Two elementary oxidation reactions of nitrogen are shown in the table below

CHEMISTRY · Multi-part · A Level · Chemistry · Energetics · Energetics

Problem

Two elementary oxidation reactions of nitrogen are shown in the table below. All enthalpy changes are standard molar values.

Reaction	Chemical equation	$\Delta H^\circ$ ( $\text{kJ}$ )
1	N₂(g) + O₂(g) → 2 NO(g)	+180
2	2 NO(g) + O₂(g) → 2 NO₂(g)	−110

Using only the information in the table, analyze the direct formation of nitrogen dioxide from the elements. (1) Part 1: (a) Write the overall balanced chemical equation that results when the two elementary reactions are combined so that the intermediate NO(g) cancels. (2) Part 2: (b) Predict whether the overall reaction you wrote in part (a) is exothermic or endothermic. Justify your answer without performing a full arithmetic calculation. (3) Part 3: (c) Without carrying out exact subtraction, decide whether the magnitude

|\Delta H^\circ|

for the overall reaction is closer to

70\,\text{kJ}

or to

290\,\text{kJ}

. State your choice and explain your reasoning. (4) Part 4: (d) Describe qualitatively how reversing, scaling, and adding reactions affect the sign and magnitude of

\Delta H^\circ

, and explicitly connect those principles to your answers in parts (b) and (c).

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