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Which of the following statements is incorrect?

1. Heat of combustion is always negative.
2. H+(aq) + OH-(aq) -> H2O(l); delta H = -13.7 kcal/mol
3. If the bond enthalpy of H-H is 436 kJ/mol, then the enthalpy of formation of an isolated gaseous hydrogen atom is 218 kJ/mol.
4. If the enthalpy of a reaction is negative, then the reaction must occur.

Correct answer: If the enthalpy of a reaction is negative, then the reaction must occur.

Solution

A: Heat of combustion is always negative (exothermic) — correct. B: Enthalpy of neutralisation of strong acid and strong base = -13.7 kcal/mol — this is the standard value, correct. C: H2(g) -> 2H(g); delta H = 436 kJ/mol (bond dissociation). Enthalpy of formation of one H(g) atom = 436/2 = 218 kJ/mol — correct. D: A negative delta H (exothermic reaction) does NOT guarantee the reaction will occur spontaneously. Spontaneity is determined by delta G = delta H - T*delta S < 0. If delta S is sufficiently negative, even an exothermic reaction may not be spontaneous. So D is the incorrect statement.

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