Explain how to determine if a reaction is endothermic or exothermic from enthalpy change values and provide signs.

• A. If ΔHrxn > 0, reaction is endothermic; if ΔHrxn < 0, reaction is exothermic.
• B. If ΔHrxn > 0, reaction is exothermic; if ΔHrxn < 0, reaction is endothermic.
• C. The sign of ΔHrxn does not indicate heat flow.
• D. ΔHrxn only indicates feasibility, not heat.

Answer: (B)

The sign of the enthalpy change for a reaction tells you heat flow at constant pressure: a positive ΔHrxn means the system absorbs heat from its surroundings, so the process is endothermic; a negative ΔHrxn means heat is released to the surroundings, so the process is exothermic. In other words, if heat flows into the system, ΔHrxn is positive; if heat flows out, ΔHrxn is negative. For example, dissolving certain salts or melting ice can have a positive ΔHrxn, while combustion or neutralization often has a negative ΔHrxn. This sign indicates heat flow, not whether the reaction is feasible or spontaneous, which is governed by other thermodynamic quantities.