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The lattice energy of NaCl(s) is 788 kJ/mol and the enthalpy of hydration is -784 kJ/mol. Calculate the enthalpy of solution of NaCl(s).

1. -1572 kJ/mol
2. -4 kJ/mol
3. 4 kJ/mol
4. 1572 kJ/mol

Correct answer: 4 kJ/mol

Solution

The Born-Haber cycle: NaCl(s) -> Na+(g) + Cl-(g), Delta_H = +788 kJ/mol (endothermic, breaking lattice). Na+(g) + Cl-(g) + H2O -> Na+(aq) + Cl-(aq), Delta_H = -784 kJ/mol (hydration, exothermic). Delta_H_solution = 788 + (-784) = +4 kJ/mol. The solution process is slightly endothermic.

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