Which statement best explains how the enthalpy of formation is used to predict whether a chemical reaction is endothermic or exothermic?

• A. If the enthalpy change (riangle H) for the reaction is negative, the reaction is exothermic; if riangle H is positive, the reaction is endothermic.
• B. A reaction is always exothermic if it involves the formation of a solid from its elements.
• C. The enthalpy of formation only predicts whether a reaction will occur spontaneously, not whether it is endothermic or exothermic.
• D. If the enthalpy change (riangle H) for the reaction is positive, the reaction is exothermic; if riangle H is negative, the reaction is endothermic.

Answer: (A)

Verified step by step guidance

1. Understand that the enthalpy of formation (\( \Delta H_f^\circ \)) is the heat change when one mole of a compound forms from its elements in their standard states.
2. Recognize that the overall enthalpy change for a reaction (\( \Delta H_{reaction} \)) can be calculated using the enthalpies of formation of reactants and products with the formula: \[ \Delta H_{reaction} = \sum \Delta H_f^\circ (\text{products}) - \sum \Delta H_f^\circ (\text{reactants}) \]
3. Interpret the sign of \( \Delta H_{reaction} \): if it is negative, the reaction releases heat to the surroundings and is exothermic; if it is positive, the reaction absorbs heat and is endothermic.
4. Note that the enthalpy of formation helps predict whether a reaction is endothermic or exothermic by indicating the heat flow based on the difference in bond energies between products and reactants.
5. Be aware that spontaneity of a reaction depends on both enthalpy and entropy changes, so enthalpy of formation alone does not determine if a reaction is spontaneous.