Multiple Choice
Which of the following best describes the difference between the heat (q) of a reaction and the reaction enthalpy (ΔH_{rxn})?
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8. Thermochemistry
Thermochemical Equations
Multiple Choice
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
Verified step by step guidance1
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.
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:
\(\Delta H^\circ = \sum \Delta H_f^\circ (\text{products}) - \sum \Delta H_f^\circ (\text{reactants})\)
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.
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.
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.
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