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Multiple Choice
Calculate ΔH⁰ for the process 1/2 N2(g) + 1/2 O2(g) → NO(g) using Hess's Law and the following reactions: 1. N2(g) + 3 H2(g) → 2 NH3(g) ΔH⁰ = -91.8 kJ 2. 4 NH3(g) + 5 O2(g) → 4 NO(g) + 6 H2O(g) ΔH⁰ = -906.2 kJ 3. H2(g) + 1/2 O2(g) → H2O(g) ΔH⁰ = -241.8 kJ
A
90.3 kJ
B
-180.6 kJ
C
180.6 kJ
D
-90.3 kJ
Verified step by step guidance
1
Identify the target reaction: 1/2 N2(g) + 1/2 O2(g) → NO(g). We need to manipulate the given reactions to achieve this target reaction using Hess's Law.
Reverse reaction 1 to get NH3 on the reactant side: 2 NH3(g) → N2(g) + 3 H2(g). This changes the sign of ΔH⁰ to +91.8 kJ.
Divide reaction 2 by 4 to match the stoichiometry of NO in the target reaction: NH3(g) + 5/4 O2(g) → NO(g) + 3/2 H2O(g). Adjust ΔH⁰ to -906.2 kJ / 4 = -226.55 kJ.
Use reaction 3 to cancel out H2O and H2: Multiply by 3/2 to get 3/2 H2(g) + 3/4 O2(g) → 3/2 H2O(g). Adjust ΔH⁰ to 3/2 * -241.8 kJ = -362.7 kJ. Combine all adjusted reactions to achieve the target reaction and sum their ΔH⁰ values to find the ΔH⁰ for the target reaction.