Which of the following best describes the reaction Fe^{3+}(aq) + SCN^{-}(aq) → FeSCN^{2+}(aq)?

• A. It is exothermic because energy is required to break the bonds in the reactants.
• B. It is endothermic because heat is absorbed during the formation of the complex ion.
• C. It is exothermic because heat is released when the complex ion forms.
• D. It is endothermic because the temperature of the solution increases during the reaction.

Answer: (C)

Verified step by step guidance

1. Step 1: Understand the nature of the reaction between Fe^{3+}(aq) and SCN^{-}(aq). This reaction forms a complex ion FeSCN^{2+}(aq), which involves the formation of new chemical bonds.
2. Step 2: Recall that bond formation releases energy, while bond breaking requires energy. In this reaction, the key process is the formation of the FeSCN^{2+} complex ion, which means bonds are being formed.
3. Step 3: Since bond formation releases energy, the reaction is exothermic. This means heat is released to the surroundings during the reaction.
4. Step 4: Analyze the given options in light of this understanding. The correct description should state that the reaction is exothermic because heat is released when the complex ion forms.
5. Step 5: Note that if the reaction were endothermic, it would absorb heat, causing the temperature of the solution to decrease, not increase. Therefore, the option stating the temperature increases but the reaction is endothermic is incorrect.