Nitrogen dioxide, NO sub 2 times g Δ⁢H_f=33.84kJ/mol , is decomposed according to the reaction: 2 NO sub 2 g rightarrow cap n sub 2 times g plus 2 cap o sub 2 g What is the enthalpy change when 2 point 5 0 mol of nitrogen dioxide decomposes? Use Δ⁢H_rxn=∑(Δ⁢H_f_1p⁢r⁢o⁢d⁢u⁢c⁢t⁢s)−∑(Δ⁢H_f_1r⁢e⁢a⁢c⁢tan⁡t⁢s)

The elements N₂(g) and O₂(g) have standard enthalpies of formation of 0. For the reaction as written, ΔH_rxn = 0 − 2(33.84 kJ/mol) = −67.68 kJ per 2 mol of NO₂. For 2.50 mol, the change is (2.50 ÷ 2)(−67.68 kJ) = −84.6 kJ. The negative sign means 84.6 kJ of energy is released, so the answer is choice C.

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