CHEMISTRY · Single correct · A Level · Chemistry · Energetics · Energetics
Problem
For the decomposition reaction 2NH3(g)→N2(g)+3H2(g), which statement most accurately predicts the sign of ΔH∘ and the reason for that sign?
(a) ΔH∘ is negative because the reaction produces more moles of gas, so the increasing disorder releases heat.
(b) ΔH∘ is negative because forming the three H–H bonds releases more energy than is required to break the N–H bonds (the N≡N bond contribution is ignored).
(c) ΔH∘ is positive because more energy is required to break the six N–H bonds in ammonia than is released when the N≡N and H–H bonds are formed.
(d) ΔH∘ is positive because breaking bonds always requires energy whereas forming bonds releases very little energy.