Open Question
Hydrogen and methanol have both been proposed as alternatives to hydrocarbon fuels. Use standard enthalpies of formation to calculate the amount of heat released per kilogram of methanol fuel and per kilogram of hydrogen fuel.
8. Thermochemistry
Enthalpy of Formation
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Find ΔHrxn for the following reaction:
N2 + 2 O2 → 2 NO2
Based on the following data.
2 NO → N2 + O2 ΔH = −180 kJ
2 NO + O2 → 2 NO2 ΔH = −112 kJ
A
68 kJ
B
−68 kJ
C
−292
D
292
E
−146
Verified step by step guidance1
Identify the target reaction: N2 + 2 O2 → 2 NO2. We need to find the enthalpy change (ΔHrxn) for this reaction.
Examine the given reactions and their enthalpy changes: 1) 2 NO → N2 + O2 with ΔH = -180 kJ, and 2) 2 NO + O2 → 2 NO2 with ΔH = -112 kJ.
Reverse the first reaction to match the formation of NO from N2 and O2: N2 + O2 → 2 NO. This changes the sign of ΔH to +180 kJ.
Add the reversed first reaction to the second reaction: (N2 + O2 → 2 NO) + (2 NO + O2 → 2 NO2). This results in the target reaction: N2 + 2 O2 → 2 NO2.
Calculate the overall ΔHrxn by summing the enthalpy changes of the modified reactions: ΔH = (+180 kJ) + (-112 kJ).
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