Change in enthalpy is always relative to the energy within a system. If the system gains energy, change in enthalpy (∆H) will be positive. If the system loses energy, the change in enthalpy (∆H) will be negative.

In lessons 2 and 3, we observed the dissolving of an ionic compound in water. A decrease in temperature of the surroundings was observed, and this was labeled as endothermic. In an endothermic reaction, the change in enthalpy (∆H) equals the energy in joules absorbed by the system from the surroundings. Because energy is absorbed into the system, ∆H is positive.

In lesson 4, we observed the synthesis of an ionic compound. An increase in temperature of the surrounding was observed, and this reaction was labeled as exothermic. In an exothermic reaction, the change in enthalpy (∆H) equals the energy in joules released by the system to the surroundings. Because energy is released into the surroundings, ∆H is negative.