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Which of the following thermodynamic statements is/are correct? (A) Given delta_f_G(298) for calcite = -1128.8 kJ/mol and for aragonite = -1127.75 kJ/mol, the calcite form of CaCO3 is thermodynamically more stable at standard conditions. (B) For reactions: (a) C(diamond) + 2H2(g) -> CH4(g) with enthalpy change delta_H1, and (b) C(g) + 4H(g) -> CH4(g) with enthalpy change delta_H2; more heat is released in reaction (b). (C) delta_f_H(I2, g) equals the enthalpy of sublimation of I2(s) at 25 deg C. (D) For the exothermic reaction 2Ag(s) + (1/2)O2(g) -> Ag2O(s) at 298 K, delta_H < delta_U.

1. A only
2. A and B only
3. A, B, and C
4. A, B, C, and D

Correct answer: A and B only

Solution

A is correct: lower G means more stable, calcite has lower G. B is correct: reaction (b) involves atomized gaseous C and H, so all C-H bonds are formed with no bonds to break (beyond what is already done), releasing more energy than (a) where C-C and H-H bonds must also be broken. C is incorrect: delta_f_H(I2, g) is the enthalpy to form I2(g) from I2(s) (sublimation) plus half the dissociation of I2(g) to 2I(g)? No - standard formation of I2(g) from I2(s) IS just sublimation since I2(s) is the standard state; so C is actually correct. D: deltaₙ_gas = -1/2 for Ag2O formation, so delta_H = delta_U + deltaₙ*RT = delta_U - (1/2)RT, meaning delta_H < delta_U. D is correct for exothermic reaction context.

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