What is the standard enthalpy change (ΔH°) for the reaction: 2 NO(g) + O2(g) → 2 NO2(g)?

• A. +57 kJ
• B. −114 kJ
• C. +114 kJ
• D. −57 kJ

Answer: (B)

Verified step by step guidance

1. Identify the given reaction: \$2 \mathrm{NO}(g) + \mathrm{O}_2(g) \rightarrow 2 \mathrm{NO}_2(g)\( and note that you need to find the standard enthalpy change \)\Delta H^\circ$ for this reaction.
2. Recall that the standard enthalpy change of a reaction can be calculated using the standard enthalpies of formation (\(\Delta H_f^\circ\)) of the reactants and products with the formula: \(\Delta H^\circ = \sum \Delta H_f^\circ (\text{products}) - \sum \Delta H_f^\circ (\text{reactants})\)
3. Look up or be given the standard enthalpy of formation values for \(\mathrm{NO}(g)\) and \(\mathrm{NO}_2(g)\). Note that the standard enthalpy of formation for elemental \(\mathrm{O}_2(g)\) in its standard state is zero.
4. Multiply the standard enthalpy of formation values by their respective stoichiometric coefficients in the balanced equation (2 for NO and 2 for NO2), then sum the products and reactants separately.
5. Subtract the sum of the reactants' enthalpies of formation from the sum of the products' enthalpies of formation to find \(\Delta H^\circ\) for the reaction.