The dissolving of ammonium nitrate in water lowers the temperature of the solution, which means that
Correct answer: D. the energy absorbed in breaking the lattice exceeds the energy released on hydrating the ions
- A. the process is exothermic
- B. no energy change occurs
- C. the lattice energy is zero
- D. the energy absorbed in breaking the lattice exceeds the energy released on hydrating the ions
Explanation
Dissolving involves two steps with opposite signs: separating the ions from the lattice costs energy while surrounding them with water molecules releases it, and the overall sign depends on which is larger. For ammonium nitrate the lattice term wins, so the process is endothermic and the solution cools, which is what an instant cold pack exploits. For calcium chloride the balance goes the other way and the solution warms.
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About Exothermic and Endothermic Reactions
Exothermic reactions release heat to the surroundings and have negative enthalpy change, while endothermic reactions absorb heat and have positive enthalpy change. The topic uses energy profiles to identify reactants, products and activation energy, and includes bond energy, enthalpy diagrams, calorimetry and Hess's law for calculating overall heat changes.
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