NY Regents Chemistry Exam Jan 2023
43 Based on Table I, which chemical equation represents a reaction with a heat of reaction that indicates a net release of energy?
(1) N2(g) + O2(g) → 2NO(g)
(2) N2(g) + 2O2(g) → 2NO2(g)
(3) 2C(s) + 3H2(g) → C2H6(g)
(4) 2C(s) + 2H2(g) → C2H4(g)
Solution: When ΔH has a positive sigh the reaction is endothermic (heat is absorbed). When ΔH has a negative sign, the reaction is exothermic (heat is released). Therefore, we are looking for a reaction with a negative ΔH sign. Choice 3 is the correct answer.