When is a chemical reaction considered to be endothermic?

A chemical reaction is considered to be endothermic when the change in enthalpy (delta H) is positive. This means that the reaction absorbs energy from its surroundings, typically in the form of heat. In an endothermic process, the energy required to break the bonds in the reactants is greater than the energy released when forming the bonds in the products. Consequently, the system needs to take in energy to proceed, leading to an increase in enthalpy.

In practical terms, during an endothermic reaction, the temperature of the surrounding environment may decrease since energy is drawn from it, often making it feel cold to the touch. Many processes such as photosynthesis and the dissolution of certain salts in water are examples of endothermic reactions, emphasizing the absorption of heat.

The other options reflect different scenarios. A negative delta H indicates an exothermic reaction, where energy is released. A zero delta H suggests that there is no net change in energy, which does not characterize a reaction that absorbs energy. An unstable delta H is not a standard term used in thermodynamics and does not accurately describe the energetics of a reaction.