Chemistry - Thermochemistry Question with Solution | TestHub
From the following data at 25°C, which of the following statement(s) is/are correct? (one or more than one correct)
1/2 H2(g) + 1/2 O2(g) → OH(g); ΔH° = 42 kJ
H2(g) + 1/2 O2(g) → H2O(g); ΔH° = -242 kJ
H2(g) → 2H(g); ΔH° = 436 kJ
O2(g) → 2O(g); ΔH° = 495 kJ
Options:(select one or more)
Answer:
Solution:
The enthalpy of formation for atomic hydrogen is Δf H(H) = 436/2 = +218 kJ/mol, and for atomic oxygen, it is Δf H(O) = 495/2 = +247.5 kJ/mol.
(a) For the reaction H₂O(g) → H₂ + ¹/₂ O₂, the enthalpy change is +242 kJ.
Then, for H₂ → 2H, the enthalpy change is +436 kJ.
And for ¹/₂ O₂ → O, the enthalpy change is +247.5 kJ.
The total enthalpy change is 242 + 436 + 247.5 = 925.5 kJ. This statement is true.
(b) For the reaction OH → ¹/₂ H₂ + ¹/₂ O₂, the enthalpy change is -42 kJ.
Then, for H → H, the enthalpy change is +218 kJ.
And for O → O, the enthalpy change is +247.5 kJ.
The total enthalpy change is -42 + 218 + 247.5 = 423.5 kJ, which is not 502 kJ. This statement is false.
(c) The enthalpy of formation for atomic hydrogen, Δf H(H), is +218 kJ/mol, not -218 kJ/mol. This statement is false.
(d) The first equation represents the formation of OH(g) from its elements, with an enthalpy change of +42 kJ/mol. This statement is true.
Therefore, the correct answers are (a) and (d).
