Which statement is true regarding the heats of reaction?

The statement that heats of reaction can be either positive or negative is accurate because it reflects the nature of exothermic and endothermic reactions. In an exothermic reaction, heat is released into the surroundings, resulting in a negative heat of reaction value. Conversely, in an endothermic reaction, heat is absorbed from the surroundings, which yields a positive heat of reaction value.

This distinction is fundamental in thermochemistry as it helps in understanding not only the energy changes during a reaction but also the direction of heat flow. While other options present limitations or misconceptions about heats of reaction—such as being exclusively positive or only measurable in a closed system—the reality is that heat can be transferred in both directions depending on the particular reaction being studied. Additionally, heats of reaction are not solely tied to temperature changes; they are related to the energy changes that accompany a change in the state or composition of reactants and products. Hence, the flexibility in the sign of the heat of reaction is crucial for correctly interpreting thermodynamic processes in chemical reactions.