Question

Which of the following equations represents enthalpy of formation of H2O?


a. 2H2(g) + O2(g)  2H2O() H = negative
b. H2(g) + O2(g)  H2O() H = negative
c. 2H2O (l)  2H2(g) + O2(g) H = positive
d. H2(g) + O2(g)  H2O(g) H =positive
e. 2H2O2 (l)  2H2O(l) + O2(g) H = positive

Answer 1

Answer: (a).

Reason: The standard enthalpy of formation is the enthalpy change for forming a compound from its elements in their standard states. For water (liquid), the formation reaction is
H2(g) + 1/2 O2(g) → H2O(l)
Option (a), 2H2(g) + O2(g) → 2H2O(l), is the same reaction multiplied by 2, and is therefore the correct choice (exothermic, ΔH < 0).

(Option b is stoichiometrically incorrect as written — it should have 1/2 O2 — and option d has the wrong sign.)