Calculate the value of ΔH°/kJ for the following reaction using the listed thermochemical equations:N₂(g) + 1/2O₂(g)→N2O(g)2NH₃(g) + 3N2O(g)→4N₂(g) + 3H₂O(l), ΔH°/kJ = -1010kJ4NH₃(g) + 3O2(g)→2N₂(g) + 6H₂O(l), ΔH°/kJ = -1531kJ
Correct answer: A. +81.5
- A. +81.5
- B. -81.5
- C. +163
- D. -163
Explanation
To find the ΔH° for the reaction N₂(g) + 1/2 O₂(g)→N₂O(g), we need to manipulate the given thermochemical equations using Hess's Law. First, reverse the second reaction to express N₂O(g) on the reactant side and change the sign of ΔH° to +1010 kJ. Then, add the reversed second equation to the first equation: 2NH₃(g) + 3N₂O(g)→4N₂(g) + 3H₂O(l) (ΔH° = +1010kJ) + 4NH₃(g) + 3O₂(g)→2 N₂(g) + 6 H₂O(l) (ΔH° = -1531 kJ). Simplifying and canceling the common terms, we find ΔH° for N₂(g) + 1/2O₂(g) → N₂O(g) is +81.5 kJ. The other options represent incorrect calculations due to errors in applying Hess's Law.
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About Thermochemistry and Energetics of Chemical Reactions
Thermochemistry measures energy changes in chemical reactions and distinguishes exothermic from endothermic processes. Work covers systems, surroundings and state functions, internal energy, the first law of thermodynamics, enthalpy and Hess's law, including the sign conventions used when heat enters or leaves a system.
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