C: +33 kJ mol⁻¹.